Development and Usability Evaluation of Inventive and Repurposable Children’s Furniture

Abstract

This study developed inventive and repurposable children’s furniture to improve functionality and extend product lifespan. Unlike typical cribs which serve a single purpose, the proposed design supports multiple functions: crib, cushioned chair, highchair, walker, toilet attachment, pull-up bar, and bed safety rail. Specific dimensions were established, and the correct material selections were made for the selected concept. This was finalised using Autodesk Inventor 2019 that was used for stress analysis, and material optimisation. Usability tests were conducted to compare the proposed invention with single-function products. These tests were discussed in terms of task completion time, space-saving ability, survey feedback, and a REBA of musculoskeletal risk. It was found that the proposed design could be repurposed in a shorter time and save more space. A majority of the survey participants agreed that it performed well in terms of repurposability, design, space-saving ability, usability, comfort, sustainability, and safety. Additionally, the proposed design is cheaper compared to single-function products. Thus, creating inventive and repurposable children’s furniture can contribute to reducing waste and extending the lifespan of children’s furniture through innovative, cost-effective design solutions.

1. Introduction

Repurposable and multifunctional furniture has gained significant attention as a sustainable solution, aiming to optimise space, reduce the amount of furniture, and extend product usability [1,2], while minimising the disposal of furniture [3,4]. Additionally, this type of furniture is useful for children, as stressed by Romadhona and Joedawinata (2023), who state that optimised furniture design is important in small living spaces to support children’s development [5]. Among all types, children’s furniture is more prone to being discarded because children outgrow their furniture items rapidly. Thus, an effective furniture item for children is one that fulfils all the developmental requirements of children of different age groups without losing its functionality as the children grow [6].

Phuah et al. (2022) proposed a repurposable children’s furniture concept that can serve multiple functions throughout a child’s growth, reducing waste and resource consumption [3]. However, prior to the research published by Phuah et al. (2022), to our knowledge, no study has been published on the conceptualization of inventive and repurposable children’s furniture for better functionality that would promote extended use. Most repurposed furniture designs are limited to ordinary and common functions between transformations, an issue that may be caused due to the underdeveloped research area [3]. This is supported by Cheng et al. (2021), who also reported on the seldom researched area of multipurpose furniture, resulting in the development of foldable furniture [7].

These studies collectively emphasise the importance of this research, while considering factors such as repurposability and multifunctionality in furniture development to promote sustainability. Thus, strategic approaches are required in developing inventive and repurposable furniture to ensure the development of useful products.

Rajan et al. (2019) reported on the design and fabrication of a multipurpose space-saving dining table for Indian middle-class homes. The authors started their design process by listing wood, plywood, glass, plastic, aluminium frame, and stainless steel-frame as commonly used materials. Among these, folding tables made of wood are said to be strong, durable, and aesthetically pleasing. However, aluminium was selected for its added values. Then, a market study on furniture was conducted, in which the market trend and arrival of new products were considered. Following that, a detailed analysis of selected concepts was conducted using Computer-Aided Design (CAD) SOLIDWORKS. The total deformation and Von misses stress analysis was performed using ANSYS under the 25 kg and 50 kg load. The digital modelling was said to provide better visualisation [8].

Romadhona and Joedawinata (2023) reported on the impact of apartment furniture design on children’s development, and provided recommendations for multifunctional furniture. The outline of the work shows that it is in the conceptual phase. The authors’ data collection methods included literature reviews, observations, and interviews. Several recommendations for materials used in the development of children’s furniture emphasise safety, durability, and suitability for children’s needs. These include lightweight materials like thistle particles, polyurethane foam, and solid wood, synthetic materials like imitation leather and plastics, and safe finishing substances such as varnish, melamine, wax, bleach, and paints. Finally, the authors recommend optimising furniture design in terms of construction system, materials, safety measures, dimensions, and age-appropriate usage [5]. This is supported by Georgeta and Goanta (2017) who reported that multifunctional furniture is the answer for optimal space utilisation [9].

While significant progress has been made in designing multifunctional and sustainable furniture, the research remains insufficiently explored. Subsequent to concept selection, current designs often lack a systematic way of selecting and discussing product dimension and product material, as well as usability tests to evaluate the feasibility of the product. This study addresses all the mentioned issues in developing inventive and repurposable children’s furniture.

2. Materials and Methods

2.1. Refinement of Conceptual Design

2.1.1. Concept Selection

Five design concepts have been developed through researching patents and journal papers that comprised various products such as children’s furniture, conventional furniture, and everyday products. Then, concept screening and scoring were conducted as previously reported [3]. As a result, Concept 1 was selected as shown in Figure 1. The primary function of this concept includes a baby’s crib as shown in Figure 1a. The crib provides a safe and comfortable space for the baby to rest and sleep in. Once the baby has outgrown the crib, the crib frame can be removed and repurposed as a cushioned seat as shown in Figure 1b. Once the storage box is removed, it reveals two repurposed chairs (two chairs of the same design) as shown in Figure 1c. The chairs provide a comfortable area for sitting and resting in general. Each of these chairs can be repurposed to a highchair as shown in Figure 1d. The highchair is suitable for a young child. It provides a safe area for the child to sit on during meals. Once the highchair has exceeded its useful life cycle, it can be repurposed into a walker as shown in Figure 1e by removing the wooden panels. The walker is appropriate for the elderly or disabled because it provides additional support in walking activities [10]. The walker can be repurposed as a toilet attachment as shown Figure 1f. The toilet attachment is mounted around the toilet bowl. It provides handlebars that allow for ease of getting on and off the toilet. In addition, it can be used to pull oneself up after slipping, as there are handlebars on the side of the toilet attachment. Once the toilet attachment has reached the end of its life cycle, it can be taken apart and reassembled to form a pull-up bar as shown in Figure 1g. The pull-up bar allows the user or even a growing child to exercise and live an active and healthy lifestyle. Lastly, the toilet attachment can also be repurposed into a safety rail as shown in Figure 1h. The safety rail attaches onto the side of a bed and prevents bed-falling accidents which may cause major injuries for a growing child or the elderly.

2.1.2. Dimension Establishment

Table 1 shows the dimensions of the selected design. Each repurposed part has its own dimensions. Thus, a range of suitable dimensions for each product must be established. The dimension is obtained from patents and existing products. This step is crucial in ensuring that each product’s dimension is not beyond or below the limit of the desired dimension. Exceeding the range may result in reduced functionality or functional errors.

2.1.3. Material Selection

Table 2 shows the common materials used for the existing furniture and products related to this study. The common materials used include wood, aluminium, and steel. Each material has its advantages and disadvantages. Thus, further selection is needed using the Ashby chart.

Using Equations (1) and (2), and accounting for all the values assumed, the required Young’s modulus is found as 67.75 GPa. Thus, only materials that exceed the minimum Young’s modulus value are considered when using the Ashby chart.

(1)$\mathrm{I}={\displaystyle \frac{{{\mathrm{b}}_1\mathrm{d}}_1^3}{12}}-{\displaystyle \frac{{{\mathrm{b}}_2\mathrm{d}}_2^3}{12}},$

(2)$\mathrm{E}={\displaystyle \frac{{\mathrm{F}\mathrm{L}}^3}{48\mathsf{\delta }\mathrm{I}}},$

where

• I = Moment of inertia,

• b₁ = d₁ = 2.5 cm,

• b₂ = d₂ = 2 cm,

• L = 50 cm,

• δ = allowable deflection = 0.1 cm,

• F = 750 N (halved weight of maximum load).

Foams, elastomers, and polymers are eliminated from the selection as these materials are not suitable for high tensile load applications, and do not meet the minimum Young’s modulus threshold. In addition, glass and ceramics are excluded as these materials are brittle and may not be able to withstand the force applied. Oak wood is not chosen in the screening process for the frame as it does not meet the minimum Young’s modulus threshold as well.

Material selection for the frame: Table 3 shows the material properties of aluminium, zinc, mild steel, and carbon fibre reinforced polymer. The performance index of each material is calculated to determine the best performing material. Carbon fibre reinforced polymer performed the best, followed by aluminium, zinc, and steel. Even though carbon fibre reinforced polymer had the best performance index, it is not considered in the further selection process due to its high cost and difficulty in manufacturing.

Table 4 shows the final selection of materials. Aluminium is chosen as the reference material as it is the most common material used in most of the repurposed products related to the project. Each material is compared based on aspects of cost, weight, strength, durability, manufacturability, and repurposability. Materials that performed better than the reference in a criterion are rated with ‘+’, while materials that performed worse than the reference are rated with ‘−’. It is found that aluminium 6061 is the best performing material. However, the performance of mild steel is just one rating below aluminium 6061. Research shows that the use of aluminium and steel are common across many repurposed products. Therefore, simulations are performed in Autodesk Inventor 2019 (USA) to compare the performance of these two materials for further decisions.

Material selection for the chair and highchair: Table 5 shows the material properties of the selected materials for the chair and highchair. The baby seat doubles as a normal chair. Thus, the material has to be able to withstand moderate tensile loads. The most common materials for the production of chairs and highchairs are wood and plastic. Thus, the performance index is calculated as shown in Table 5. Oak wood performed the best, followed by plastic and aluminium.

Table 6 shows the selection table of materials for the chair. Oak wood is chosen as the reference material as it is the most common material used in chairs. Plastic and aluminium are compared to oak wood based on criteria such as cost, weight, strength, durability, manufacturability, and repurposability. Oak wood and plastic are more promising materials for the chair and highchair rather than aluminium. Thus, simulations are performed in Autodesk Inventor 2019 for further decisions.

