Magnetically Engineered Conductivity of Soft Liquid Metal Composites for Robotic, Wearable Electronic, and Medical Applications

Abstract

Stretchable composites comprising liquid metal (LM) inclusions and silicone elastomers (LME composites) are of great interest for soft electronics and wearable devices. LME composites consisting of highly deformable materials and low conductive filler ratios offer high stretchability and good strain-tolerant conductance, while not compromising the functionality of their host systems. Despite advances, actively achieving electrical conductivity for LME composites with a low ratio of fillers is challenging, especially in highly deformable elastomers. Herein, a new fabrication strategy that turns nonconductive LME composites with highly deformable elastomers into conductive ones using a small amount of magnetic Ni-doped LM is introduced. By actively manipulating conductive fillers with an external magnetic field, electrically conductive traces can sustainably be achieved at any desired location. Experimental results show that conductive traces have high conductivity of 2.55 × 10⁵ S m⁻¹, high stretchability (>450%), good strain-tolerant conductance (R/R₀ ≈ 1.56 at 250% strain), and especially a tensile modulus as low as 60.1 kPa at a very low loading ratio (9.7% by volume). The noncontacting magnetic fabrication also enables the creation of diverse configurations in 1D, 2D, and 3D, offering a broad range of potential applications from robotics, stretchable electronics, wearable devices, smart garments to biomedical systems.

Introduction

Liquid metals (LMs), especially gallium-based low melting point alloys such as galinstan (68% Ga, 21.5% In, and 10% Tin) and gallium indium (EGaIn, 75.5% Ga and 24.5% In by mass), set a new class of material systems with high electrical conductivity ($\sigma =3.4\times {10}^4{\text{S cm}}^{-1})$, and excellent mechanical deformability.^[1] LMs are the building blocks for the synthesis of stretchable composites with unprecedented properties such as high thermal conductivity,^[2] improved dielectric constant,^[3] electrical conductivity,^[4] and self-healability (depending on matrix materials).^[5] Among these materials, highly stretchable and electrically conductive composites from LM and silicone elastomers (LME)^[6] have been attracting most attention due to their unique strain-tolerant conductance, which means under a tensile strain, the resistance of the composites can remain relatively stable. This unique property allows for the construction of unusual electronics that are able to undergo extreme mechanical deformation for use in many emerging fields such as soft robotics^[4b,7] and stretchable as well as wearable electronics.^[5a,6b,8] Although there are also other methods to achieve conductive stretchable composites such as metal thin films or carbon-based fillers,^[9] LME composites are more favorable because LMs possess the high conductivity of metals, while exhibiting excellent mechanical deformability of liquid.

Silicone elastomers, such as polydimethylsiloxane (PDMS), poly(styrene-butadiene-styrene) (SBS), or styrene-ethylene-butylene-styrene (SEBS), have been reported as the most common matrix materials for LME composites. LMs are normally doped into elastomers via mixing techniques^[5c,6a] and stabilize within the matrix as micro- or nanodroplets, which consist of a passivating oxide shell (1–5 nm thick) wrapping bare LM in the core.^[1] Inherently, LME composites are not electrically conductive, even at a great volume fraction,^[3,6a,6c] due to the barriers of nonconductive oxide shell and thin silicone walls that separate LM droplets within the matrix. To achieve electrical conductivity, an extra sintering process (e.g., mechanical pressurization^[4a,6a,6c] or thermal expansion of LMs at a low temperature^[6b,10]) is required to break down the barriers and allows bare LM to flow and coalesce with each other. Although electric fields could also be employed to directly assemble LM droplets into electrically conductive microwires,^[11] this method has limitations on wire length and the use of high voltage. Due to its simplicity, the mechanical sintering technique has been employed in numerous studies to achieve stretchable conductors from LME composites. Especially, when pressing by a stylus^[4b] or an embossing stamp,^[5d] it is possible to directly produce arbitrary conductive lines on cured LME sheets, thereby enabling the creation of complex, stretchable circuitries. While mechanical sintering can facilely achieve electrical conductivity in LME composites made from PDMS,^[4a] SBS,^[5c] styrene isoprene styrene (SIS),^[5d] or SEBS,^[6a] it is a direct contact method, meaning that is ill-suited for several applications where preplacement of microelectronics or functional components is desired. In addition, it is still challenging to obtain electrical conductivity with softer and more deformable materials such as Ecoflex 00-30.^[6a,12] Highly conductive and stretchable composites with low tensile moduli are desired to enable a seamless integration between functional elements and the hosting substrate (e.g., soft artificial skins or soft robots).

The technical challenges of forming conductive LM and Ecoflex composite can possibly be attributed to either: (1) silicone walls separating LM droplets are too deformable to be ruptured under mechanical loads, which would prevent bare LM from coalescing with each other or (2) the applied pressure gets absorbed by the highly deformable elastomer matrix and the force transmitted to LM droplets is insufficient to rupture their oxide shell and release the bare LM for coalescence. Due to such challenges, most approaches have been conducted around the easy to rupture elastomers such as Sylgard 184 (Down Corning Inc., USA), which has a low stretchability compared to Ecoflex Series.^[6a] Recent studies suggested an alternative approach to achieve an electrically conductive LM-Ecoflex composite by controlling LM droplet size and sedimentation via a slow curing process. This technique creates a closely packed region of large LM droplets so that mechanical loads were sufficient to break the barriers and sinter electrically conductive paths.^[6a] While electrically conductive LM-Ecoflex composites (with 50% volume of LM or 10% volume of LM for composites diluted by 10% weight of toluene) were obtained, the reliance on the sedimentation of LM droplets makes this approach passive and unstable (i.e., the sedimentation rate varies with droplet sizes and the viscosity of the matrix). In addition, large LM droplets also pose a high risk of undesirable sintering due to their high chance of being ruptured and coalescing. In addition, the electrical conductivity of the Ecoflex composite presented in ref. [6a] was all obtained via large LM droplets (mixed at 200 rpm over 10 s) and global sintering of the entire samples or prototypes, instead of selectively sintered traces (or patterns) achieved by a stylus or stamps like other composites of stiffer elastomers. It can be inferred that by choosing a suitable processing condition, it would be possible to have a suitable LM droplet size that would allow selectively sintering patterns for the Ecoflex composite. However, no study has reported that capability so far, and, therefore, we believe that a new fabrication method that can achieve selectively electrical sintering with LME composites of highly deformable elastomers such as Ecoflex 00-30 is still of great interest.

