Advances in medical devices using nanomaterials and nanotechnology: Innovation and regulatory science

Abstract

Nanomaterials and nanotechnology are emerging as promising strategies for medical devices due to their advantageous properties, including the ability to effectively interact with biomolecules and tissues, as well as enhance therapeutic efficacy and biocompatibility. This has resulted in approved and candidate devices in fields, such as orthopedics, dentistry, wound care, and neurology. However, the overall progress in translating medical devices using nanomaterials has been relatively slow, highlighting the urgent need to advance regulatory science. Regulatory authorities and organizations, such as the National Medical Products Administration in China and the European Union, have issued essential guidance documents for these devices safety and efficiency evaluation. These documents include special requirements and considerations for physicochemical characterization, biological evaluation, and other aspects. Although some evaluation paths have been defined, ongoing advancements in technologies and methods are expected to enhance safety evaluation practices, reduce burdens on the medical device industry, and accelerate the clinical translation of medical devices using nanomaterials. Herein, we review the current state of regulatory science related to medical devices using nanomaterials, suggest the feasibility of using in vitro alternative methods to advance regulatory science, and offer forward-looking insights to inspire new ideas and technologies for accelerating clinical translation.

Headnote

ABSTRACT

Nanomaterials and nanotechnology are emerging as promising strategies for medical devices due to their advantageous properties, including the ability to effectively interact with biomolecules and tissues, as well as enhance therapeutic efficacy and biocompatibility. This has resulted in approved and candidate devices in fields, such as orthopedics, dentistry, wound care, and neurology. However, the overall progress in translating medical devices using nanomaterials has been relatively slow, highlighting the urgent need to advance regulatory science. Regulatory authorities and organizations, such as the National Medical Products Administration in China and the European Union, have issued essential guidance documents for these devices safety and efficiency evaluation. These documents include special requirements and considerations for physicochemical characterization, biological evaluation, and other aspects. Although some evaluation paths have been defined, ongoing advancements in technologies and methods are expected to enhance safety evaluation practices, reduce burdens on the medical device industry, and accelerate the clinical translation of medical devices using nanomaterials. Herein, we review the current state of regulatory science related to medical devices using nanomaterials, suggest the feasibility of using in vitro alternative methods to advance regulatory science, and offer forward-looking insights to inspire new ideas and technologies for accelerating clinical translation.

Keywords: Nanomaterials Medical device Regulatory science Safety and efficiency evaluation In vitro alternative

1. Introduction

Medical devices using nanomaterials and nanotechnology contain or are manufactured using materials within the nanoscale range (approximately 1 nm-100 nm) or materials that exhibit dimension-dependent ti h [1,2]. N terials offer broad licati proper les or P enomena 241]. Nanomateria so er broa app ication prospects in medical devices compared to traditional materials due to their superior nano effects. The engineering principles of nanomaterials in medical devices include structural regulation [3-5], cell behavior regulation [6-8], and immunoregulation [9,10]. These principles are reflected in improvements to mechanical properties and biocompatibility, accelerated repair and regeneration, and new functions to medical devices, such as scalability and interactive interfaces in flexible wearable devices [11,12].

The potential of nanomaterials to enhance the quality of care and treatment has garnered significant industry attention, creating a market with substantial growth potential. According to Global Market Insights, the global market for nanotechnology in medical devices was valued at over $3 billion in 2021 and is projected to grow at a compound annual growth rate (CAGR) of more than 8% from 2022 to 2030, reaching $7 billion by 2030. Despite these advances, the translation from research to clinical use is still slow for medical devices employing nanomaterials and nanotechnology. To date, only a few medical devices using nanomaterials or nanotechnology have received regulatory approvals, including those from the U.S. Food and Drug Administration (FDA), European Medicines Agency (EMA), and the National Medical Products Administration (NMPA) in China, with a notable focus on nano-silver dressings and dental fillings. For example, several nano-silver dressings have been certified by NMPA and have played a role in reducing infection of the wound surface. Nano Hybrid Universal Restorative is widely used to restore front and rear teeth directly. The limited nature of medical devices using nanomaterials or nanotechnology is mainly attributed to the uncertainties regarding the risks associated with nanomaterials, such as toxicology and exposure limits, which continue to pose challenges for safety evaluation and regulation [1,13].