2.1.4. Simulation and Optimisation

Table 7 shows the simulation results on repurposed parts of the invention. The simulation and optimisation of the repurposed parts are crucial in ensuring that the products are not overengineered. Stress simulations are run on the walker, toilet attachment, pull-up bar, and safety rail to determine the von Mises stress, displacement, and factor of safety. During the stress simulation of the invention made of mild steel hollow bars with a thickness of 2.5 mm, the repurposed parts had a safety factor range of 8.02 to 15. This factor of safety is considered high, indicating that the design might be overengineered. Thus, the material of the repurposed parts is replaced with aluminium with a thickness of 2.5 mm. The results indicated that the walker still had a minimum safety factor of 13.91 which is still high. Therefore, further optimisation is performed by reducing the thickness of mild steel and aluminium from 2.5 to 1 mm. Mild steel at 1 mm thickness resulted in a safety factor range of 2.76 to 4.9 which is an acceptable range for the safety factor. Similarly, the aluminium bar with a thickness of 1 mm provided a safety factor range of 2.53 to 4.37 which is also an acceptable range.

The maximum displacement of the steel and aluminium with a thickness of 1 mm is 0.6571 mm and 2.514 mm, respectively, which is negligible. Thus, according to the simulation results, aluminium and mild steel with a thickness of 1 mm are both suitable for the project. However, due to the cost constraints and material availability of aluminium, it is decided that steel (thickness of 1 mm) would be a better option for this project.

Walker and toilet attachment simulation: Table 8 shows the simulation results of the walker and toilet attachment. The simulation for the walker and toilet attachment are performed simultaneously as both products need to withstand the same amount of load at similar points. The maximum load applied on the repurposed parts is 1500 N which simulates an extreme case of loading. Four different simulations are run with varying materials and thicknesses. The first simulation is performed with mild steel with a thickness of 2.5 mm. Since the safety factor is high, the material is changed to aluminium. However, a hollow aluminium bar with a thickness of 2.5 mm still resulted in an unacceptable safety factor of 13.91. Thus, the material is changed to steel and aluminium with a thickness of 1 mm, producing safety factors of 4.9 and 4.37, respectively. Mild steel with a thickness of 1 mm provided an acceptable maximum von Mises stress of 65.09 MPa, while aluminium with a thickness of 1 mm provided an acceptable maximum von Mises stress of 65.21 MPa. The maximum deformations of 0.2247 mm and 0.7154 mm are recorded for mild steel and aluminium, respectively. The displacements can be neglected since the maximum deformation is small compared to the size of the invention.

Pull-up bar simulation: Table 9 shows the simulation results of the pull-up bar. The pull-up bar needs to be lightweight yet able to withstand the loading from the user. By assuming an extreme loading of 1500 N from the user, four different simulations are run with varying materials and thicknesses. The simulation with mild steel (2.5 mm thickness) presented a high safety factor. Therefore, the material is changed to aluminium. However, a hollow aluminium bar (2.5 mm thickness) still resulted in an unacceptable safety factor. Hence, the thickness of the mild steel and aluminium bars are reduced to 1 mm, and the safety factors were recorded as 2.76 and 2.53, respectively. Maximum deformations of 0.1793 mm and 0.5761 mm are found for mild steel and aluminium, respectively, which are negligible values.

Safety rail simulation: Table 10 shows the simulation results of the safety rail. The safety rail has to be lightweight yet able to withstand the load applied by the weight of the user. The maximum load applied on the repurposed parts is 1000 N, which simulates the extreme cases of usage. Four different simulations are run with varying materials and thicknesses. The first simulation is performed with mild steel with a thickness of 2.5 mm. Due to the safety factors being too high, the structure is optimised by changing the material to aluminium, though a hollow aluminium bar with a thickness of 2.5 mm still resulted in an unacceptable safety factor. Hence, the thicknesses of the mild steel and aluminium bars are modified to 1 mm, resulting in acceptable safety factors of 4.81 and 3.28, respectively. Maximum deformations of 0.6571 mm and 2.514 mm are found for mild steel and aluminium, respectively, which are negligible values.

Crib and chair simulation: Table 11 shows the simulation results of the crib and chair. The crib and chair have to be lightweight yet able to withstand the load applied by the weight of the user. The maximum load applied on the repurposed parts is 1000 N, which simulates the extreme cases of usage. Two different simulations are run with varying materials. The first simulation is performed with ABS plastic. Due to the safety factors being too high at 9.94, the structure is optimised by changing the material to oak wood. The oak wood produced a safety factor of 2.04 which is acceptable. The maximum displacement is 0.2297 mm, which can be neglected as it is small compared to the overall size of the invention.

Cushioned chair: Table 12 shows the simulation results of the cushioned chair. The cushioned chair has to be able to withstand the load applied by the weight of the user. The maximum load applied on the repurposed parts is 1500 N, which simulates the extreme cases of usage. Two different simulations are run with varying materials. The first simulation is performed with ABS plastic. It resulted in a safety factor of 15 which is considered overengineered. The next simulation involved oak wood. Using oak wood as the chair’s material resulted in a safety factor of 3.26 which is acceptable. The maximum von Mises stress of 1.606 MPa and maximum displacement of 1.9 mm are also acceptable.

After the simulations and optimisations, it is found that mild steel at a thickness of 1 mm is the most suitable material for the prototype. Due to manufacturing restrictions, aluminium 6061 is not chosen as the material for the invention. Although it is a common material used in products such as walkers, pull-up bars, and chairs, the cost to manufacture an aluminium prototype is high. Thus, mild steel at a thickness of 1 mm is the most suitable material to be used for the proposed invention.

Oak wood is chosen in comparison to ABS plastic for the crib and seat material. Oak wood is chosen due to its manufacturability and accessibility. In addition, oak wood is a more environmentally friendly material as compared to plastic.

2.2. Finalised Design

Figure 2 shows the finalised design after the optimisation process. The materials used for the final design are aluminium with a 1 mm thickness for the frame, and oak wood for the crib and seat. The finalised design starts off as a crib and can be repurposed into a cushioned chair, regular chair, highchair, walker, toilet attachment, pull-up bar and safety rail. Table 13 shows the functions of each repurposed product.

2.3. Mechanisms and Components

Table 14 presents the advantages and disadvantages of the baby chair mechanism. The baby chair mechanism functions to repurpose the baby chair from its previous life cycle of a chair. The mechanisms considered for the baby chair are a nut and bolt mechanism and a hinge mechanism. The mechanisms are compared based on ease of repurposability, stability, manufacturability, and requirement of tools.

The foldable hinge is chosen because it allows for the repurposing process to be performed without the need of any additional tools. Although it has a moderate stability compared to the fixed mechanism, this criterion holds less importance because the weight exerted by a baby or toddler is relatively small. In addition, the foldable hinge can be bought and used easily.

Table 15 presents the advantages and disadvantages of the walker frame mechanism. The walker frame mechanism functions to securely hold all the components of the walker in place. In addition, the mechanism has to be removable to allow the walker to be split and repurposed into another life cycle. The mechanisms considered for the baby chair are the brackets with butterfly nuts, and the hole and bolt mechanism. The mechanisms are compared based on ease of implementation, ease of repurposability, stability, manufacturability, and requirement of tools.

Both mechanisms are very similar in function. However, the butterfly nut is more advantageous in ease of repurposing. The butterfly nut allows for removal of the nut without any tools such as a wrench. In addition, the bolt-hole mechanism is harder to implement as the female end of the bolt has to be welded into the frame. Each beam of the frame will also be attached to each other, potentially causing several dimension problems apart from looking aesthetically unappealing.

Table 16 shows the advantages and disadvantages of the crib mechanism. The crib mechanism functions to secure all the components of the crib in place. The mechanisms considered for the crib are the solid part and the locking mechanism. The solid part is integrated into the child’s crib and consists of a wooden block with grooves that allows the components to be slotted in and secured as shown in Figure 3. The locking mechanism is a simple lock to hold two components in place. The mechanisms are compared based on ease of implementation, cost, ease of repurposability, stability, manufacturability, and requirement of tools.

The solid part is chosen because it provides good stability in holding all the parts in place. In addition, it allows for easy repurposing for the next life cycle’s function since there are no nuts and bolts involved, hence requiring no tools for the repurposing process.

2.4. Usability Test Plan

2.4.1. Test Plan 1: T-Test (Time Usage)

For test 1, the time usage and efficiency of the proposed invention is tested. The test mainly focuses on the conversion time of the proposed invention in comparison with conventional furniture. This is significant for evaluating the practicality and convenience of the proposed design. The result will help determine whether the process of repurposing the furniture is efficient enough to encourage the adoption. Two scenarios are created to test the invention. Each scenario is tested with a control and test group. Figure 4 shows the illustration of the test scene. Room 1 contains a highchair and a crib. Room 2 contains a cushioned seat and two chairs. Lastly, the test room contains the proposed invention and a chair.

• Case A (Control group): Performs the tests with conventional furniture.

• Case B (Test Group): Performs the tests with the proposed invention.

The time taken to complete the tasks by the control and test group are tabulated and compared. Minitab 19 is then used to carry out a t-test to determine whether there is a significant difference between the two groups regarding the time in completing the tests.

Scenario 1: This scenario is made to test the usability of the crib, chair, and highchair. The scenario is firstly tested with a control group, and then the test group. Due to COVID-19 restrictions, a doll is used to represent the toddler. The following steps are adhered to when testing the control and test groups.

• Control Group:

• 1.. The timer starts when the participant picks up the doll at point A.

• 2.. The participant places the doll on a single-function chair at point B.

• 3.. The doll is left on the chair for 5 s (for any possible toddler activity).

• 4.. The participant then walks into room 1 to retrieve the single-function highchair.

• 5.. The participant then carries the single-function highchair into the test room and places the single-function highchair on point X.

• 6.. The participant retrieves the doll from the chair and places the doll on the highchair.

• 7.. The experimenter pretends to feed the doll three spoons of food as a means to simulate the related toddler activity before the timer is stopped.