Here, this article introduces a new magnetically assisted mechanical sintering approach to achieve electrical conductivity for a highly stretchable Ni-EGaIn Elastomer (NEGE) composite. The present approach utilizes a noncontact patterning technique where localized magnetic field is used to drive Ni-doped LM droplets within elastomeric matrix into conductive traces (namely, activated areas in this article), whereas inactivated areas (i.e., areas not exposed to the magnetic field) of the composite will remain insulating. It is noteworthy to mention that the use of both LM droplets and rigid metal fillers (e.g., Ag flakes,^[13] magnetic Ni flakes,^[5a] or magnetic Fe particles^[14]) to create stretchable conductive composites with good strain-tolerant conductance or positive piezoconductivity has been reported in the literature. However, rigid fillers and LM droplets in these composites existed as two separate fillers inside the elastomeric matrix, where rigid fillers directly contributed to conductive pathways of the composites and LM droplets functioned as deformable electrical bridges between rigid fillers. As a result, these composites required a relatively high content of the rigid metal filler, e.g., >20% weight ratio (28.7%,^[5a] 23.7%,^[13b] 25%,^[13a] or 64%^[14]), which adversely increase the stiffness of the composites. In addition, none of the matrix materials used in these studies (except one^[13a]) was as soft as Ecoflex 00-30. Different from these conventional methods, our study presents the use of magnetic Ni particles as a means to magnetically manipulate LM droplets, and, therefore, its loading ratio can be reduced to as low as 3.6% by weight (or 0.6% by volume), which is among the lowest ratio reported in the literature. This low content of rigid Ni particles mitigated the stiffening effect on the composite and helped preserve the softness of the elastomer. As a result, the created trace within the NEGE composite exhibits high electrical conductivity, relatively high stretchability, low modulus, and good strain-tolerant conductance. In fact, alignment using magnetic field has been previously reported to create either vertically conductive vias from Ag-coated Ni particles^[15] or conductive microwires from LM-coated magnetic particles.^[16] However, in these studies, due to the use of static magnetic fields, only vertically conductive regions (not transversely conductive)^[15] or short microwires^[16] (e.g., 1 mm long) were created. In the latter study, as the magnetic field strength decreased with increasing length, the formation of microwires became slower and their electrical conductivity dramatically degraded. In addition, to fabricate larger circuitries with longer wires and more connections, the arrangement of magnets will become more complicated. On the contrary, instead of a static magnetic field, our proposed fabrication strategy employs a moving magnetic field to actively manipulate magnetic LM droplets to create arbitrary conductive traces or patterns. In addition to 2D patterns, our approach can be used to create stretchable conductors with both 1D and 3D patterns, expanding its potential for a wide range of applications in stretchable electronics, smart garments, and biomedical devices such as soft surgical robots. Figure 1 shows the idea behind the new fabrication strategy with the NEGE composite in this study and its potential application areas as stretchable conductors.

Figure 1. Application areas of the proposed fabrication strategy based on the noncontact magnetic fabrication and the new LME composite of magnetic LM and highly deformable elastomers. The composite can be patterned in 1D, 2D, and 3D configurations that can be used for a wide range of applications, including stretchable electronics, wearable devices, smart garments, and biomedical devices.

Results and Discussion Preparation of the Soft Ni-EGaIn Elastomer Composites

The preparation of the NEGE composite starts with the mixture of Ni and EGaIn (NEG). Magnetic Ag-coated Ni microparticles with a mean diameter of 31 μm were dispersed into a bulk EGaIn volume. EGaIn was selected as the liquid filler due to its high electrical conductivity and liquid phase at room temperature with low viscosity.^[1b,4b] Due to the presence of the LM oxide layer, Ni microparticles tend to stay on the EGaIn surface instead of diffusing into the bulk volume. To assist the dispersion of Ni microparticles into EGaIn, both of them were transferred into a jar and vigorously mixed using a pestle for 3 min. After continuous stirring, Ni microparticles gradually entered into the EGaIn volume, forming a magnetic NEG mixture (Figure S1, Supporting Information). Scanning electron microscope (SEM) of the NEG mixture and its energy-dispersive X-ray spectroscopy (EDX) (Figure 2B) show an even distribution of Ni microparticles within EGaIn volume, similar to reported results in the literature.^[17] Once an apparently homogeneous NEG mixture was formed, Ecoflex 00-30 elastomer (Smooth-On Inc., USA) was added so that the volume fractions of EGaIn and Ni microparticles were 9.1% and 0.6%, respectively (the weight ratio was 35.7% wt EGaIn and 3.6% wt Ni), and the composite was shear mixed on a magnetic stirrer. During mixing, the NEG mixture was broken down into microscale magnetic LM droplets (or NEG droplets), which consist of Ni microparticles coated in a thin layer of EGaIn. This core–shell structure of NEG droplets can be inferred by observing optical micrographs of NEG droplets (Figure S2, Supporting Information) where the NEG droplets have a dark silver color of EGaIn instead of a yellowish color of Ag-coated Ni microparticles, as shown in Figure S1, Supporting Information. In addition, wrinkles can also be observed on the outer surface of these droplets, which can be attributed to the typical passivating oxide shell formed on the LM surface. It is also this passivating oxide shell that helps the NEG droplets suspend within the elastomeric matrix, resulting in a magnetic NEGE composite. When exposed to a magnetic field (e.g., a permanent magnet), NEG droplets were attracted and moved toward the magnet, confirming the core–shell structure of NEG droplets with Ni microparticles coated by a layer of EGaIn. Similarly, the NEGE composite can also be created with a composite precursor (part A’ in Figure 2) formed by first mixing the NEG mixture with part A of Ecoflex 00-30. Part B of Ecoflex 00-30 can then be added during fabrication to initiate the curing process. Figure 2A summarizes the preparation process for NEGE composites by both approaches.

Figure 2. A) Illustration of the fabrication process of the soft, conductive NEGE composite from magnetic NEG mixture and highly deformable silicone elastomers. B) SEM and EDX micrographs of the NEG mixture, showing an even distribution of Ni microparticles in EGaIn volume. Scale bar—200 μm. C) Conductive trace patterning mechanism utilizing a miniature magnet controlled by an automated XYZ stage. D) NEGE composite deposited on a substrate before being exposed to a permanent magnet. E) Conductive line is formed within the NEGE composite when the permanent magnet is moved underneath the substrate. The inset figures in (D) and (E) show the schematic mechanism of using a magnetic field to actively create a closely packed structure of NEG droplets, which can then become electrically conductive.

Pattern Mechanism for the Conductive Trace

As NEG droplets contain ferromagnetic Ni microparticles, which can be manipulated using a magnetic field. We utilized this feature to actively create high-density regions of NEG droplets (in contrast to passive LM droplet sedimentation reported here^[6a]) where closely contact NEG droplets can be easily ruptured and form coalescence between neighboring droplets, resulting in electrically conductive networks via subsequent mechanical sintering. In addition, the closely packed structure of NEG droplets created under magnetic forces could potentially mitigate the presence of the highly deformable silicone barrier between adjacent NEG droplets, which has been a daunting challenge in the conventional mechanical sintering method.^[6a,10,12] In addition, NEG droplets can follow the trajectory of the magnet to form complex 2D patterns of continuous conductive traces, as shown in Figure 2C.