The key regulatory measures for this area are outlined in the timeline shown in Fig. 1. Currently, regulating medical devices using nanomaterials presents a twofold challenge. On the one hand, regulatory authorities and organizations must provide precise risk assessments and regulatory pathways, with only the European Union (EU) and NMPA issuing clear regulatory guidance [14]. At the same time, the FDA has also issued NANOMATERIAL GENOTOXICITY TESTING ROADMAP [15], and the EU has established BIORIMA, a risk management framework for nano-biomaterials used in medical devices [16]. In addition, countries such as the Republic of Korea and Brazil are actively involved in related projects and have proposed relevant testing guidance for nanomaterials [17]. On the other hand, standardization efforts for safety and efficiency evaluations require further refinement and demonstration of applicability. The regulatory science process must be accelerated to boost industry confidence by enhancing policy transparency and predictability to facilitate the rapid translation and clinical application of medical devices using nanomaterials.

We list some medical devices using nanomaterials and nanotechnology currently in clinical use under the existing regulatory framework and briefly describe how they are integrated. Our focus is on the regulatory requirements for these devices, as clinical translation must adhere to these policies. In the following section, we highlight recent advances in regulatory science, emphasizing modern in vitro alternative methods such as organoids, organs-on-a-chip (OOAC), and computational modeling and simulation (M&S) in the context of medical devices using nanomaterials. We also provide perspectives and outlooks on the challenges and future direction for translating these devices. This review aims to address the difficulties and improve the efficiency of clinical translation of medical devices using nanomaterials.

2. Medical devices using nanomaterials and nanotechnology used in clinical therapy

Nanotechnology is an emerging technology that gained prominence in the 1990s. The nanomaterials represent a unique class of materials whose composition or surface dimensions fall within the nanometer range (1-100 nm). The materials may exhibit novel optical, electrical, and mechanical properties with a decrease in size. Notably, nanomaterials smaller than cellular dimensions can demonstrate new biological functions, creating significant potential for medical diagnosis, therapy, and regenerative medicine advancements. This section will explore the mechanisms of integrating nanomaterials and nanotechnology with medical devices while reviewing the current market and clinical applications of medical devices that utilize these innovative materials.

2.1. The integration of nanomaterials and nanotechnology with medical devices enhances their performance

The growing interest in developing novel medical devices encourages innovative approaches incorporating nanomaterials or nanotechnology. The most clinically relevant applications include the addition of components, the formation of nanocoatings, and the construction of nanosurface structures [18]. Depending on the integration methods, nanomaterials and nanotechnology serve various roles in medical devices.

One practical approach to impart nanomaterials' unique properties to medical devices is the direct incorporation of nano-components. For instance, the controlled release of silver ions from silver nanoparticles can damage bacterial membranes and subcellular structures, demonstrating a broad-spectrum antimicrobial effect that positions it as a hot material for innovative dressings [19]. Before 2017, NMPA approved several dressings containing nano-silver to reduce wound infections. However, caution is advised when using silver nanoparticles in implanted medical devices, as they may produce toxic effects in various body organs. Multiple factors, including particle size and application site, influence these poisonous characteristics. Another appealing material commonly used as a nanocomponent directly added to medical devices is nano-hydroxyapatite because hydroxyapatite is the main inorganic component of human bones and teeth and has a directional nanocrystalline structure in its natural form [20,21]. Representative commercially available medical device products include medical nano-hydroxyapatite/polyamide 66 composite bone filling material, NanoStim Synthetic Bone Paste, and Desensibilize Nano P, etc.

Coating nanomaterials on the surfaces of medical devices to create functional coatings is a key strategy for enhancing their performance. The specific application requirements of medical devices may impose conflicting constraints on the materials used. For instance, bone implants often utilize metal-based materials due to their load-bearing properties; however, their susceptibility to corrosion increases the risk of biological incompatibility, infection, and mechanical wear. Functional nanocoatings effectively solve this challenge [22-24]. Currently, functional nanocoatings in medical devices serve various purposes, including antibacterial properties, promoting repair, and anti-adhesion. For example, the Ployzene F nanocoating of COBRA PzF Nan°Coated Coronary Stent Systempromotes healing after stent implantation and reduces the attachment of platelets and validation cells. The TiN nano-coating of the left atrial appendage Occlader Systempromotes high mobility of surface endothelial cells [25].LigaSure Sealer/Divider Nano-coated devices, compared to uncoated conventional devices, create more than 50% less sticking, reduce eschar buildup by 15%, result in fewer cleanings, and make cleaning more efficient. More and more examples show that nanocoatings can be used to modify and functionalize medical devices.