• Test Group:

• 1.. The timer starts when the participant picks up the doll at point A.

• 2.. The participant places the doll on the proposed invention in its chair-transformed state at point B.

• 3.. The doll is left on the chair for 5 s (for any possible toddler activity).

• 4.. The participant removes the doll from the proposed invention and places the doll at point Y.

• 5.. Instead of retrieving a highchair, the participant converts the proposed invention into a highchair.

• 6.. The participant retrieves the doll and places it on the highchair.

• 7.. The experimenter pretends to feed the doll three spoons of food as a means to simulate the related toddler activity before the timer is stopped.

Scenario 2: This scenario is made to test the usability of the crib, cushioned seat, and chair. The scenario is firstly tested with the control group, followed by the test group. Once again, due to COVID-19 restrictions, a doll was used to represent the toddler. The following steps are adhered to when testing the control and test groups.

• Control Group:

• 1.. The timer starts when the participant goes into room 1 to retrieve the doll from the crib.

• 2.. The participant then places the doll on point A in the test room.

• 3.. The participant walks from the test room into room 2.

• 4.. The participant then carries a cushioned seat and returns to the test room to place it on point X.

• 5.. The participant then goes into room 2 to retrieve another chair, and places it on point C.

• 6.. Step 5 is repeated with another chair placed on point Z.

• 7.. The participant goes to point A to retrieve the doll.

• 8.. The timer stops after the participant sits on the chair while carrying the doll at point Z for 5 s.

• Test Group:

• 1.. For this group, the crib and doll are already in the test room.

• 2.. The timer starts when the participant retrieves the doll from the crib and places the doll on point A.

• 3.. Instead of retrieving the single-function items, the participant proceeds to convert the crib into a cushioned seat and 2 chairs.

• 4.. The participant then carries the cushioned seat and places it on point X.

• 5.. The participant then places the 2 chairs on point Y and point Z, respectively.

• 6.. The participant goes to point A to retrieve the doll.

• 7.. The timer stops after the participant sits on the chair while carrying the doll at point Z for 5 s.

Scenario 1 hypotheses: With a significance value, α, set at 0.05, the hypotheses for test 1—scenario 1 are formulated as such:

• Null hypothesis, H_01.1—There is no significant difference in the time to complete the tasks between Case A1 and Case B1.

• Alternative hypothesis, H_a1.1—There is a significant difference in the time to complete the tasks between Case A1 and Case B1.

Scenario 2 hypotheses: With a significance value, α, set at 0.05, the hypotheses for test 1—scenario 2 are formulated as such:

• Null hypothesis, H_01.2—There is no significant difference in the time to complete the tasks between Case A2 and Case B2.

Alternative hypothesis, H_a1.2—There is a significant difference in the time to complete the tasks between Case A2 and Case B2.

2.4.2. Test Plan 2: Span Availability Test

Repurposable furniture saves space because the furniture can be separated into many other components from its initial setup. The repurposed parts can be stored in the footprint of a crib when not in use, saving even more space than before and avoiding clutter. Each component can then be repurposed whenever necessary.

To test the space-saving capability of the repurposable invention, the size of each repurposed parts is collected. The repurposed parts are then drawn in Autodesk Inventor 2019 to visualise the amount of space needed for each individual component in a 4 × 4 m² room. The crib is also drawn to visualise its footprint in the room. The space taken up is then compared with the space consumed by the proposed invention.

2.4.3. Test 3: Usability Survey and Feedback

The participants involved in the control and test groups are also requested to fill out a questionnaire comprising various usability questionnaire items regarding the proposed invention. The questionnaire items are classified under variables such as repurposability, price, design, space-saving effectiveness, usability, comfortability, sustainability, and safety. A 5-point Likert scale ranging from strongly agree to strongly disagree is used to score each individual questionnaire item. The survey ends with an unstructured feedback section where the participants are able to voice their positive and negative opinions on the invention, depending on their needs. This is important for a specific style furniture design as highlighted by a furniture teaching plan [22].

2.4.4. Research Ethics

All the participants had filled out a consent form prior to the experiments. In addition, all procedures and protocols have been approved by the Research Ethics Committee (REC) from the Technology Transfer Office (TTO) of Multimedia University. The research ethics approval for the project has been granted on 7 March 2022 with the approval number of EA0172022.

2.4.5. Test 4: Benefit of Repurposability

Sustainability score sheet: Repurposable products are generally more eco-friendly than conventional products due to the multiple life cycles and functions. A repurposable product should last longer compared to a conventional product and result in less waste generation.

To test the sustainable aspect of the proposed invention, the invention is compared with other eco-friendly products in the market. Each design is judged based on a variety of criteria that relates to sustainability, including material selection, manufacturing process, effective lifespan, waste generation, and disposal impact. In addition, a life cycle assessment is performed on the proposed invention. The proposed invention is assessed by its material selection, manufacturing process, use, and end of life to determine its sustainability and impact to the environment.

The main benefit of repurposing is to extend a product’s effective lifespan. Repurposing a product allows the product to obtain a new function and be useful for a longer period of time. Thus, it is relevant to compare the total effective lifespan of the proposed invention with the lifespan of conventional products. The comparison includes the use of online sources in indicating a product’s expected lifespan. The total lifespan of all the products is then added up and compared to the effective lifespan of the proposed invention.

2.4.6. Test 5: REBA

The Rapid Entire Body Assessment (REBA) is used to rapidly evaluate risks of musculoskeletal disorders (MSD) associated with certain job tasks. It is suitable in evaluating the risk posed by the invention onto the user.

The REBA focuses on the postural analysis of the user when performing various tasks. It assesses the risks on the upper arms, lower arms, wrist, trunk, neck, and legs when performing tasks. There are a few objectives to be achieved with the use of the REBA, namely:

1. To provide a simple postural analysis system sensitive to musculoskeletal risks in a variety of tasks.

2. To divide the body into segments to evaluate individually with reference to postures and movement planes.

3. To provide a scoring system for muscle activity caused by static, dynamic, rapid changing or unstable postures.

4. To give an action level output with an indication of urgency.

5. To provide a user-friendly assessment tool that requires minimal time, effort, and equipment.

To perform the REBA, the participants are directed to perform specific tasks with the proposed invention. Photos of the participants performing the tasks are then captured. The posture of the participant in the pictures are analysed using the REBA tool as shown in Figure 5 [23]. In addition, a computer software for the REBA tool is used to in calculating the REBA scores. Each of the scores are inspected to determine the relevant risks.

3. Results

3.1. Proposed Invention Functions and Conversions

Crib to cushioned chair and chair: The primary form of the invention includes a baby crib as shown in Figure 6. In addition, the wooden crib acts as a locking mechanism that holds every component in place. To convert the crib into the cushioned chair, the wooden crib is lifted, flipped around, and placed on the floor as shown in Figure 7. The cushioning used in the crib is removed and placed on the flipped wooden crib to form the cushioned chair. In addition, the wooden crib is lifted in the repurposing process to reveal the two chairs underneath as shown in Figure 7a and Figure 8. These chairs can be seats for family and guests in the home.

Chair to highchair: To convert the chair in Figure 9a to a highchair, the hinged part of the chair is lifted as shown in Figure 9b. The wooden plank is then secured to the metal frame of the invention with a locking mechanism as per Figure 9c. Thus, the repurposing process of the highchair is performed as shown in Figure 9d and Figure 10. The highchair can be used to safely seat a child during mealtimes and other activities.

Chair to walker and toilet attachment: To convert the chair into the walker and toilet attachment, the user has to lift the wooden seat as shown in Figure 11a,b. Once the wooden seat is removed, the walker and toilet attachment are functional. The walker as shown in Figure 12a can be used by the elderly as an assistive tool to be more mobile outdoors and around the house. The toilet attachment as shown in Figure 12b can also be used by the elderly to get in and out of the toilet easily. The handlebars of the invention can enhance their support when in the toilet.

Walker to safety rail: The walker is held together by joints as shown in Figure 13a. The joints can be loosened for the individual frame parts to be disassembled, and to be reassembled into a bed safety rail. To convert the walker into the safety rail, one side of the walker has to be removed as shown in Figure 13b. Once removed, the two remaining parts should be placed in the orientation as shown in Figure 13c,d. The safety rail will be fitted between the bed frame and bed as shown in Figure 14. The safety rail prevents the user from falling off the bed. It is useful for children and the elderly in preventing bed-falling accidents.

Walker to pull-up bar: The walker is held together by joints as shown in Figure 15a. The joints can be loosened for the individual frame parts to be reassembled into a pull-up bar. Firstly, the two long frame tubes are removed as shown in Figure 15b,c to be used as the handle and mounting bar of the pull-up bar. Subsequently, all the other frame parts are removed, leaving only the bended bars as shown in Figure 15d. Customised brackets are then used to secure the two long frame tubes from before with the bended bars as shown in Figure 15e,f. The finished assembly of the pull-up bar can be seen in Figure 16a,b. Once assembled, the pull-up bar can be hooked and mounted at the doorway as shown in Figure 16c–e. This mode allows the user to exercise from the comfort of their own homes.

3.2. Usability Test Results

3.2.1. Test 1: Two-Sample T-Test Results

Scenario 1: Table 17 shows the results of the two-sample t-test for scenario 1 obtained from Minitab-19. It was found that the results of the control group and test group differed significantly [t(10) = 12.01, p < 0.05], with samples from Case B1 completing tasks in a shorter amount of time compared to samples from Case A1 (M_B1 = 45.33 s, M_A1 = 70.92 s). Therefore, H_a1.1 was accepted.

Scenario 2: Table 18 shows the results of the t-test for scenario 2 obtained from Minitab-19. It was found that the results of the control group and test group differed significantly [t(10) = 29.32, p < 0.05], with samples from Case B1 having a shorter completion time as compared to samples from Case A1 (M_B1 = 48.02 s, M_A1 = 9349 s). Therefore, H_a1.2 was accepted.