It is also possible that the movement of the magnet dragged and forced the NEG droplets to collide with each other, which resulted in more contacts along the trajectory of the magnet. As a consequence, electrical conductivity could be facilely achieved along magnetically drawn traces within the NEGE composite (Figure 2C,E). To enable a high-throughput process, we directly deposited the NEGE composite onto a soft silicone substrate using either spin coating or thin film applicator. Mounting a miniature permanent magnet on an automated XYZ stage under the substrate and generating a moving path for this stage using computer-aided design (CAD) designs can rapidly create conductive traces in different configurations. As the NEG droplets aggregate toward the bottom of the composite layer due to magnetic attraction, our fabrication method also offers self-encapsulation architectures with electrically conductive traces covered by a layer of insulating silicone. As shown in Figure 2C,E, when the NEG droplets are attracted toward the magnet, they also create two insulating silicone-rich regions on both sides of the conductive traces. This feature is useful as it provides an extra protection for conductive traces from unintended shorting. Simultaneous formation of conductive path and insulating regions are highly suitable for large-scale and automatic processing that can be used to create large surface areas of skin-like electronics. Figure 2C,E shows the NEGE composite deposited onto a silicone substrate by a thin film applicator and NEG droplets being attracted toward a magnet moving under the substrate to form high-density regions (i.e., conductive trace as shown in Figure 2C) in contrast to randomly scattered NEG droplets in areas not exposing to the magnet (i.e., nonconductive areas as shown in Figure 2C) (Video S1, Supporting Information).

Electrically Conductive Traces from Ni-EGaIn Elastomer Composites

It is noted that the sizes of LM droplets highly depend on mixing conditions that may have a significant impact on the conductive property of their composites. To investigate the size effect, we varied the size of NEG droplets using different mixing conditions that resulted in three sets of samples: (1) 50 revolutions per minute (rpm) for 30 s, (2) 200 rpm for 30 s, and (3) 400 rpm for 30 s. The selection of these mixing conditions was based on settings available on the magnetic stirrer (see Experimental Section). The conductive traces of these samples were patterned in a line-shaped configuration using a magnet and then connected to an light-emitting diode (LED) as a means for a simple demonstration. The average Feret's diameters of NEG droplets created by the three mixing conditions were approximately 266.9 ± 172.1, 144.3 ± 53.8, and 130.7 ± 44.9 μm, respectively (Figure S2, Supporting Information). Evidently, increasing the mixing speed would result in smaller droplets and the NEG droplets apparently exhibited irregular shapes rather than the spherical shape. This could be attributed to the mixing process by the magnetic stirrer, in which a rotating magnetic field could cause magnetic NEG droplets to deform. It is also noted that due to the irregular shapes, the Feret's diameter, which is the maximum distance between two points of a NEG droplet, was used as a representative quantity for the droplet sizes instead of spherical diameter to show the effect of mixing conditions on NEG droplet sizes (see Note S1, Supporting Information).

We further characterized the sintering capability and electrical conductivity for the three types of the as-synthesized NEGE composites. It is noted that the de novo NEGE composites are nonconductive and, therefore, only the magnetically-induced traces are electrically conductive (see Figure 2 and Note S2, Supporting Information). Experimental results revealed that after sintering, the magnetically drawn traces of all three NEGE composites could confer good electrical conductivity, as shown by the LED demonstration in Figure 3A. However, there was a difference between the three types in terms of sintering process and the resultant conductivity. For the first type of NEGE composites (left panel, Figure 3A, 50 rpm for 30 s), NEG droplets had the largest size, and the magnetically drawn trace could achieve electrical conductivity right after activation without further mechanical sintering. Here, the activation is referred as the magnetically drawing process. When the traces were peeled off the substrates, there was a further reduction in their electrical resistance due to extra mechanical sintering resulted from the local stress during the peeling process and, therefore, leading to the rupture and coalescence of the NEG droplets. In contrast, the conductive traces made from the second and third NEGE composite types (middle and right panels of Figure 3A, 200 rpm for 30 s and 400 rpm for 30 s, respectively) were nonconductive after the activation. However, upon being peeled off, they all became conductive due to the effect of local stress. Finally, when mechanically sintered by stretching to 100% strain, all composite traces became slightly more conductive. We selected 100% strain to ensure that there is sufficient mechanical sintering applied to the traces.

Figure 3. Characterization of the NEGE composites. A) Conductive traces of three NEGE composite types powering LEDs (scale bar—10 mm). B) Optical micrographs showing the NEG droplet sizes corresponding to three mixing conditions. Left to right: 50 rpm—30 s, 200 rpm—30 s, and 400 rpm—30 s (scale bar—500 μm). C) Electrical conductivity of inactivated areas (i.e., not exposed to the magnetic field) and conductive traces fabricated from three NEGE composite types measured at three phases: after activating, after peeling, and after sintering by stretching to 100% strain. Note: NC stands for not conductive (N = 3). D) SEM micrograph showing the dome-shaped cross section of a NEGE conductive trace (scale bar—400 μm). E) Top-view micrograph of a 200 rpm NEGE conductive trace, showing the trace width bounded by the green rectangle (scale bar—1 mm). F,G) NEGE conductive traces in parallel (F) and in series (G) with different designed spacing values.

Figure 3C shows the electrical conductivity calculated from the dimensions of the drawn traces (i.e., length and cross-sectional area) for the NEGE composite types at three phases: (1) after the traces were cured and left on their substrates (i.e., after activation), (2) after they were peeled off their substrates, and (3) after they were stretched to 100% strain. The results show a significant increase in conductivity (or decrease in resistance) after conductive traces were peeled off their substrates due to the coalescence of closely packed NEG droplets under the local stress. Stretching the traces to 100% strain slightly improved their conductivity, possibly because most of NEG droplets had been already ruptured in the previous step. Figure 3D shows a cross-sectional view of a conductive trace, which follows a dome shape or a half of an ellipse due to the spreading nature of the magnetic field from a magnet. As a result, the cross-sectional area of conductive traces was estimated as half of the cross-sectional area of an ellipse where its major diameter and minor radius are the width and height of the trace, respectively (see Figure S3, Supporting Information). Sintered electrical conductivity of the NEGE composites in this work could be as high as 4.60 (±1.03) × 10⁵, 2.55 (±0.32) × 10⁵, and 1.36 (±0.68) × 10⁴ S m⁻¹ for the three NEGE composite types, respectively. These values are comparable to other reported works^[4b,5d,6a] with a lower loading fraction of conductive fillers.