Another common mechanism for integrating nanotechnology with medical devices is the fabrication of nanosurface structures. In recent years, nanomaterials have been recognized for their biomimetic effects, as their size closely resembles the microphysical structure of the extracellular matrix and natural tissue in biological systems. Additionally, surfaces with nanoscale characteristics play a crucial regulatory role in cellular responses, promoting tissue repair and regeneration. For instance, biochemical modifications of surfaces and the micro- and nanoscale structures of nerve conduits facilitate the directional elongation of axons and the guided migration of Schwann cells [26,27]. Furthermore, microneedles designed using the advantages of micro-nanomaterials exhibit minimally invasive properties, enhancing the convenience of drug delivery and improving patient compliance. For example, Adaptix Interbody System with Titan nanoLOCK Surface Technology is produced using a laser melt additive manufacturing process. It was the first technology to demonstrate elements that qualify as nanotechnology for spinal devices. This technology indicates that rough titanium alloys enhance osteoblasts' production of angiogenic factors [28]. Similarly,EIT Cellular Titanium is the first 3D-printed interbody fusion device with nanoscale features to receive approval from the U.S. FDA. Titanium materials with nanoscale features lead to increased adhesion of osteoblasts compared to conventional titanium materials.

2.2. The diverse development of medical devices necessitates the advancement of regulatory science

Currently, the global industry for medical devices using nanomaterials and nanotechnology is rapidly developing, driven by policy guidance and market stimulation. North America is expected to hold a significant market share due to substantial research and development investments and key industry players' presence. From a demand perspective, many medical devices are being enhanced by nanomaterials and nanotechnology, aiming to address unmet clinical needs. Globally, marketed medical devices using nanomaterials are primarily found in orthopedics, dentistry, and wound care, with examples listed in Table 1 (non-exhaustive) To be specific, in the field of dentistry, Nano Hybrid Universal Restorative, which contains nanoscale fillers such as 20 nm silica and 4-11 nm zirconia, has received approval from the FDA, EMA, and NMPA. Incorporating nanomaterials provides excellent polishing capabilities and an extremely low wear rate, making this product widely utilized for direct anterior and posterior restorations. Additionally, Spectrum Bond, Tetric N, and DENTE have also been approved by the EMA and NMPA.

The clinical development of medical devices using nanomaterials and nanotechnology is progressing rapidly, as illustrated in Table 2. These clinical trials' extensive advancement highlights the potential of nanomaterials and nanotechnology in medical devices. Notably, the sirolimus-eluting coronary stent system (NANO Plus) features nanosized pores that serve as drug carriers. The NANO Plus stent exhibits a more uniform distribution on the adluminal surface than microporous or textured rough surface stents [29,30]. Additionally, clinical trials conducted by Cairo University on nano silver fluoride demonstrated that nanosilver particles possess antimicrobial activity against Mutans Streptococci and Lactobacilli, which are the primary pathogens involved in the formation of carious lesions. Its fluoride component is well-known for enhancing remineralization and inhibiting bacterial activity [31]. These innovations are designed to improve surgical efficiency, create a bionic healing environment, and enhance the properties of medical devices.

However, the application of approved and clinical developed medical device using nanomaterials and nanotechnology primarily focuses on antimicrobial dressings, orthopedic and dental fillings, and implants, including orthopedic, neurological, and cardiovascular devices. In comparison to the broader landscape of medical devices, the use of nanomaterials and nanotechnology in this field remains relatively limited. This situation arises from a variety of complex factors, including the insufficient technological capabilities of enterprises, varying perceptions of the risks associated with nanomaterials among the public (including patients and healthcare professionals), and inadequate methods for assessing the safety and effectiveness of medical devices using nanomaterials and nanotechnology. To address these challenges and promote the broader application of medical devices using nanomaterials and nanotechnology, several regulatory agencies have conducted regulatory scientific research and subsequently introduced relevant measures. For instance, advice on whether an FDA-regulated product involves the application of nanotechnology has been issued by FDA to help identify products. Additionally, the NANOMATERIAL GENOTOXICITY TESTING ROADMAP has been proposed to evaluate the safety of these medical devices. For the registration of medical devices using nanomaterials and nanotechnology, the FDA requires applicants to submit data to answer questions related to the product's safety, effectiveness (where applicable), or regulatory status. Additionally, the FDA will continue to conduct post-market monitoring. The EMA and NMPA have established clear guidance, which is discussed in detail in Section 3. From a regulatory standpoint, the requirements for medical devices using nanomaterials and nanotechnology are relatively consistent across various regulatory agencies, focusing on the evaluation of safety and effectiveness based on risk identification. Consequently, these measures may effectively enhance the confidence of both enterprises and the public in the development and application of medical devices using nanomaterials and nanotechnology, while also providing valuable tools for regulatory agencies to scientifically and objectively assess the safety and effectiveness of these device.