Power analysis: A power analysis was performed for the two-sample t-tests with the statistical power set at 80% to predict the actual sample size needed for the usability tests. Before performing the power analysis, the mean, mean difference and standard deviation were calculated with the following equations. Table 19 shows the calculation results of the mean, mean difference, and standard deviation of all the scenarios.

(3)$\mathrm{M}\mathrm{e}\mathrm{a}\mathrm{n},\mathrm{M}={\displaystyle \frac{{\sum }_0^{\mathrm{n}}{\mathrm{x}}_{\mathrm{i}}}{\mathrm{n}}},$

(4)$\mathrm{M}\mathrm{e}\mathrm{a}\mathrm{n} \mathrm{D}\mathrm{i}\mathrm{f}\mathrm{f}\mathrm{e}\mathrm{r}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{e}={\mathrm{M}}_2-{\mathrm{M}}_1.$

(5)$\mathrm{S}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{a}\mathrm{r}\mathrm{d} \mathrm{D}\mathrm{e}\mathrm{v}\mathrm{i}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n},\mathsf{\sigma }=\sqrt{{\displaystyle \frac{{{\sum }_0^{\mathrm{n}}{|\mathrm{x}}_{\mathrm{i}}-\mathrm{M}|}^2}{\mathrm{n}}}}.$

(6)$\mathrm{P}\mathrm{o}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{d} \mathrm{S}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{a}\mathrm{r}\mathrm{d} \mathrm{D}\mathrm{e}\mathrm{v}\mathrm{i}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n},\mathrm{S}=\sqrt{{\displaystyle \frac{{{\mathsf{\sigma }}_1}^2+{{\mathsf{\sigma }}_2}^2}{2}}}.$

Note:

• x_i—total time taken to complete the test for case A or B in each scenario,

• n—number of samples,

• M₁—mean value of time taken for case A in each scenario,

• M₂—mean value of time taken for case B in each scenario.

Using the power and sample size estimator in Minitab 19, the predicted sample sizes for all the cases were computed. Table 20 shows the results of the predicted sample sizes and actual power level for the usability tests. Based on the actual statistical power from Minitab 19, it was found that every scenario had a statistical power of over 80% with only a predicted sample size of two samples. Thus, the current sample size of six participants used to produce the datasets is deemed sufficient. The study proceeded with the two-sample t-tests.

3.2.2. Test 2: Space-Saving Ability Results

Table 21 shows the average dimension of repurposed products collected from online resources such as patents and journals. This information was needed in visualising and comparing the space-saving effectiveness of the proposed invention.

The dimension of each product is drawn in Autodesk Inventor 2019 as shown in Figure 17 to visualise the space taken up by all the products. It was observed that the crib, pull-up bar, chair, walker, toilet attachment, and bed safety rail took up the majority of the space in the 4 × 4 m room.

In Figure 18, it was found that the footprint of the crib is significantly smaller compared to all the repurposed parts. The proposed crib design allows all the repurposed parts to be stored within the footprint of the crib when not in use. Thus, the proposed invention demonstrated an effective space-saving capability.

The total area taken up by all the conventional products is 2.95 m² while the total area taken up by the proposed invention is 0.6 m². Thus, the repurposed invention is able to save approximately five times more space than the conventional products. This finding supports the space-saving ability of the proposed invention.

3.2.3. Test 3: Usability Survey and Feedback Results

Demographics: According to the demographic data obtained during the experiments, as shown in Table 22, the participants were equally accounted for at 50% for ages 20 to 40 years and 60 to 80 years. The wide range of participants age is due to the varying intended users for the repurposed furniture, whereby the repurposed chair can be used for all ages, the pull up bar can be used by adults, while the walker and toilet attachment is appropriate for the elderly. Males and females were also equally accounted for at 50%, respectively. In terms of repurposability, 83.34% of the participants agreed that the invention performed well. Additionally, the participants also affirmed that the proposed invention performed well in terms of space-saving ability (88.89% agree), usability (75.00% agree), comfort (55.56% agree), and safety (55.56% agree).

Unstructured feedback: Table 23 shows the summary of the participants’ unstructured feedback on the proposed invention. Based on the feedback, the participants stated that the proposed invention is creative, possesses good stability, possesses numerous repurposable functions, and is reasonably priced. In addition, the repurposing process of the invention is smooth, and it can be stored in a smaller space when not in use as compared to conventional products. However, it was also highlighted that the proposed invention was not easy to move around the home as it was bulky and heavy. Furthermore, the baby chair had holes that were too large, which may be dangerous to the toddler if left unmonitored. Moreover, some repurposing processes may require two people for the full execution of the processes.

Survey analysis: Spearman’s correlation analysis was carried out among the variables surveyed (usability, repurposability, comfortability, space-saving effectiveness, and safety). Table 24 shows the results involving the correlations. It was found that a few variables are strongly and significantly correlated with each other, namely:

• usability and repurposability (r = 0.822, p < 0.05),

• comfortability and space-saving effectiveness (r = 0.861, p < 0.05),

• comfortability and safety (r = 0.833, p < 0.05).

Repurposability and usability are strongly and significantly correlated with each other partly because these two criteria were emphasised considerably during the design and conceptualisation phase. The repurposing processes were designed to be user-friendly and straightforward for the end user. In addition, the numerous repurposable functions provide users with a very practical invention that can be used for various daily tasks. This understanding is related to Husein’s [2] study, who found that easily convertible furniture provides users who live in small apartments or small spaces a higher level of satisfaction when compared to conventional furniture (23.2% satisfaction).

Space-saving effectiveness and comfortability are also strongly and significantly correlated with each other due in part to these criteria being carefully considered during the design process. The proposed invention is designed to be space-saving because of the many joined and interlinked parts which allow the invention to be interchangeable across different functions. However, during the design of these parts, the comfortability was not neglected, and features such as cushioning were introduced to increase user comfort. In addition, space-saving effectiveness provides comfort to users living in a small space as the user would not feel cramped or claustrophobic. This notion is aligned with Husein’s [2] study, who suggested that conventional furniture would not necessarily provide comfort in small-space living, and that only 19.4% of users in the study indicated that they felt comfortable with conventional furniture in their apartments.

Finally, comfortability is also found to correlate strongly and significantly with safety because the invention was designed to provide the end user with the best user experience. During the design process, the cushioning was used to cover the sharp edges of the proposed invention and to ensure the user’s safety. This suggestion can be related with Fawley’s [24] study who suggested that the consideration of furniture safety is important in the pursuit of designing comfortable and aesthetically pleasing furniture.

However, several variables are also not significantly correlated with each other (p > 0.05). These variables included:

• usability and comfortability,

• usability and space-saving effectiveness,

• usability and safety,

• repurposability and comfortability,

• repurposability and space-saving effectiveness,

• repurposability and safety,

• space-saving effectiveness and safety.

The usability aspect may not correlate with comfortability, space-saving effectiveness, and safety, as the usability aspect may prioritise function over the form of the invention. Thus, the prioritisation of functionality in the invention may have resulted in the slight neglect of comfortability, space-saving effectiveness, and safety.

The repurposability aspect may not correlate with comfortability because the implementation of repurposability requires very specific design choices such as unavoidable sharp angles at some sections of the invention which may result in decreased comfort. The repurposability aspect does not correlate with space-saving effectiveness in the proposed invention because, during the design phase, some of the overlapped dimensions had to be chosen to accommodate all the repurposed functions. Thus, the repurposed functions may have been supported by a bulkier structure or feature, which takes up more space than a usual single-function product. However, as a whole (considering all functions in one), the invention still saves more space compared to several single-function products.

The repurposability criteria also does not correlate with safety because fitting all the repurposed functions into one design required specific design decisions and selections in terms of the dimensions. However, some of these dimension selections or features might create risks to the user, such as the presence of large holes at the sides of the baby chair which might result in the toddler falling out of the chair if left unmonitored.

3.2.4. Test 4: Benefit of Repurposability Result

Sustainability score sheet: As shown in Table 25, the proposed invention was compared with different eco-friendly and convertible furniture in terms of material selection, manufacturing process, effective lifespan, waste generation, and disposal impact. The proposed invention was compared with a few sustainable products. Product A is a multipurpose table that can be used as a chair, desk, or rocking chair by changing its orientation [25]. Product B is a rack made out of paper composite resulting in less harm to the environment when discarded [26]. Lastly, product C is an adaptive furniture that can be converted into a bed, chair, or storage unit that enables the product to have a long effective lifespan [27]. Ultimately, the overall performance of the proposed invention surpassed all the other products at a weighted score of 4.2.

Life cycle assessment: Table 26 shows the effective lifespan of each conventional product that can be repurposed into from the proposed invention. The effective lifespan of each product not only represents the rate of deterioration, but also the expected usage life of the product. These single-function items may easily be discarded over time because its function is no longer useful or needed.

The lifespan of the proposed invention was obtained by adding up the effective lifespans of all the products and subtracting this total with an estimated duration of deterioration.

Taking into account the wear and tear of the proposed invention, the effective lifespan was reduced by 15 years. Even with this reduction, it was found that the proposed invention has a much longer effective lifespan of 19 years as compared to each individual conventional product.

3.2.5. Test 5: REBA Result

The proposed invention was tested using the REBA as shown in Figure 19. The invention was tested when it was in the form of a chair, walker, and toilet attachment. These activities were chosen as they may contribute to some adverse postural effects towards the user. The risk levels of each activity can be seen in Table 27.

Each of the scores were analysed to indicate the relevant risk levels as shown in Table 28. It was found that the chair induced a negligible risk which required no further corrective actions, while the walker and toilet attachment induced low risks. Thus, the walker and toilet attachment may be modified in the future to further reduce the risks, such as reducing the height of the invention to improve the wrist and lower arm posture. The modification of the design ensures that the posture is not maintained during future uses and reduces the risk of musculoskeletal disorders.