The difference in electrical properties between NEGE composite types can be attributed to several factors, such as the Tresca equivalent stress ${\sigma }_{\mathrm{e}}$ on the oxide layer of NEG droplets in the elastomeric matrix, which can be expressed by[Image Omitted. See PDF]where k is a constant term, $E_{\mathrm{m}}$ is Young's modulus of the elastomer, ε is the strain of the substrate, R is the radius of the droplet, and t is the thickness of the gallium oxide layer. During a mechanical sintering process, forces can be transmitted to NEG droplets via the elastomeric matrix when the composite gets stretched or pressed. When ${\sigma }_{\mathrm{e}}$ becomes larger than the yield stress of gallium oxide, the oxide layer will rupture, and NEG droplets can coalesce with each other.

From Equation (1), ${\sigma }_{\mathrm{e}}$ is proportional to the droplet size, i.e., larger droplets are easier to break during a mechanical sintering process. As a result, the 50 rpm NEGE composite with larger NEG droplets can have their oxide shell layer easily ruptured just by the external magnetic force. In contrast, the other type of NEGE composites have NEG droplets in smaller sizes, which result in a lower Tresca stress, thereby requiring an additional mechanical sintering step such as peeling to create enough stress for droplet coalescence. Larger NEG droplets are also expected to incorporate more magnetic Ni microparticles, which can generate a large attraction force (under the same magnetic field) in comparison to smaller droplets. In addition, because coalescing networks formed from smaller NEG droplets can contain more “narrow connection necks” between NEG droplets, which is a significant source of resistance, the electrical conductivity of NEGE composites with smaller droplet sizes will also be affected.

Despite having the highest conductivity value, the 50 rpm NEGE composites were found to occasionally suffer from unintentional connections between inactivated areas (i.e., not exposed to the magnetic field, which should stay electrically insulating) and conductive traces, which are possibly due to large NEG droplets within the composite that can be ruptured easily. In contrast, inactivated areas of the 200 and 400 rpm NEGE composites were electrically insulating even after being mechanically sintered (Note S2 and Figure S4, Supporting Information). As a result, to avoid the unintentional sintering and undesirable shorting between traces while maintaining a good conductivity, the 200 rpm NEGE composite was used for the fabrication of all stretchable conductive traces below unless otherwise stated. We used miniature cylindrical magnets (NdFeB, N52 grade) of 1/16” or 1.5875 mm in diameter throughout this study and, therefore, the obtained conductive traces are around 2.41 (±0.21) mm wide (N = 3). Figure 3E shows a top-view micrograph of a NEGE conductive trace with its width bounded by a green rectangle. Smaller traces could also be achieved by either: (1) using smaller magnets; (2) reducing the thickness of the deposited NEGE composite layer; or (3) prestretching the silicone subtract and then releasing it once the magnetically patterning process is completed (See Note S3 and Figure S5, Supporting Information). However, it is worth noting that the goal of this study is not to create high-density electrical circuitries with high-resolution traces. Instead, the purpose of this study is to introduce a new approach to actively achieve magnetically induced conductive traces with nonconductive LME composites of highly deformable elastomers that were previously challenging to be sintered electrically conductive. A systematic optimization for traces with higher resolution will be an interesting topic for future research. Figure 3F,G shows the spacing between adjacent NEGE traces to prevent intertrace shorting. For the current approach, circuit designs require a minimum of 3 mm center-to-center spacing for parallel traces and 2.5 mm end-to-end spacing for traces in series. It is noted that the spacing values were determined as the distance measured from the center of the cylindrical magnets, as shown in Figure 3F,G.

Mechanical and Electrical Properties of the Ni-EGaIn Elastomer Composites with Magnetically Induced Conductive Traces

To evaluate the effect of conductive fillers on the mechanical properties of the NEGE composite, we benchmarked samples of Ecoflex 00-30, conductive traces, and inactivated areas of the NEGE composite using tensile stress test. Figure 4A,B shows the tensile modulus and strain at break values for the composites, respectively. Due to the low loading volume of rigid Ni microparticles, both conductive traces and inactivated areas of the NEGE composite possessed a relatively similar softness with an average tensile modulus of 60.1 (±4.6) and 61.2 (±4.3) kPa, respectively. These results are not much different from the tensile modulus of pure Ecoflex 00-30 samples as 65.8 (±12.4) kPa, as shown in Figure 4A. It is also shown in Figure 4B that there is a small reduction in the strain at break values of the NEGE composite compared to Ecoflex 00-30, possibly due to the addition of LM, which resulted in slightly less elastomeric material in composite samples. Nevertheless, both conductive trace and inactivated area samples of the NEGE composite can maintain their stretchability well over 450%, while retaining a good electrical conductivity. There was also no observation of any loss in the electrical conductivity of the NEGE traces before mechanical failure (Figure S6, Supporting Information). Figure 4C shows a LED circuit remaining lit with a NEGE trace, while the trace was stretched from 0 to 450% strain.

Figure 4. Electromechanical characterization of the NEGE composite (200 rpm—30 s). A) Tensile modulus and B) strain at break of the Ecoflex 00-30 elastomer (E30), NEGE conductive traces (NEGE-a), and NEGE inactivated areas (NEGE-in). C) Optical images showing a NEGE conductive trace powering a LED, while being stretched up to 450% strain (scale bar—10 mm). D) The normalized resistance (R/R0) is a function of tensile strain of a NEGE conductive trace (blue line). The red line shows the R/R0 of a LM microtubule, which is in good agreement with the black dashed line that represents the predicted R/R0 change by the Pouillet's law. E) The robust electromechanical performance of a NEGE conductive trace under a tensile strain to 250% for 500 cycles. F) Mechanical cyclic loading–unloading curves of a NEGE conductive trace to 250% strain for 500 cycles.

To characterize the electromechanical coupling behavior, strip samples of NEGE traces were tested with uniaxial tension, while changes in resistance were monitored (Figure S8, Supporting Information). The ratio between instantaneous (R) and initial (R₀) electrical resistance values, which is also known as the normalized resistance (R/R₀), was used to evaluate changes in resistance. Initially, the R/R₀ value was equal to 1 as there was no difference between R and R₀. When a strain was applied, the R/R₀ slowly increased with increasing strain and reached a value of 1.79 at 450% strain before its mechanical failure occurred (Figure 4D). The average R/R₀ value at 450% strain of three NEGE trace samples was 1.56 ± 0.22. This electromechanical coupling of the NEGE composite sharply contrasts with the response of a conventional LM microchannel (i.e., a soft silicone microtubule filled with EGaIn, see Figure S7, Supporting Information), which reached an R/R₀ value of ≈9 at only 200% strain and with the theoretical prediction by Pouillet's law for incompressible conductors, which should reach a R/R₀ value of 30.25 at 450% strain (Note S4, Supporting Information). Figure 4D shows the electromechanical curves of a representative NEGE conductive trace, a LM microtubule, and the theoretical prediction plotted as a function of tensile strain, showing the contrary electromechanical coupling characteristics of the NEGE composite. This electromechanical coupling, which is known as a typical feature of conductive composites based on LM inclusions, also validates the hypothesis about the core–shell structure of NEG droplets by which the conductive networks of NEGE composites should come from the coalescence of LM instead of the alignment of only rigid Ag-coated Ni microparticles (by which the resistance of the composites will increase more significantly under tensile strain). Figure 4E,F shows a cyclic test of a NEGE conductive trace being strained up to 250% for 500 cycles. The result shows a robust performance of the NEGE trace in repetitive operation with a stable electromechanical coupling behavior. Overall, the resistance change of the trace is very low and actually decreases gradually over cycles, as shown by the shrinkage in R/R₀ values with peak-to-valley changes in R/R₀ < 0.5 during final cycles (which accounts for R/R₀ of ≈1.56 compared to R/R₀ of 12.25 according to the Pouillet's law). Low hysteresis mechanical cyclic curves after the first cycle (where a Mullins effect is observable) in Figure 4F show a stable and good elastic recovery of the NEGE composite although the cycles do not overlap totally, indicating some of the composite's elasticity has been lost during cycling, possibly due to the viscoelastic creep of the uncrosslinked elastomer under large deformation up to 250% strain. Nevertheless, this phenomenon did not affect the conductivity of the NEGE composite samples.