In summary, medical devices using nanomaterials and nanotechnology and their applications are limited within the current regulatory framework, with a relatively narrow range of products. Stringent regulations in various countries restrict market growth. The United States, the European Union, China, and other regions are intensifying regulatory research to address the global challenge of inadequate regulation of medical devices using nanomaterials. Therefore, it is essential to establish a globally harmonized regulatory framework and requirements for the application of medical devices using nanomaterials and nanotechnology, led by international regulatory coordination organizations such as the IMDRF.

3. Regulatory requirements and considerations specific to medical devices using nanomaterials

Regulatory science is the science of developing new tools, standards, and approaches to assess safety, efficacy, quality, and performance [32]. Regulatory authorities in various countries have conducted scientific research on regulation to address the challenges posed by emerging technologies, such as nanotechnology. The international academic community has not reached a consensus on the safety of nanomaterials. However, all regulatory authorities agree on the necessity of evaluating and researching the safety of nanomaterials in drugs and medical devices before they enter the market. In particular, long-term monitoring studies on medical nanomaterial implants are essential.

In particular, when medical devices using nanomaterials and nanotechnology are presented as a combination of drugs and devices-such as drug-coated or impregnated devices, including drug-eluting stents with nanostructures, catheters with antimicrobial nano-coatings, and microneedle transdermal patches-they must first be classified. Although the FDA, NMPA, EU, and MHLW (Ministry of Health, Labor and Welfare, Japan) have slightly different definitions of combination products, they all fundamentally adhere to the concept of "primary mode of action" (PMOA). Specifically, the FDA determines an Investigational New Drug (IND) or an Investigational Device Exemption (IDE) based on the PMOA [33]. The NMPA has issued the Principles for the Classification and Definition of Medical Device Products with Nanomaterials (Draft for Comment) [34], and the Guiding Principles for the Registration Review of Combination Products of Drugs and Devices with a Primary Function as Medical Devices, which stipulates that pharmaceutical and device combination products containing nanomaterials or nanotechnology are classified as either primarily drugs or medical devices according to their PMOA and are regulated by the corresponding administrative departments. The EMA classifies medications based on their complementary or primary roles, while the MHLW classifies combination products according to their primary functions.

The nano-related risks of medical devices containing nanomaterials primarily concern the potential release of free nanoparticles from the device and their potential toxic effects [35]. Although the toxicology of nanomaterials and their environmental impact have been extensively studied over the past 20 years, and despite efforts to develop robust systems for evaluating health and/or environmental impacts, as well as strategic programs for relevant risk-focused research, the safety and effectiveness evaluation of these applications in the biomedical field, especially in medical devices, remains unresolved [36-38]. Regulatory authorities worldwide have issued guidance for the safety and effectiveness assessment of medical devices using nanomaterials and nanotechnology. Fig. 2 outlines the specific procedures required by the Center for Medical Device Evaluation (CMDE), NMPA, for the safety assessment of medical devices using nanomaterials. Additionally, Fig. 3 highlights the specific considerations and requirements in regulatory science compared to traditional medical devices. From a translation perspective, a thorough understanding of regulatory policies, including regulations, guidance, and standards, is essential to translate medical devices successfully.