3.3. Cost Accounting

Cost accounting is used in this study to aid in future ventures such as the manufacturing, marketing, and commercialisation of the proposed invention. The cost accounting provides future investors or researchers with a rough idea of the manufacturing costs of the product as well as the possible revenue to be made.

Assuming that the business is small, it would be treated as a micro-enterprise which is the smallest enterprise in Malaysia. A micro-enterprise comprises either less than five full-time employees or an annual sales turnover of less than RM 250,000.

3.3.1. Variable Cost and Sales Price (Per Unit)

Variable cost: Variable cost per unit refers to the amount of labour, materials, and other resources required to produce a product [34]. Table 29 shows the material costs needed to manufacture one unit of the repurposable crib. The total material cost is around RM 610. Furthermore, there may be unexpected additional costs incurred due to defect. Thus, a 5% defect material cost was included into the total material costs of the proposed invention.

3.3.2. Sales Price per Unit

The sales price per unit of the repurposable crib is RM 900, which is a respectable profit margin of around 50% above the material costs of a single unit. As a general rule of thumb, a 10% net profit margin is considered average, a 20% margin is considered high (or “good”), and a 5% margin is low on general products [35]. However, furniture businesses seem to be able to stretch the profit margin to around 50% [36]. Thus, the proposed invention may benefit from having a higher profit margin. Future researchers may increase or decrease the profit margin depending on the situation as well, especially whenever mass production is involved.

3.3.3. Fixed Cost

The estimated fixed costs tabulated in Table 30 include labour costs and rent. The company is assumed to spend RM 1500 per month on the minimum wages of two workers, RM 900 on workshop or factory rent, RM 400 on advertisements, RM 500 on utility fees, and RM 300 on monthly tools and maintenance equipment fees. The total monthly fixed cost was found to be RM 5100. Thus, the estimated yearly fixed cost is RM 68400.

3.3.4. Break-Even Analysis

A break-even analysis is a financial calculation that weighs the costs of a new business, service, or product against the unit selling price to determine the point at which the business will break even [37]. Thus, it was used in the cost accounting to determine the number of units needed to cover all the aforementioned costs such as fixed cost and variable costs.

(7)$\mathrm{B}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{k} \mathrm{E}\mathrm{v}\mathrm{e}\mathrm{n} \mathrm{P}\mathrm{o}\mathrm{i}\mathrm{n}\mathrm{t}={\displaystyle \frac{\mathrm{F}\mathrm{i}\mathrm{x}\mathrm{e}\mathrm{d} \mathrm{C}\mathrm{o}\mathrm{s}\mathrm{t}\mathrm{s}}{\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l} \mathrm{S}\mathrm{a}\mathrm{l}\mathrm{e}\mathrm{s} \mathrm{R}\mathrm{e}\mathrm{v}\mathrm{e}\mathrm{n}\mathrm{u}\mathrm{e}-\mathrm{C}\mathrm{o}\mathrm{s}\mathrm{t}\mathrm{t}\mathrm{o} \mathrm{M}\mathrm{a}\mathrm{k}\mathrm{e} \mathrm{P}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}}}.$

The break-even quantity was calculated using the above-stated, and was found to be 17.59 (around 18 units). Thus, a profit can be made after the sale of the 18th proposed invention every month. The break-even analysis chart was plotted in Microsoft Excel spreadsheet (version 2019) as shown in Figure 20. Based on the chart, profits will be made if the sales exceeded 18 units per month as well. The selling price and sales target of the proposed invention are considered practical seeing as the total price of the combined functions from existing individual products is usually around RM 2000.

With the assumption that the workers work normal hours in Malaysia, which is from 9 a.m. to 5 p.m. (8 h) every Monday to Friday, the minimum time required to earn profit or produce 18 units of products is calculated using the following equations:

(8)$\mathrm{M}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{m}\mathrm{u}\mathrm{m} \mathrm{T}\mathrm{i}\mathrm{m}\mathrm{e}={\displaystyle \frac{(\mathrm{B}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{k} \mathrm{e}\mathrm{v}\mathrm{e}\mathrm{n} \mathrm{u}\mathrm{n}\mathrm{i}\mathrm{t}\times \mathrm{T}\mathrm{i}\mathrm{m}\mathrm{e} \mathrm{t}\mathrm{a}\mathrm{k}\mathrm{e}\mathrm{n} \mathrm{p}\mathrm{e}\mathrm{r} \mathrm{u}\mathrm{n}\mathrm{i}\mathrm{t})}{(\mathrm{N}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r} \mathrm{o}\mathrm{f} \mathrm{w}\mathrm{o}\mathrm{r}\mathrm{k}\mathrm{e}\mathrm{r}\mathrm{s}\times \mathrm{W}\mathrm{o}\mathrm{r}\mathrm{k}\mathrm{i}\mathrm{n}\mathrm{g} \mathrm{h}\mathrm{o}\mathrm{u}\mathrm{r}\mathrm{s} \mathrm{p}\mathrm{e}\mathrm{r} \mathrm{w}\mathrm{e}\mathrm{e}\mathrm{k})}}.$

(9)$\mathrm{M}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{m}\mathrm{u}\mathrm{m} \mathrm{t}\mathrm{i}\mathrm{m}\mathrm{e}={\displaystyle \frac{(18 \mathrm{u}\mathrm{n}\mathrm{i}\mathrm{t}\times 5 \mathrm{h}/\mathrm{u}\mathrm{n}\mathrm{i}\mathrm{t})}{(2 \mathrm{w}\mathrm{o}\mathrm{r}\mathrm{k}\mathrm{e}\mathrm{r}\times 40 \mathrm{h}/\mathrm{w}\mathrm{e}\mathrm{e}\mathrm{k})}}=1.125 \mathrm{w}\mathrm{e}\mathrm{e}\mathrm{k}\mathrm{s}.$

The business would take about 8 days (1.125 weeks) to gain a profit in an optimal financial situation where the product sells at maximum production rate. The maximum production rate per month and the maximum profit of the first start-up year is calculated using the following equation:

(10)$\mathrm{M}\mathrm{a}\mathrm{x}\mathrm{i}\mathrm{m}\mathrm{u}\mathrm{m} \mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e} \mathrm{p}\mathrm{e}\mathrm{r} \mathrm{y}\mathrm{e}\mathrm{a}\mathrm{r}={\displaystyle \frac{(\mathrm{h}\mathrm{o}\mathrm{u}\mathrm{r}\mathrm{s} \mathrm{w}\mathrm{o}\mathrm{r}\mathrm{k}\mathrm{e}\mathrm{d} \mathrm{p}\mathrm{e}\mathrm{r} \mathrm{y}\mathrm{e}\mathrm{a}\mathrm{r}\times \mathrm{n}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r} \mathrm{o}\mathrm{f} \mathrm{w}\mathrm{o}\mathrm{r}\mathrm{k}\mathrm{e}\mathrm{r}\mathrm{s})}{\mathrm{T}\mathrm{i}\mathrm{m}\mathrm{e} \mathrm{t}\mathrm{a}\mathrm{k}\mathrm{e}\mathrm{n} \mathrm{p}\mathrm{e}\mathrm{r} \mathrm{u}\mathrm{n}\mathrm{i}\mathrm{t}}},$

$\mathrm{M}\mathrm{a}\mathrm{x}\mathrm{i}\mathrm{m}\mathrm{u}\mathrm{m} \mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e} \mathrm{p}\mathrm{e}\mathrm{r} \mathrm{y}\mathrm{e}\mathrm{a}\mathrm{r}={\displaystyle \frac{(1920 \mathrm{h} \mathrm{w}\mathrm{o}\mathrm{r}\mathrm{k}\mathrm{e}\mathrm{d} \mathrm{p}\mathrm{e}\mathrm{r} \mathrm{y}\mathrm{e}\mathrm{a}\mathrm{r}\times 2 \mathrm{w}\mathrm{o}\mathrm{r}\mathrm{k}\mathrm{e}\mathrm{r}\mathrm{s})}{\begin{array}{c}5\mathrm{h}\mathrm{per}\mathrm{unit}\\ =768\mathrm{unit}\end{array}}}.$

(11)$\begin{array}{l}\mathrm{M}\mathrm{a}\mathrm{x}\mathrm{i}\mathrm{m}\mathrm{u}\mathrm{m}\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{f}\mathrm{i}\mathrm{t}\left(\mathrm{f}\mathrm{i}\mathrm{r}\mathrm{s}\mathrm{t}\mathrm{y}\mathrm{e}\mathrm{a}\mathrm{r}\right)\\ =\left(\mathrm{S}\mathrm{e}\mathrm{l}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{p}\mathrm{r}\mathrm{i}\mathrm{c}\mathrm{e}\mathrm{p}\mathrm{e}\mathrm{r}\mathrm{u}\mathrm{n}\mathrm{i}\mathrm{t}\times \mathrm{M}\mathrm{a}\mathrm{x}\mathrm{i}\mathrm{m}\mathrm{u}\mathrm{m}\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{p}\mathrm{e}\mathrm{r}\mathrm{y}\mathrm{e}\mathrm{a}\mathrm{r}\right)-\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{C}\mathrm{o}\mathrm{s}\mathrm{t}\mathrm{s}\mathrm{p}\mathrm{e}\mathrm{r}\mathrm{y}\mathrm{e}\mathrm{a}\mathrm{r}.\end{array}$

$\begin{array}{l}\mathrm{M}\mathrm{a}\mathrm{x}\mathrm{i}\mathrm{m}\mathrm{u}\mathrm{m}\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{f}\mathrm{i}\mathrm{t}\left(\mathrm{f}\mathrm{i}\mathrm{r}\mathrm{s}\mathrm{t}\mathrm{y}\mathrm{e}\mathrm{a}\mathrm{r}\right)\\ =\left(\mathrm{R}\mathrm{M}900\times 768\mathrm{u}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{s}\right)-\left(\right(\mathrm{R}\mathrm{M}5100\times 12)+(\mathrm{RM}610\times 768\left)\right)\\ =\mathrm{RM}161520\mathrm{per}\mathrm{year}\end{array}$

In an ideal situation, the business or company will be able to earn RM 161,520 for the first start-up year with a maximum production rate of 768 units per year. Using the obtained profits, the company may be able to further expand the business to incorporate mass production or further improvements on the product design. Furthermore, the material costs per unit can be reduced from bulk purchasing. Thus, the break-even quantity can be further lowered, and profit could be made sooner.