Applications Stretchable Electronics and Smart Garments

Based on the low electromechanical coupling (i.e., good strain-tolerant conductance) and the ability to create arbitrary patterns using a magnetic field, the NEGE composites with magnetically induced conductive traces could be adapted to produce soft and stretchable circuits for electronic applications. Figure 5A,B shows a stretchable LED circuit at its original and “stretched” stages, respectively. To demonstrate the fabrication capability for 2D circuit designs, we created a conductive flower pattern as an electrical circuit for powering LEDs. The low mechanical modulus of the NEGE composite allows the LED circuit to be soft, highly flexible, and easily stretched to approximately 150% strain by hand, while maintaining the LED array powered on without any noticeable changes in brightness (Video S2, Supporting Information).

Figure 5. Stretchable electronic applications with the magnetically induced conductive traces within the NEGE composites. A,B) The LED circuit with the flower pattern at 0% strain and approximately 150% strain, respectively (scale bar—10 mm). C) A proof-of-concept sensing glove with NEGE wires and its potential biomedical applications, such as abdominal palpation, breast check, and rectal examination (left to right). The tactile data could be used to replicate haptic experience in medical training or recorded for long-term monitoring. D) Tactile signal from the sensing glove showing no noticeable changes during finger bending, but clear fluctuations during fingertip pressing against a surface, and then no changes again during finger bending (left to right). E) An example demonstration of using the sensing glove in the rectal examination procedure with a colon and rectum phantom.

To further demonstrate applications of our method with the NEGE composites, a proof-of-concept prototype of a sensing glove was designed and fabricated, as shown in Figure 5C. The sensing glove can conceptually have multiple types of sensors attached to its fingertips, while incorporating stretchable wiring made from the NEGE composites (or NEGE wires as shown in Figure 5C) on the back of the fingers. The use of NEGE wires can minimize the undesirable resistance change due to the movement of finger and allows for precise measurement of the specific stimuli applied at the fingertip. As a proof-of-concept, we attached a spiral-shaped piezoresistive tactile sensor made from a LM-filled microtubule^[7a] to the tip of the glove's index finger. When pressing the fingertip against a surface, we observed clear output signals as a result of the resistance change in the spiral-shaped tactile sensor. In contrast, vigorously bending motion of fingers barely had any effect on the sensing signal (Figure 5D and Video S3, Supporting Information), owing to the good strain-tolerant conductance of NEGE wires (Figure 5D and Video S3, Supporting Information). The results suggest the promise of our flexible sensing and wiring platform for several biomedical applications, especially palpation-related procedures such as breast cancer screening or rectal examination for prostate cancer screening.^[18] For instance, it is possible to engineer the sensitivity of the spiral-shaped tactile sensor by changing the sheath's constituent materials to differentiate abnormal tissues from healthy ones, as well as eliminating any artifacts from finger motions by utilizing NEGE wiring. This new capability is highly valuable for palpation-related procedures, where quantitative metrics offered by tactile sensors would be more accurate and consistent than the conventional method based on practitioners’ own sensation and assessment. In addition, the tactile information can be useful for haptics replication in medical training,^[8b,19] where medical students may find it challenging to know whether they are palpating an abnormality correctly or not if they have not felt it before, and, therefore, need a reference to learn from (Figure 5C). Figure 5E shows a demonstration of using the sensing glove for the rectal palpation procedure with a phantom of rectum and colon, where the sensing signal could remain stable, regardless of finger insertion and movements in a tight space of the rectum, until the finger palpated the rectal wall (Video S4, Supporting Information). This glove is also a conceptual example of a smart garment that will be capable of sensing (such as tactile force, shear force, vibration, or temperature depending on the equipped sensors), which can be used to replicate sensations to wearers and collect environment information in teleinteraction activities (such as medical teletraining), in exosuit^[8c,8d] or protection garments for extreme environments such as high temperature, underwater, or in space.

In addition to 2D circuitries, it is also possible to use our fabrication technique with NEGE composites to create stretchable and conductive fibers, which can have lower electromechanical coupling compared to conventional resistive sensors (e.g., microtubules filled with liquid metals^[20]). For example, Figure 6A shows a NEGE conductive fiber (1.58 ± 0.01 mm in diameter and 0.64 m in length) with a good electrical conductivity, powering a LED with a voltage of 5 V. A microscopic image of a conductive fiber (Figure 6B) shows the asymmetric accumulation of NEG droplets to one side of the fiber due to the magnet's attraction. Nevertheless, that asymmetric assembly of the conductive line does not impede the performance of the conductive fiber. By rotating the magnetic field along the axis of the fiber during fabrication, it was also possible to create conductive fibers with a 3D helical trace (or NEGE helical fiber), which could potentially further improve their strain-tolerant conductance at the cost of higher initial resistance. An example of a helical conductive fiber with a diameter of 1.94 ± 0.01 mm and a helical pitch of 4 mm is shown in Figure 6C. While being stretched, thanks to the helical shape, the helical fiber's conductance exhibited a better tolerance to strain, and, therefore, lower change in resistance compared to the normal fiber (Figure S9, Supporting Information). The helical fiber's good strain-tolerant conductance was also demonstrated by lighting a LED at a small voltage of 3.3 V, while the fiber was being stretched. There was no noticeable change in the LED brightness observed when the helical fiber was at 0% and 300% strain (Figure 6D). With the 1D form factor, these NEGE conductive fibers can be a potential candidate as stretchable conductors in integration with textile for a variety of smart garment applications,^[21] as shown in Figure 6E,F. Although winding microtubules filled with LM around soft fibers can also create conductive fibers with similar strain-tolerant conductance as the NEGE conductive fibers, the fabrication process for such fibers will be more complicated with more steps. In contrast, our fabrication strategy with NEGE composites allows a simpler process to create monolithic conductive fibers with intrinsic strain-tolerant conductance.