3.1. Physicochemical characterization might provide the fundament for the evaluation of medical devices using nanomaterials

Medical devices using nanomaterials aim to improve performance by utilizing the special properties of nanomaterials, such as surface and quantum size effects etc. The expected improved performance includes mechanical properties [39,40], electrical properties [41,42], magnetic properties [43,44], biological effects [45,46], repair effects [47-49], and so on. These special properties differ from those of conventional materials depending on their size, structure, and surface properties, which is why the physicochemical characterization of nanomaterials involved in medical devices is the first step in risk assessment and biological evaluation. In other words, accurate characterization of the physicochemical properties of nanomaterials, both in vitro and in vivo, is crucial for understanding and managing potential risks and impacts on human health. This is a considerably complex task, especially because the nanomaterials involved in medical devices in the process of manufacturing, such as cleaning, disinfection, sterilization, and even storage, may lead to changes in size, size distribution, composition, morphology, aggregation state, surface properties and other physicochemical properties of nanomaterials and affect their intended properties in medical devices. Therefore, the challenges in the physicochemical characterization of nanomaterials reflect the challenges in the quality control of medical devices using nanomaterials. Additionally, the complex interactions of nanomaterials in the human body will also affect their risk, safety, and efficacy assessment. As discussed in later sections, the physicochemical characterization of nanomaterials involved in medical devices is a technique challenge.

The physicochemical characterization of nanomaterials used in medical devices heavily focuses on (i) stability and uniformity and (ii) the risk of nanomaterials entering the body. Unlike medical devices without nanomaterials, those containing nanomaterials must be assessed for exposure risks related to these materials. First, it is essential to classify medical devices based on their exposure routes, including surface, external communication, and implantable devices. This assessment aims to determine whether systemic exposure or local effects are possible. If there is no systemic exposure or local effects risk, the devices can be characterized using traditional physicochemical methods. If nanoparticles are released, however, it is necessary to study the total amount released, including particles and ions. This phase focuses on determining whether the released nanoparticles could enter systemic circulation. If systemic exposure is not a concern, only biological evaluations to assess local effects are needed. Otherwise, physicochemical characterization of the nanomaterials within biological tissues is required. The total amount, form, and material structure of nanomaterials in biological tissues influence their activity and toxicity [50-52].

Based on this, the physicochemical characterization of nanomaterials can be divided into three levels: raw materials, final product, and biological tissue. In particular, for nanomaterials that enter the body and are absorbed by tissues or migrated to biological tissue samples, it is necessary to evaluate how nanomaterials perform in biological tissue or fluid environments and identify their metabolites. It is crucial because nanomaterials can form dynamic protein coronas upon in vivo contact, leading to varying biological effects [53-55]. Because of their special effects and properties, physicochemical characterization of nanomaterials presents challenges for parameters and methods, most of which do not have stable reference materials and few harmonized methods for physicochemical assessment of nanomaterials to aid the development of a robust test plan.

The parameters for characterizing nanomaterials in the development of medical devices depend on the product's initial state and intended use. These key parameters can include physical properties (such as particle size, morphology, aggregation/agglomeration, and surface area/specific surface area), chemical composition (including purity, viscosity, and surface chemistry), and extrinsic properties (including surface charge, solubility, dispersibility, and hydrophilicity or hydrophobicity). These parameters help answer three fundamental questions about nanomaterials: What does it look like? What is it made of? How does it interact with the surrounding environment or media? It is crucial to emphasize that once the parameters of raw materials are characterized, it is essential to provide evidence that the characterization data corresponds to the same nanomaterial used in the final product. This is important because the severity of nanomaterial toxicity is related to particle size and other specific characteristics; therefore, the properties of the nanomaterials in the final product must match those of the raw material [56]. This step is critical for regulatory risk assessment; otherwise, additional physicochemical characterization of the final product is required to demonstrate the stability and homogeneity of the nanomaterial.

Another challenge in the physicochemical characterization of medical devices using nanomaterials is the methodology. The difficulty in characterizing nanomaterials in raw materials and final products stems from the current lack of standards and guidance necessary for a comprehensive evaluation of the physicochemical properties of nanomaterials in medical devices. Furthermore, the absence of standardized materials for nanomaterials can lead to potentially controversial evaluation results. Although ISO/TC 229 and its representative specification, ISO/TS 16195, have introduced concepts and related specifications for representative test materials, significant technical gaps remain in the field of nanomaterials due to a lack of consensus, considerable technical challenges, and economic factors [57-59]. Fortunately, many methods of key parameters for nanomaterials have been widely recognized and applied. For instance, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM), which utilize high-resolution electron beams, have been used to characterize the surface morphology and topography of nanomaterials in raw materials and medical device products [60]. However, standardized physicochemical characterization methods for raw materials and products based on different types of nanomaterials and medical devices still require further discussion and consolidation.