3.3.5. Cost Comparison

The average price of each individual product was obtained through online resources such as online catalogues and were tabulated as shown in Table 31. The total cost of all the existing products was found to be RM 2020 and the selling price of the proposed invention is RM 900. Thus, a stark difference in price can be seen between the proposed invention and the repurposed products. Customers are able to save more than RM 1000. Thus, the product is deemed to be cost effective.

4. Conclusions

The main aim of this study was to develop inventive and repurposable children’s furniture for improved functionality and extend the product lifespan. Consequently, a repurposable crib concept with seven uses was proposed: crib, cushioned chair, highchair, walker, toilet attachment, pull-up bar, and bed safety rail. The finalised concept used 1 mm thick aluminium for the frame and oak wood for the crib and seat. Through the usability tests, it was found that the proposed invention performed significantly better than conventional products in terms of efficiency of use. The invention could be repurposed in a short amount of time without requiring any tools. After measuring the total space taken up by the proposed invention and the total space taken up by all the conventional products, it was found that the proposed invention was able to save up to about five times more space. Additionally, the parts can be stored in the footprint of the crib when not in use. Therefore, the proposed invention has an effective space-saving ability. Through the survey, it was found that a majority of the participants agreed that the proposed invention performed well in terms of repurposability, price, design, space-saving ability, usability, comfort, sustainability, and safety. Thus, it can be concluded that the proposed device is well-rounded invention by design. The proposed invention was conceptualised and designed to be repurposable into seven different functions after the effective lifespan of each function completes. Existing repurposable products normally have only two transformations. Hence, the proposed invention presents improved repurposability. Using the REBA, it was found that the proposed invention did not induce any risks to the user. Thus, the proposed invention is not likely to cause any musculoskeletal disorders such as sprains and muscle tears. Finally, the proposed design is found to be two times cheaper compared to single-function products, making it substantially a cost-effective product. The invention’s versatility and space-saving design make it ideal for urban households, addressing modern needs for affordability, functionality, and sustainability. Nevertheless, some improvements can be made onto the product. This includes modifications on the walker and toilet attachment to improve the wrist and lower arm posture for further reduction in risks. This can be performed specifically by reducing the height of the invention. Besides that, the cushion design for the crib can be improved by using two separate pieces. This configuration allows one cushion piece to be used independently for the repurposed chair, while both pieces can be combined to provide full coverage and comfort when used in the crib configuration. In conclusion, this research could serve as a foundational study for further research in the field of inventive and repurposable furniture to improve functionality and extend the product lifespan.

Author Contributions

Conceptualization, Z.Y.P., P.K.N., B.K.L., K.W.L. and P.L.C.; methodology, Z.Y.P., P.K.N., B.K.L., K.W.L. and P.L.C.; software, Z.Y.P., P.K.N., B.K.L. and K.W.L.; validation Z.Y.P. and P.K.N.; formal analysis, Z.Y.P. and P.K.N.; investigation, Z.Y.P.; resources, P.K.N., B.K.L. and K.W.L.; data curation, Z.Y.P. and C.H.T.; writing—original draft preparation, Z.Y.P.; writing—review and editing, C.H.T.; supervision, P.K.N., B.K.L. and K.W.L.; funding acquisition, P.K.N., B.K.L. and K.W.L. All authors have read and agreed to the published version of the manuscript.

Informed Consent Statement

Informed consent was obtained from all subjects involved in the study. Written informed consent has been obtained from the patient(s) to publish this paper.

Data Availability Statement

Data available on request due to restrictions.

Acknowledgments

This study was part of the main author’s final year project for his B.Eng. (Hons) Mechanical degree. The researchers appreciate the permission given by the Faculty of Engineering and Technology at Multimedia University (MMU) to conduct this investigation.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Designs for repurposed parts of Concept 1.

Figure 2. Repurposed parts of finalised design.

Figure 3. Solid part integrated into crib.

Figure 4. Test scene layout.

Figure 5. REBA worksheet.

Figure 6. Baby crib.

Figure 7. Cushioned chair repurposing process.

Figure 8. Repurposed chair.

Figure 9. Highchair repurposing process.

Figure 10. Repurposed highchair (with doll representing the toddler or baby).

Figure 11. Walker and toilet attachment repurposing process.

Figure 12. Walker and toilet attachment.

Figure 13. Bed safety rail repurposing process.

Figure 14. Bed safety rail.

Figure 15. Pull-up bar repurposing process.

Figure 16. Door-mounted pull-up bar.

Figure 17. Dimension of all repurposed products in test room.

Figure 18. Dimension of proposed invention in test room.

Figure 19. REBA on chair, walker, and toilet attachment.

Figure 20. Break-even analysis chart.

Table 1

Design dimension table.

Product	Citation	Dimensions
		Length (cm)		Width (cm)		Height (cm)
		Range	Chosen	Range	Chosen	Range	Chosen
Crib	[11]	96–132	100	61–71	60	86–109	110/96
Chair	[12]	48–60	50	48–60	60	>48	48
Walker	[13]	40–50	50	50–60	60	77–96	96
Toilet Attachment		>50	50	>43	60	>41	48
Pull-up bar	[14]	93–110	110	>42	60	30–35	32
Safety Rail	[15]	48–60	50	>42	50	61–71	60

Table 2

Common product materials.

Product	Common Materials	Citation
Crib	Wood, Aluminium	[16]
Chair	Wood, Aluminium	[17]
Walker	Wood, Aluminium	[18]
Toilet Attachment	Aluminium	[19]
Pull up bar	Aluminium, Steel	[20]
Safety Rail	Aluminium, Steel	[21]

Table 3

Material properties of selected materials for frame.

Material	Young’s Modulus (GPa)	Density (kg/m3)	Performance Index (E1/2/p)	Tensile Strength (MPa)	Yield Strength (MPa)
Aluminium alloy(Aluminium 6061)	68	2700	1.0046	324	276
Zinc alloys	90	6700	1.0019	283	-
Steels (Mild steel)	210	7850	1.0017	440	250
Carbon fibre reinforced polymer	110	1550	1.0082	600	3000

Table 4

Selection table of materials for the frame.

Material	Cost	Weight	Strength	Durability	Manufacturability	Repurposability
Aluminium 6061	0	0	0	0	0	0
Zinc	-	-	-	0	-	0
Mild steel	+	-	+	0	0	-

Table 5

Material properties of selected materials for the chair.

Material	Young’s Modulus (GPa)	Density (kg/m3)	Performance Index (E1/2/p)	Tensile Strength (MPa)
Oak Wood	12.3	593	1.0198	5.5
Plastic (ABS)	3.4	1450	1.0076	15
Aluminium alloy(Aluminium 6061)	68	2700	1.0046	324

Table 6

Selection table of materials for the chair.

Material	Cost	Weight	Strength	Durability	Manufacturability	Repurposability
Oak Wood	0	0	0	0	0	0
Plastic	-	+	-	+	-	+
Aluminium 6061	-	-	+	+	-	0

Table 7

Simulation results on repurposed parts.

Part	Simulation	Material and Dimension
		Mild Steel (2.5 mm)		Aluminium (2.5 mm)		Mild Steel (1 mm)		Aluminium (1 mm)
		Min	Max	Min	Max	Min	Max	Min	Max
Walker	Von Mises stress (MPa)	0	19.62	0	19.77	0	65.09	0	65.21
	Displacement (mm)	0	0.0546	0	0.1738	0	0.2247	0	0.7154
	Safety factor	15	15	13.91	15	4.9	15	4.37	15
Toilet Attachment	Von Mises stress (MPa)	0	19.62	0	19.77	0	65.09	0	65.21
	Displacement (mm)	0	0.0546	0	0.1738	0	0.2247	0	0.7154
	Safety factor	15	15	13.91	15	4.9	15	4.37	15
Pull-up bar	Von Mises stress (MPa)	0	58.54	0	57.49	0	108.4	0	106.2
	Displacement (mm)	0	0.1597	0	0.4479	0	0.1793	0	0.5761
	Safety factor	8.02	15	4.68	15	2.76	15	2.53	15
Safety Rail	Von Mises stress (MPa)	0	45.31	0	45.34	0	66.98	0	84.09
	Displacement (mm)	0	0.5199	0	1.4560	0	0.6571	0	2.5140
	Safety factor	10.98	15	6.29	15	4.81	15	3.28	15
		Material and Dimension
		ABS Plastic				Oak Wood
		Min		Min		Min		Min
Crib and Chair	Von Mises stress (MPa)	0		25.97		0		37.2
	Displacement (mm)	0		0.2268		0		0.2297
	Safety factor	9.94		15		2.04		15
Cushioned Chair	Von Mises stress (MPa)	0		1.375		0		1.606
	Displacement (mm)	0		1.658		0		1.9
	Safety factor	15		15		3.26		15

Table 8

Simulation results of the walker and toilet attachment.

Scenarios	Von Misses Stress	Displacement	Safety Factor
Mild Steel (2.5 mm)	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]
Aluminium (2.5 mm)	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]
Mild Steel (1 mm)	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]
Aluminium (1 mm)	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]

Table 9

Simulation results of pull bar.