Figure 6. Conductive fibers created from NEGE composites and magnetic field. A) Illustrated structure of the NEGE fiber. B) Optical image of a LED circuit being illuminated with a 0.64 m long NEGE conductive fiber at a voltage of 5 V and optical micrograph of a NEGE conductive fiber, which shows the asymmetric assembly of NEG droplets on one side of the fiber (scale bar—500 μm). C) A NEGE conductive fiber with 3D helical trace (scale bar—4 mm). The inset shows a micrograph of the helical trace (scale bar—1 mm). D) Optical images of an LED lit up via a NEGE helical fiber with a 3.3 V power supply (scale bar—20 mm). No apparent change in the LED's brightness was noticeable between 0% and 300% strain stages. E) An illustration of potential application of NEGE conductive fibers with textile for smart garments. F) Proof-of-concept integration of NEGE conductive fibers in a compression garment with a zoom into the helical fiber. G) The application of magnetically induced conductive trace with NEGE composites as stretchable conductors for a soft surgical robotic arm. H) The prototype of the soft electrosurgical robotic arm with the NEGE conductive sleeve and ablation electrode (or cutting electrode). I) The electrosurgical robotic arm prototype successfully performing dissection on a piece of apple. J) 3D conductive trace of “UNSW” letter induced along a soft NEGE fiber.

Electrosurgical Robotic Arm

Being among conductive composites with the least stiffness, our approach with the NEGE composites can also be useful for creating stretchable conductive wiring in 3D for small-scale soft robotic systems such as soft surgical robotic arms or robotic catheters, which normally have limited force generation, with minimum obstruction to the host device's motions. As a proof-of-concept, a continuum electrosurgical robotic arm equipped with a 3D conductive helical trace on a soft sleeve comprising our NEGE composite was designed and fabricated (Figure 6G,H) to be employed as the transmission line to supply electrical power to the robotic arm's cutting tip for dissecting operations. To induce bending motion, the robotic arm was comprised of three hydraulic-driven soft artificial muscles, which were developed by our group.^[21,22] These soft artificial muscles were arranged in a triangular configuration and covered by the soft sleeve (Figure 6H and S10, Supporting Information). By elongating one of the three soft artificial muscles, the arm could achieve omnidirectional bending up to 90° (Figure S10, Supporting Information), thanks to the low stiffness of the conductive sleeve. A metal electrode was attached at the tip of the robotic arm and functioned as a monopolar mode electrosurgery cutting tip.

For electrosurgical robotic arms, it is crucial to maintain the resistance of the conducting path lower than that of the tissue so that tissue dissection could be performed, while motions of the robotic arm should also be preserved. Helically routing a rigid wire around the bending segment of the robotic arm would preserve the electrical power transmitted to the cutting tip. However, this approach has a drawback that the rigid wire being used should have a small gauge as possible to not increase the stiffness at the bending segment, which will limit the arm's motions. Smaller wires, however, normally come with higher resistance, are more fragile, and prone to breaking, which make handling small-gauge wires more challenging. In contrast, the conductive sleeve with 3D helical conductive trace made from the NEGE composite appears to be a more suitable candidate for this task, thanks to its low stiffness (i.e., little obstruction to the robotic arm's bending motions), which offers a safe interaction with the tissue during the insertion, high initial conductivity, and high strain-tolerant conductance, which will help maintain the resistance of the conducting path, while the robotic arm is bending. In addition, as the size of the conductive trace can be controlled by the size of the magnet, wide traces can be fabricated to lower the initial resistance of the sleeve (Figure 6H). As a result, minute changes in the resistance of the conductive sleeve were observed during the robotic arm's motions, with the resistance values being 0.2520 Ω (±0.0006 Ω) at the initial stage, 0.2633 Ω (±0.0010 Ω), 0.2549 Ω (±0.0004 Ω), and 0.2556 Ω (±0.0005 Ω) when the arm bent in three directions, and 0.2571 Ω (±0.0025 Ω) when the arm extended to ≈50% strain (Figure S10, Supporting Information) (N = 3 measurements). The conductive sleeve was then connected to a long wire (0.5 m) that ran through the flexible body of the robotic arm and connected to an electrosurgical unit, resulting in a transmission line with a total resistance of 0.5607 Ω (±0.0007 Ω, N = 3 measurements). Figure 6I and Video S5, Supporting Information, demonstrate the use of the electrosurgical robotic arm to successfully perform electrosurgical dissection on a piece of apple. These results revealed the potential use of our NEGE conductive composites for soft surgical robotic arms and potentially other small-scale soft robotic systems.

Discussion

The reduced electromechanical coupling of the conductive traces within the NEGE composite with the magnetically assisted drawing method in this study is hypothesized to be attributed to several factors. It can be the calabash networks of coalesced LM droplets and the pressure-induced opening of connections between incompressible LM droplets. This plays a major role in preserving their end-to-end electrical resistance, and, therefore, resulting in only modest electromechanical coupling under tensile strain, similar to recent reports by refs. [6b,13b,23]. The good affinity between EGaIn and Ag-coated Ni microparticles (possibly due to the formation of intermetallic compounds such as AgIn₂^[13b] or Ga₅Ni^[24]), which lead to a good adhesion between liquid and solid fillers, could also be another contributor to the modest electromechanical coupling of the conductive trace in the NEGE composite by reducing the separation between the two conductive components. In addition, it is hypothesized that rigid Ag-coated Ni microparticles inside the LM networks could also function as pillars that support the networks from collapsing under tensile strain, and, therefore, keeping the cross section of the conductive path from significant changes and contributing to the modest electromechanical behavior of the composite (Figure S11 and Note S5, Supporting Information). A combination of three mechanisms discussed above is hypothesized as the main reasons for a comparable electromechanical coupling (of the magnetically induced conductive lines of the NEGE composite compared to other conductive LME composites at a much lower loading fraction of conductive fillers (9.7% volume). In addition, the conductive traces of the NEGE composite in this study also exhibit a relatively high initial conductivity (2.55 × 10⁵ S m⁻¹ for the 200 rpm composite, which is higher than some conductive LME composites^{[4a,4b,5d,6a]}), while remaining as soft as Ecoflex 00-30 elastomer (Figure S12 and Table S1, Supporting Information). Despite being possible factors contributing to the good strain-tolerant conductance of NEGE composites, intermetallic compounds formed by direct contact between EGaIn and Ag-coated Ni microparticles may have an adverse effect on mechanical properties of the composites via gradual solidification.^[13b,24] In contrast, compared to the volume fraction of EGaIn, the volume content of Ag-coated Ni microparticles is very small, and, therefore, the formation of solid intermetallic compounds is hypothesized to be small as well and may not have a significant effect on the composites. As a result, understanding this effect is of great importance for long-term use of the composite, and, therefore, will be investigated in future research.