Due to the complex interactions between nanomaterials and biological organisms, the methodological challenges previously mentioned are evident in the physicochemical characterization of nanomaterials within biological tissues. These challenges include but are not limited to, the following questions: How can the physicochemical properties of nanomaterials in living organisms and tissues be characterized? How can nanomaterials be quantitatively or qualitatively analyzed and imaged in vivo? How can nanomaterials' stability and chemical presence in organisms and tissues be evaluated? According to the recommendations outlined in The Safety and Effectiveness Evaluation of Medical Devices Using Nanomaterials and Nanotechnology, the amount, form, and structure of nanomaterials accumulation in biological tissues are expected to be characterized (see Fig. 4). The total amount of information can be obtained through quantitative and semi-quantitative analyses, which typically include mass spectrometry, isotope labeling, chromatographymass spectrometry, and in vivo imaging. Inductively Coupled Plasma Mass Spectrometry (ICP-MS) is a crucial detection technology; notably, the enhanced Single Cell-ICP-MS (SC-ICP-MS) allows for the rapid detection of nanomaterials within individual cells [61,62]. Isotope analysis techniques can facilitate in vivo tracing and quantitative studies of nanomaterials through radioisotope or stable isotope labeling. Isotope-Ratio Mass Spectrometry (IRMS) and imaging technologies, such as Single-Photon Emission Computed Tomography (SPECT) and Computed Tomography (CT), are employed for this analysis [63,64]. Regarding the forms of nanomaterials present in biological tissues, synchrotron radiation sources are expected, with X-ray absorption Fine Structure, Extended X-ray Absorption Fine Structure (EXAFS), and X-ray Absorption Near-Side Structure (XANES) being recognized as powerful detection methods. Additionally, the interaction between nanomaterials and biomolecular surfaces, particularly the protein corona analysis [65], is also considered. In cases where characterizing nanomaterials proves challenging, a combination of methods-including the characterization of starting raw materials, the manufacturing process, product performance, and toxicokinetic testing-may be utilized.

In summary, the field is focused on developing standardized protocols for manufactured nano-objects. Standard documents, such as ISO/ TR 10993-22, which addresses the physicochemical characterization of nanomaterials used in medical devices, have been summarized and issued based on the fundamental principles outlined in ISO/TR 13014 and other related standards. Although the parameters and methods proposed by these standards are not sufficiently specific or comprehensive, they still provide significant guidance for risk assessment of medical devices. Given the limitations of current physicochemical characterization methods for medical devices using nanomaterials, a methodological demonstration of the adopted methods is necessary to evaluate their suitability.

3.2. Biological evaluation might be the core step to reveal the mechanism of interaction between nanomaterials and organisms

The biological evaluation of nanomaterials, particularly their nanotoxicology, is crucial for their application in nanomedicine [66]. Regulatory science involves conducting biological evaluations, including toxicokinetics and toxicology, for medical devices that might release nanomaterials [67,68]. Toxicology testing typically includes assessments of cytotoxicity, acute toxicity, irritation, delayed-type hypersensitivity, genotoxicity, hemocompatibility, and chronic toxicity.

3.2.1. Toxicokinetics

Toxicokinetics integrates pharmacokinetics and toxicology to describe how nanomaterials behave in the body, considering absorption, distribution, metabolism, and excretion (ADME) when introduced through various exposure routes in medical devices [68]. Nanomaterial properties may alter toxicokinetics and tissue distribution compared with their non-nano counterparts [54]. The exposure route, including the form and position of medical devices in contact with the human body, significantly affects the toxicokinetics of nanomaterials. For example, in surface medical devices in contact with intact skin, the primary concern is the penetration of nanomaterials from transdermal devices, such as skin patches. Generally, nanoparticle uptake through the skin is minimal due to the stratum corneum's barrier effect [69]. However, advances in skin penetration technologies, such as microneedles and delivery systems [70-72], may influence the toxicokinetics data of these devices. Therefore, if the technology allows, it is crucial to determine whether nanomaterials accumulate in specific tissues or cells and specify their exposure at targeted locations.