Scenarios	Von Misses Stress	Displacement	Safety Factor
Mild Steel (2.5 mm)	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]
Aluminium (2.5 mm)	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]
Mild Steel (1 mm)	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]
Aluminium (1 mm)	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]

Table 10

Simulation results of safety rail.

Scenarios	Von Misses Stress	Displacement	Safety Factor
Mild Steel (2.5 mm)	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]
Aluminium (2.5 mm)	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]
Mild Steel (1 mm)	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]
Aluminium (1 mm)	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]

Table 11

Simulation results of crib and chair (full body).

Scenarios	Von Misses Stress	Displacement	Safety Factor
ABS Plastic	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]
Wood	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]

Table 12

Simulation results of Cushioned Chair.

Scenarios	Von Misses Stress	Displacement	Safety Factor
ABS Plastic	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]
Oak Wood	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]

Table 13

Functions of each repurposed product.

Product	Function
Crib	Provides a safe environment for a baby to rest and sleep
Cushioned Chair	Provides a comfortable seat area
Chair	A lightweight chair that can be used around the house
Highchair	Provides a secure seat area for a child during meals
Walker	Aids the elderly or people with mobility issues during walks
Toilet Attachment	Provides a handle for the elderly or people with mobility issues to grab onto to easily get in and out of the toilet Provides handles for a people who have slipped to pull themselves up
Pull-up Bar	Allows the user to exercise from the comfort of the home Enables the user to live a healthier lifestyle
Safety Rail	Prevents any bed-falling accidents that may bring detrimental effects to children or the elderly

Table 14

Mechanisms for baby chair.

Fixed Mechanism (Nut and Bolt)		Foldable Mechanism (Hinge)
Advantages	Disadvantages	Advantages	Disadvantages
Good stability	Many parts to disassemble and reassemble Requires the use of a wrench Hassle to repurpose	Repurposing without disassembling and reassembling Does not require any tools Easy to purchase and use Inventive	Moderate stability

Table 15

Mechanisms for walker.

Customised Brackets with Butterfly Nuts		Hole and Bolt Mechanism
Advantages	Disadvantages	Advantages	Disadvantages
Easy of manufacturing Ease of repurposing Allows for level fit of beams	Adds additional bulk and width	Repurposable without tools	Additional dimension considerations Hassle to implement

Table 16

Mechanisms for crib.

Solid Part		Locking Mechanism
Advantages	Disadvantages	Advantages	Disadvantages
Already implemented into design Can be repurposed Low cost No screws and tools needed High stability	Precise dimensioning accuracies have to be followed during manufacturing	Inventive design	Hassle to implement Less stable

Table 17

Results of the t-test for scenario 1.

Scenario	Mean, M (s)	Standard Deviation, SD	t-Value	df	p-Value
Case A1	70.92	3.72	12.01	10	0.000
Case B1	45.33	3.66

Table 18

Results of the two-sample t-test for scenario 2.

Scenario	Mean, M (s)	Standard Deviation, SD	t-Value	df	p-Value
Case A1	93.49	3.34	29.32	10	0.000
Case B1	48.02	1.8

Note: N = 6; M—Mean; SD—Standard deviation; df—degrees of freedom.

Table 19

Results of all t-test scenarios.

Scenario	Case	Mean (s)	Mean Difference	Standard Deviation	Pooled Standard Deviation, S
1	A	70.92	25.59	3.72	3.69
	B	45.33		3.66
2	A	93.49	45.47	3.34	2.68
	B	48.02		1.8

Table 20

Predicted sample size and actual power of usability tests.

Scenario	Predicted Sample Size	Actual Power
1	2	1.000
2	2	0.909

Note: Sample size—6 participants.

Notes: * 5—most important, 4—important, 3—neither important nor unimportant, 2—less important, 1—least important. PI—proposed invention; A—Multipurpose table; B—Paper-made rack; C—Transforming rack; S—Score; WS—Weighted score.

Table 21

Dimension of repurposed products.

Product	Citation	Dimension (W × L cm)	Dimension (W × L m)	Area (m2)
Crib	[11]	100 × 60	1.0 × 0.6	0.6
Cushioned chair	[12]	100 × 60	1.0 × 0.6	0.6
Chair		50 × 60	0.5 × 0.6	0.3
Walker	[13]	50 × 60	0.5 × 0.6	0.3
Toilet attachment		50 × 60	0.5 × 0.6	0.3
Pull-up bar	[14]	110 × 60	1.0 × 0.6	0.6
Bed safety rail	[15]	50 × 50	0.5 × 0.5	0.25
Total				2.95

Table 22

Survey results.

No	Items	Levels (Scale)	No of Scorers	Mean Rating
1	Age	1: 20–40	3 (50.00%)	-
		2: 40–60	0
		3: 60–80	3 (50.00%)
2	Gender	1: Male	3(50.00%)	-
		2: Female	3(50.00%)
3	Repurposability(3 items)	1: Strongly Disagree	0	4.00
		2: Disagree	0
		3: Neutral	3 (16.67%)
		4: Agree	12 (66.67%)
		5: Strongly Agree	3 (16.67%)
4	Space-Saving Ability(3 items)	1: Strongly Disagree	0	4.22
		2: Disagree	1 (5.56%)
		3: Neutral	1 (5.56%)
		4: Agree	9 (50.00%)
		5: Strongly Agree	7 (38.89%)
5	Usability(4 items)	1: Strongly Disagree	0	3.79
		2: Disagree	1 (4.16%)
		3: Neutral	5 (20.83%)
		4: Agree	16 (66.67%)
		5: Strongly Agree	2 (8.33%)
6	Comfort(3 items)	1: Strongly Disagree	0	3.39
		2: Disagree	3 (16.67%)
		3: Neutral	5 (27.78%)
		4: Agree	10 (55.56%)
		5: Strongly Agree	0
7	Safety	1: Strongly Disagree	0	3.50
		2: Disagree	2 (11.11%)
		3: Neutral	6 (33.33%)
		4: Agree	9 (50.00%)
		5: Strongly Agree	1 (5.56%)

Table 23

Unstructured feedback.

Type of Feedback	Feedback
Positive	The invention is creative.The invention has great stability (sturdy and strong).The invention possesses numerous repurposable functions.The price is reasonable for the functions it provides.The repurposing processes are smooth and seamless.The invention can be stored in a small footprint or space.
Negative	The invention is bulky and heavy.The holes of baby chair are too large and may pose injury risks to the toddler.

Table 24

Spearman’s correlation analysis.

Variables		Repurposability	Comfortability	Space-Saving Effectiveness	Safety
Usability	Correlation Coefficient	0.822	−0.217	−0.423	−0.317
	Sig. (2-tailed)	0.045	0.680	0.404	0.541
Repurposability	Correlation Coefficient		0.000	−0.257	0.000
	Sig. (2-tailed)		1.000	0.623	1.000
Comfortability	Correlation Coefficient			0.861	0.833
	Sig. (2-tailed)			0.028	0.039
Space-Saving Effectiveness	Correlation Coefficient				0.548
	Sig. (2-tailed)				0.260

Note: Sample size—6 participants.

Table 25

Competitive profile matrix.

Competitive Profile Matrix (CPM)			PI		A		B		C
Key Factors	Rank	Weightage	S *	WS	S *	WS	S *	WS	S *	WS
Material Selection	3	3/15 × 100%	4	0.80	4	0.80	4	0.80	4	0.80
Manufacturing Process	2	2/15 × 100%	3	0.40	4	0.53	3	0.40	4	0.53
Effective Lifespan	5	5/15 × 100%	5	1.67	2	0.67	5	1.67	2	0.67
Waste Generation	1	1/15 × 100%	4	0.27	4	0.27	4	0.27	4	0.27
Disposal Impact	4	4/15 × 100%	4	1.07	4	1.07	4	1.07	4	1.07
Total	15	100%		4.20		3.33		4.20		3.33

Notes: * 5—most important, 4—important, 3—neither important nor unimportant, 2—less important, 1—least important. PI—proposed invention; A—Multipurpose table; B—Paper-made rack; C—Transforming rack; S—Score; WS—Weighted score.

Table 26

Product effective lifespan.

Product	Effective Lifespan	Source
Crib	3	[28]
Cushioned Chair	5	[29]
Chair	5
Highchair	3	[30]
Walker	4	[31]
Toilet Attachment	4
Pull-up Bar	7	[32]
Safety Rail	3	[33]
Proposed Invention (Sum of lifespan for all repurposed products)—(Estimated duration of deterioration)	34 − 15 = 19

Table 27

Risk levels by invention.

Activity	Score
Sitting on chair	1
Using Walker	2
Using Toilet Attachment	3

Table 28

REBA risk levels.

Score	Level of MSD Risk
1	Negligible risk, No action required
2–3	Low risk, Change may be needed
4–7	Medium risk, Further investigation, Change soon
8–10	High risk, Investigate and implement change
11+	Very high risk, Implement change

Table 29

Material costs for a single repurposable crib.

Materials	Units	Cost (MYR)
Steel tubes	4	350
Joints	10	80
Hinges	1	10
Wooden Planks	1	120
Nuts and Bolts	20	20
Total		580 + (5% defect cost) = 609

Table 30

Estimated monthly fixed costs.

Fixed Costs	Monthly Costs (MYR)
Labour Costs (Salary of two workers)	3000
Factory/Workshop Rent	900
Advertisement	400
Utility Fees	500
Tools and maintenance equipment	300
Total	5100

Table 31

Product cost.