Although in this article, the fabrication strategy for the NEGE composite was demonstrated with millimeter-scale electrical wiring, there can be several approaches to further improve the resolution of the fabrication method to achieve smaller features in future research. In addition to attempts discussed in the Supporting Information, using magnetic particles with smaller sizes can also be another way to improve the resolution of the fabricated features by reducing the size of NEG droplets. However, smaller NEG droplets will possibly have an effect on conductivity and possibly the electromechanical properties of conductive traces. Therefore, a systematic study will be essential to optimize the fabrication process and understand possible effects that smaller magnetic particles may have on the composite's performance.

In this study, Ecoflex 00-30 silicone elastomer was selected for the main matrix material as an example for materials that are too deformable to achieve electrical conductivity with LM. However, due to the actively induced accumulation of NEG droplets with an external magnetic field, we envision that this fabrication strategy of NEGE composite could also be extended to a wide range of materials (obviously, with necessary modifications) to produce stretchable conductive composites. As the NEG droplets are coated in a layer of LM, it is also possible to modify this LM surface with functional groups such as thiols^[5c] or disulfide^[5b] as in other reports to enhance functionality and be an enabler for future applications of the composites for stretchable electronics and soft robotic systems. It is worth noting that the creation of 3D conductive structure such as helical wire can be done with LM-filled silicone tube, which is wrapped around a silicone fiber. However, this process is a manual method, which involves: (1) the injection of LM into a hollow fiber, (2) sealing process to secure its end, and (3) winding the conductive tube around the soft silicone fiber. However, this method is not suitable with highly deformable silicone elastomer such as Ecoflex 00-30 where the creation of long fiber made from Ecoflex 00-30 is much more challenging. However, our new approach allows a simpler yet efficient technique to create monolithic and 3D conductive fibers with good strain-tolerant conductance with highly deformable silicone elastomer, which is not limited at Ecoflex 00-30. For example, the 3D conductive trace with “UNSW” letter created along a soft silicone fiber shown in Figure 6J demonstrated that our technique is unique to create such complex 3D structures. Finally, the practical use of our new magnetic fabrication method with LM elastomers is not limited in several robotic and medical applications presented in this article. It can be used to create conductive trace on demands of silicone and fabrics-based robotic systems or medical devices such as cardiac catheter, thermal heating and cooling for compression garments, health monitoring, as well as soft wearable exoskeletons for rehabilitation and human augmentation.

Conclusion

In summary, this article reported a new fabrication strategy with NEGE conductive composites formed from magnetic Ni-doped EGaIn droplets (i.e., NEG droplets) and the Ecoflex 00-30 silicone elastomer (which is a material known for being so deformable that it was previously challenging to form conductive composites). By using an external magnetic field, the fabrication method proposed in this article can actively accumulate magnetic NEG droplets suspended in the elastomeric matrix into traces that can then sustainably achieve electrical conductivity via mechanical sintering processes. The NEGE conductive traces obtained by this fabrication strategy were demonstrated to be intrinsically soft (as soft as pristine Ecoflex 00-30), highly conductive, highly stretchable, and have a relatively good strain-tolerant conductance. These features of the presented NEGE composites are of great usefulness for soft functional materials in several emerging fields, such as stretchable electronics, smart garments, or soft robotic systems, where conductance of electrical wiring should be stably maintained under tensile deformation. In terms of applicability, NEGE composites also demonstrated great versatility by being able to be used in many forms for different applications, such as 1D conductive fibers for compression garments, 2D circuits for an LED array and a sensing glove, and a conductive sleeve with a 3D helical trace for a soft electrosurgical robotic arm. We believe that these demonstrations are just a few examples of the potential uses of the NEGE conductive composite and its fabrication strategy to create soft and stretchable conductors for stretchable electronics, smart garments, wearable devices, and soft robotics applications.

Experimental Section Materials

Gallium (99.99%) and indium (99.99%) were purchased from RotoMetals Inc., USA. Magnetic Ag-coated Ni microparticles (SN325P25-ALT1 with 23% wt of Ag) were obtained from Potters Beads LLC, USA. The mean diameter of the magnetic microparticles was 31 μm according to its datasheet.^[25] EGaIn (75.5% Ga and 24.5% In by weight) was prepared by heating and stirring the metals on a magnetic stirrer at 80 °C for 2 h. Ecoflex 00-30 elastomer was purchased from SmoothOn Inc., USA. NdFeB permanent magnets were purchased from K&J Magnetics Inc., USA and SuperMagnetMan Inc., USA. The magnetic field strength applying to the NEGE composites by the magnets was estimated by magnetic field calculators.^[26] (see Note S3, Supporting Information).

Ni-EGaIn Elastomer Composite Synthesis

First, the NEG mixture was prepared by mixing Ag-coated Ni microparticles into EGaIn at a weight ratio of 1:10 (Ni: EGaIn) in a jar using a pestle for 3 min. Ecoflex 00-30 elastomer (mixed at 1:1 weight ratio of part A: part B) was added to the NEG mixture at a ratio so that the volume fraction of the NEG mixture in the final composite was 9.7% (9.1% volume EGaIn and 0.6% volume Ni microparticles). Then, the mixture was shear mixed by a magnetic stirrer at the desired setup (e.g., 50 rpm—30 s, 200 rpm—30 s, or 400 rpm—30 s) to achieve the final NEGE composite. For the precursor composite approach, part A’ was prepared by mixing NEG mixture and part A of Ecoflex 00-30 at the desired conditions. During fabrication, part B of the Ecoflex 00-30 was added to part A’ at a calculated weight ratio of 3:7 (part B: part A’) so that the weight ratio of part A: part B of Ecoflex 00-30 within the composite was 1:1. Gently stirring part A’ prior to mixing with part B was necessary to ensure a uniform suspension of NEG droplets in the composite.

Droplet Size Characterization

Samples of three NEGE composite types were spin coated on acrylic substrates and allowed to cure before they were investigated under a digital microscope (SubaScope 2021 4 K USB, Suba Engineering, Australia) for assessing morphology and droplet size. Sizes of NEG droplets were estimated in the Fiji image processing software [ImageJ, National Institutes of Health (NIH), USA, https://imagej.nih.gov/ij/index.html) and then analyzed using MATLAB (MathWorks, USA) (see Note S1, Supporting Information).

Scanning Electron Microscope Imaging

SEM and EDS analyses were performed on the Nova NanoSEM SEM at a voltage of 20 kV.

Trace Patterning

2D conductive traces were created by using a desktop router (SainSmart Genmitsu 3018-PROVer Router). Permanent magnets were fixed on the router table, which could move in X–Y directions. Substrates with NEGE composites were attached to the Z-axis via a customized holder. Programs to move the magnets underneath substrates to create conductive traces were developed in Gcode and run on the GRBL-based open-source candle computer numerical control (CNC) control software. 1D conductive fibers were fabricated by injecting the NEGE composites into polytetrafluoroethylene (PTFE) tubes and a magnet was used to create a conductive trace along the tubes. The tubes were then allowed to cure at 70 °C for 15 min and demolded from the PTFE shells. The conductive fibers were then dip coated in Ecoflex 00-30, and premixed with 10% weight of silicone thinner (SmoothOn Inc., USA) to form a coating layer. Conductive fibers with a 3D helical trace were fabricated using the same method. Instead of a fixed magnet, a specially designed setup was used to simultaneously translate and rotate the PTFE tubes over a magnet, resulting in a helical conductive trace. Due to limited volume inside the tube, the NEGE composites used for fiber fabrication were prepared with a higher volume fraction of approximately 17.7% of EGaIn and Ni microparticles.

Volumetric Conductivity

Samples of three NEGE composite types were fabricated by casting composites into layers of approximately 900 μm by a thin-film applicator (ZUA 2000, Zehntner Inc., Switzerland) on ≈230 μm thick Ecoflex 00-30 substrates and conductive traces were created by moving a cylindrical permanent magnet (NdFeB, N52, 1.5875 mm in diameter) underneath the substrates. The traces were drawn with two drawing cycles at the speeds of 50 and 10 mm min⁻¹, respectively. The samples were then removed from the magnetic field and cured in an oven at 70 °C for 15 min. The electrical conductivity of the fabricated conductive traces was then examined at three moments: (1) after they were cured, (2) after they were peeled off the substrates, and (3) after they were stretched to 100%. Volumetric conductivity (σ) of NEGE composites was calculated using the length of conductive traces (L) and their cross-sectional areas (A) estimated by the Fiji image processing software. Three samples were fabricated for each NEGE composite type, and each sample was measured for its resistance (R) by a four-wire probe with three repetitions. The volumetric conductivity value of each composite was then calculated by the equation: $\sigma =L/RA$.

Tensile Test

For tension testing, strip samples (25 mm × 10 mm) of the 200 rpm NEGE composite were fabricated and mechanically activated by stretching to 100% strain. To prevent slipping and stress concentration, samples were glued to 3D-printed anchors using Sil-poxy (SmoothOn Inc., USA) and clamped to a customized uniaxial tension setup based on a Zaber translational stage (Zaber Technologies Inc., Canada) and a miniature loadcell (10 lb, Futek Inc., USA) (Figure S8, Supporting Information). To mitigate resistance fluctuations at the interfaces between samples and external wires, copper tape was adhered underneath each end of the samples, and small drops of EGaIn were added to connect samples with the copper tape. External wires were directly soldered to the copper tape. The resistance values during tension tests were measured using a four-wire probe and a Quanser QPIDe data acquisition device (Quanser Inc., Canada). Samples of pristine Ecoflex 00-30 and NEGE inactivated areas were fabricated in the same manner and tested using the same setup. All samples were preloaded to 0.1 N (0.0984 ± 0.0086 N; N = 9 samples in total) before being subjected to uniaxial tensile loadings at 300 mm min⁻¹. For stress–strain characterization, tensile force was recorded by the miniature loadcell, and strain was measured using an optical encoder. Tensile modulus values were calculated with a linear fitting up to 5% strain. For cyclic testing, the same experimental setup was used to repetitively stretch a NEGE conductive trace from 0 to 250% strain for 500 cycles.

Stretchable Circuit Fabrication

The stretchable LED circuit was fabricated by first spin coating a layer of silicone elastomer (Ecoflex 00-30, SmoothOn Inc., USA) onto a PET substrate. When the silicone layer was cured, a layer of NEGE composite was cast onto it by a thin-film applicator. The substrate was then mounted to a customized holder on the desktop CNC machine and a Gcode file that had been developed in advance was run to move the magnet following a flower pattern. The circuit was then allowed to cure at 70 °C for 15 min before it was peeled off the substrate and sintered. As the circuit was only approximately 1 mm thick, it was encapsulated with additional silicone layers before connecting electronic components to make the circuit thicker and easier for manipulation by hand. LEDs were then connected to the circuit by directly penetrating their pins through the circuit thickness. It is noted that surface-mount devices (SMD) components can also be connected by making tiny incisions on the circuits to expose the conductive LM. Finally, Sil-poxy silicone adhesive (SmoothOn Inc., USA) was used to secure the LEDs and their connected pins.

Sensing Glove

The sensing glove was fabricated by attaching a prefabricated module, containing stretchable NEGE wires and a spiral-shaped tactile sensor, on top of a fabric glove using Velcro strips. This design allows users to quickly attach the prefabricated module to any glove and turn it into a sensing glove. The NEGE wires in this application were fabricated using the same patterning mechanism for 2D patterns described above, but with an additional stretchable fabric layer for improved durability and compatibility with apparel techniques. The tactile sensor was created by injecting EGaIn into a silicone microtubule (Saint-Gobain, France) and sealing its two ends with rigid wires as electrodes. Copper tape adhered on nonstretchable fabric substrates was used as interconnects between the NEGE wires and the tactile sensor. Connection between the NEGE wires and the copper tape was formed by small drops of EGaIn and Sil-poxy silicone adhesive, while external wires and the tactile sensor’ electrodes were soldered directly onto the copper tape.

Electrosurgical Robotic Arm

The electrosurgical robotic arm was fabricated based on three hydraulic-driven soft artificial muscles that elongate when they are pressurized. The fabrication process and working principle of the artificial muscles have been recently reported by our group.^[22a] The NEGE conductive sleeve was fabricated by injecting the NEGE composite into a mold with a center rod, and a magnet was moved helically outside the mold to create a helical trace. Once the trace was formed, the sleeve was let cure at 70 °C for 15 min before it was demolded and dip coated in Ecoflex 00-30 to form a coating layer. Rigid wires were then employed to create electrical connections from the conductive sleeve to the robotic arm's cutting tip (0.1 mm wire) and to an external electrosurgical unit (0.5 mm wire). The head of the robotic arm was then covered by a silicone cap (exposing only the cutting tip), while the wire connecting to the electrosurgical unit and transmission tubes for three artificial muscles were routed inside the robotic arm's flexible body.

Acknowledgements

The authors acknowledge support from the UNSW Start-Up Grant (PS58173), the UNSW Scientia Fellowship Grant (PS46197), the Vanguard Grant from the National Heart Foundation of Australia (RG204224), and the GROW Early Career Academics Grant (PS66730). Trung Thien Hoang, Mai Thanh Thai, and Chi Cong Nguyen would like to acknowledge the support from the Science and Technology Scholarship Program for Overseas Study for Master's and Doctoral Degrees, Vin University, Vingroup, Vietnam. The authors acknowledge Dr Shuhua Peng and Zhao Sha for their help with SEM imaging.

Conflict of Interest

The authors declare no conflict of interest.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.