Products	Average Price (RM)	Citation
Crib	900	[38]
Cushioned Chair	400	[39]
Chair (x2)	100	[40]
Highchair	80	[41]
Walker	200	[42]
Toilet Attachment	200	[43]
Pull-up Bar	100	[44]
Safety Rail	40	[45]
Total cost of all existing products	RM 2020
Price of proposed invention	RM 900

References

1. Domljan, D.; Vlaović, Z.; Grbac, I. Design Concepts of Multifunctional Furniture for Sitting and Lying Related to the Industry. Proceedings of the 25th International Scientific Conference New Materials and Technologies in the Function of Wooden Products; Posjedovanje, Sadržaj, 17 October 2014.

2. Husein, H.A. Multifunctional Furniture as a Smart Solution for Small Spaces for the Case of Zaniary Towers Apartments in Erbil City, Iraq. Int. Trans. J. Eng.; 2021; 12, 12A1H. [DOI: https://dx.doi.org/10.14456/ITJEMAST.2021.8]

3. Phuah, Z.Y.; Ng, P.K.; Lim, B.K.; Nathan, R.J.; Ng, Y.J.; Yeow, J.A. The Conceptualisation of Inventive and Repurposable Children’s Furniture. Forests; 2022; 13, 2053. [DOI: https://dx.doi.org/10.3390/f13122053]

4. Gajwani, N.; Deepti Pande, R. Optimizing Residential Spaces: The Role of Multifunctional Furniture in Enhancing Functionality. Int. J. Hous. Hum. Settl. Plan.; 2024; 10, pp. 32-46.

5. Romadhona, M.; Joedawinata, A. The Impact of Apartment Furniture Design on Children’s Development and Recommendations for Multifunctional Furniture. Dewa Ruci J. Pengkaj. Dan Pencipta. Seni; 2023; 18, pp. 82-94. [DOI: https://dx.doi.org/10.33153/dewaruci.v18i1.5189]

6. A Comprehensive Guide to Choose Children Furniture. Available online: https://www.ibenma.com.sg/news/A-Comprehensive-Guide-to-Choose-Children-Furniture.html (accessed on 15 January 2025).

7. Cheng, H.Y.; Ng, P.K.; Nathan, R.J.; Saptari, A.; Ng, Y.J.; Yeow, J.A.; Ng, K.Y. The Conceptualisation and Development of a Space-Saving Multipurpose Table for Enhanced Ergonomic Performance. Inventions; 2021; 6, 67. [DOI: https://dx.doi.org/10.3390/inventions6040067]

8. Rajan, A.R.P.; Elavarasan, D.; Balaji, S.; Dinesh, A.; Gowtham, K. Design and Fabrication of Multifunctional Furniture. Int. J. Res. Eng. Sci. Manag.; 2019; 2, pp. 442-447.

9. Georgeta, H.; Goanta, A. FEA Analysis and Design Optimization for a Multifunctional Piece of Furniture. MATEC Web Conf.; 2017; 112, 06009. [DOI: https://dx.doi.org/10.1051/matecconf/201711206009]

10. Hung, Y.C.; Lee, Y.L.; Von, S.Y. Design and Improvement of Mobility Aid Walker by Using QFD and TRIZ Method. J. Eng. Technol. Appl. Phys.; 2024; 6, [DOI: https://dx.doi.org/10.33093/jetap.2024.6.1.2]

11. Crib Mattresses | Toddler Beds Dimensions & Drawings|Dimensions.Com. Available online: https://www.dimensions.com/element/crib-mattresses-toddler-beds (accessed on 5 December 2024).

12. Side Chairs | Dining Chairs Dimensions & Drawings|Dimensions.Com. Available online: https://www.dimensions.com/collection/side-chairs-dining-chairs (accessed on 5 December 2024).

13. Chamberlain, M. Walking Aids: How to Use a Walker Correctly|OT Flourish. Available online: https://otflourish.com/walkers-how-to-use-a-walker-correctly/ (accessed on 5 December 2024).

14. Karailiev, B. What Is the Best Width, Diameter, Pipe, Size for a Pull-Up Bar?. fittylife.com; 28 March 2022.

15. GOV.UK Consultation on Guidance on the Safe Use of Bed Rails. Available online: https://www.gov.uk/government/consultations/consultation-on-guidance-on-the-safe-use-of-bed-rails (accessed on 5 December 2024).

16. Moskowitz, N.; Moskowitz-Parness, E. Baby Soft Crib 2016. U.S. Patent; 20160296036A1, 13 October 2016.

17. US Patent Application for Adjustable Seating and Furniture Patent Application (Application #20150130241 Issued May 14, 2015)—Justia Patents Search. Available online: https://patents.justia.com/patent/20150130241 (accessed on 5 December 2024).

18. Trout, W.G. Assistive Walker Apparatus. U.S. Patent; No. 7,963,294, 21 June 2011.

19. Kelleher, H.J. Toilet Grab Rail Support. U.S. Patent; No. 6,349,423, 26 February 2002.

20. Balentine, F.A. Adjustable Mount Pull-Up Bar. U.S. Patent; No. 9,427,611, 30 August 2016.

21. Fichner-Rathus, L. Safety Rail for Sleeping Beds. U.S. Patent; No. 5,640,726, 24 June 1997.

22. Zou, W.; Li, Z.; Sun, D. The Teaching Content of Shanghai Style Furniture Design for Chinese Art Design Speciality. Heliyon; 2022; 8, e11627. [DOI: https://dx.doi.org/10.1016/j.heliyon.2022.e11627] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/36406677]

23. Middlesworth, M. A Step-by-Step Guide to the REBA Assessment Tool. Available online: https://ergo-plus.com/reba-assessment-tool-guide/ (accessed on 10 December 2024).

24. Health Design the Importance of Safety, Comfort, and Design in Behavioral Health Furniture, an Interview with Suzanne Fawley. Available online: https://www.healthdesign.org/insights-solutions/importance-safety-comfort-and-design-behavioral-health-furniture-interview (accessed on 10 December 2024).

25. Usagani, B. Multi-Functional Furniture. U.S. Patent; No. 6,857,696, 22 February 2005.

26. Wu, C.-C. Paper-Made Composite Furniture. U.S. Patent; Application No. 14/741,030, 16 June 2015.

27. O’connell, J.F., Jr. Adaptive Furniture 2016. US Patent; US9585472B2, 7 March 2017.

28. Jones, A. Your Baby Should Stay in Their Crib for Even Longer Than You Think, Experts Say. Available online: https://www.romper.com/p/how-long-do-babies-sleep-in-cribs-its-longer-than-you-think-experts-say-19446196 (accessed on 10 December 2024).

29. The Lifespan of Cheap Office Chairs: A Comprehensive Guide. Available online: https://sihoooffice.com/blogs/guide/the-lifespan-of-cheap-office-chairs-a-comprehensive-guide (accessed on 10 February 2025).

30. LaBracio, J. Best High Chairs of 2024. Available online: https://www.babylist.com/hello-baby/best-high-chairs (accessed on 10 December 2024).

31. Cooper, D.; Skelton, A.; Moynihan, M.; Allwood, J. Component Level Strategies for Exploiting the Lifespan of Steel in Products. Resour. Conserv. Recycl.; 2014; 84, pp. 24-34. [DOI: https://dx.doi.org/10.1016/j.resconrec.2013.11.014]

32. Fitness Expo How Long Does Home Gym Equipment Last|Fitness Expo. Available online: https://www.fitnessexpostores.com/how-long-does-home-gym-equipment-last/ (accessed on 10 December 2024).

33. Consumer Reports Safety Bed Rails for Kids—Consumer Reports. Available online: https://www.consumerreports.org/cro/news/2010/05/safety-bed-rails-for-toddlers-and-young-children/index.htm (accessed on 10 December 2024).

34. Becker, B. Variable Cost Explained in 200 Words (& How to Calculate It). Available online: https://blog.hubspot.com/marketing/variable-cost (accessed on 10 December 2024).

35. Zin, S. Net Profit Margin: Definition & How to Calculate It. Available online: https://www.tide.co/blog/business-tips/net-profit-margin/ (accessed on 10 December 2024).

36. Postolovski, T. How Profitable Is Running a Furniture Store? Key Factors to Consider. Available online: https://businessingmag.com/22730/equipping/furniture-store/ (accessed on 10 February 2025).

37. Ali, R. What is Break-Even Analysis and How to Calculate it for Your Business?. Available online: https://www.netsuite.com/portal/resource/articles/financial-management/break-even-analysis.shtml (accessed on 1 April 2022).

38. IKEA GONATT Cot with Drawer, White, 60 × 120 cm. Available online: https://www.ikea.com/my/en/p/gonatt-cot-with-drawer-white-70488926/ (accessed on 10 December 2024).

39. IKEA HEMNES Bench, White, 83 cm. Available online: https://www.ikea.com/my/en/p/hemnes-bench-white-30484299/ (accessed on 10 December 2024).

40. IKEA ADDE Chair, White. Available online: https://www.ikea.com/my/en/p/adde-chair-white-90219179/ (accessed on 10 December 2024).

41. IKEA ANTILOP Highchair with Tray, White/Silver-Colour. Available online: https://www.ikea.com/my/en/p/antilop-highchair-with-tray-white-silver-colour-s99067497/ (accessed on 10 December 2024).

42. Ciancanelli, J. Mobees Walking Frame: FST8207. Available online: https://www.homecareshop.com.my/categories-2/mobility-aids/mobees-walking-frame-fst8207 (accessed on 10 February 2025).

43. Cronkleton, E. 5 Best Toilet Safety Rails That You Can Order Online. Available online: https://www.healthline.com/health/toilet-safety-rails (accessed on 10 December 2024).

44. Locking Door Mount Pull-Up Bar BT 150-70 cm. Available online: https://www.decathlon.my/p/locking-door-mount-pull-up-bar-bt-150-70-cm-corength-8588490.html (accessed on 10 February 2025).

45. IKEA Vikare. Available online: https://www.ikea.com/my/en/p/vikare-guard-rail-white-50167672/ (accessed on 1 April 2022).