Invasive medical devices require particular focus in biological evaluations of nanomaterial toxicokinetics. This phase should concentrate on particle distribution and persistence, including the potential for tissue accumulation and the dissolution or degradation of nanomaterials. In addition, to evaluate the toxicokinetics in the target tissue or organ, it is essential to consider the potential for particle distribution to other organs over time. As a likely entry point for systemic circulation, the lung should be assessed for particle distribution [73,74]. Additionally, organs rich in phagocytes, such as the liver [75,76] and spleen [77,78], are critical considerations in the toxicokinetics of medical devices using nanomaterials. Notably, while blood clearance rates are faster than those in other tissues and organs, blood levels are less significant than the final tissue and organ concentrations [50,79].

3.2.2. Toxicology/biocompatibility

Nanotoxicology began with the establishment of the British journal Nanotoxicology in 2005. Nanotoxicology aims to uncover the potential effects of nanomaterials on human health. In medical devices using nanomaterials, it is widely accepted that a device's toxicity depends on its exposure potential, which aligns with the exposure assessment route (Fig. 2). To enhance the accuracy of toxicity and risk assessment for nanomaterials, it is crucial to address factors that may interfere with test results during toxicological evaluations. Key concerns in nanotoxicological/biocompatibility evaluations are as follows: 1) Sample Preparation: The characteristics and properties of nanomaterials are influenced by the type of test media and the state of the nanomaterials (e.g., aggregation or dispersion) [80]. 2) Concentration/Dose: The concentration or dose is impacted by the dispersion state of the nanomaterials. Ensuring stability and uniformity is essential. 3) Dose Unit: Due to the size and surface effects of nanomaterials [81], more appropriate dose units (such as specific surface area or particle number concentration) should be used based on the situation. 4) Interference with the test system: Factors such as endotoxins and biomolecule interactions can interfere with test results [82]. 5) Possible Mechanisms of toxicity: Nanomaterial size and morphology can affect toxicity outcomes [83]. For example, phagocytosis can influence the final results [84,85].

In the nanotoxicological evaluation of medical devices using nanomaterials, Endpoints to be addressed in a biological risk assessment (general safety assessment) proposed by ISO 10993-1 must be implemented. For example, for implanted medical devices in prolonged contact with human blood, all relevant endpoints-including cytotoxicity, sensitization, irritation or intracutaneous reactivity, material-mediated pyrogenicity, acute systemic toxicity, subacute toxicity, subchronic toxicity, chronic toxicity, implantation effects, hemocompatibility, genotoxicity, and carcinogenicity-should be evaluated. Subsequently, the appropriate assessment route is selected based on regulatory requirements. For instance, CMDE, NMPA implement a three-tier assessment method. If toxicity is evident from the general safety assessment or if there are concerns about reproductive system toxicity and immunotoxicity, a third-level additional toxicity test is conducted. This trial encompasses chronic toxicity, carcinogenicity, reproductive/developmental toxicity, central nervous system toxicity, immunotoxicity, and endocrine disruption.

In particular, nanomaterials are mainly cleared by mononuclear phagocytic system (MPS) phagocytic cells after entering the body, and they tend to accumulate in the tissues and organs of the MPS, such as the liver, spleen, and lymphoid tissue. Additionally, nanoparticles readily interact with various components of body fluids to form biomolecular canopies (such as protein canopies), which are subsequently recognized, captured, and phagocytosed by immune cells or stored within the MPS system. This interaction can lead to irritation and acute inflammation, chronic inflammation, immunosuppression, immunostimulation, hypersensitivity, and autoimmunity, resulting in immunogenicity and immunotoxicity. Immunogenicity's effects on nanomaterials' toxicity are described in detail below. 1) At the initial stage, when immunogenic nanomaterials enter the human body, the immune system is activated, resulting in an immune stimulation effect, and these materials are recognized as foreign bodies. During this phase, immune cells congregate at the site of action of the nanomaterials, leading to acute inflammation and toxic effects [86]. 2) With continuous stimulation, the immunogenicity of the nanomaterials may trigger a memory response from the immune system, resulting in a persistent chronic inflammatory state and, ultimately, chronic toxicity [87]. Various immune cells are mobilized throughout this ongoing stimulation and participate in the body's immune response. 3) Prolonged immune stimulation may damage surrounding normal tissues and further compromise the body's ability to defend against other pathogens, manifesting as a toxic body reaction. 4) However, continuous stimulation from nanomaterials, combined with the regulatory role of the immune system, may lead to immunosuppression [88]. This condition diminishes the immune system's capacity to recognize and eliminate pathogens and abnormal cells, increasing the body's vulnerability to infections and producing more pronounced toxic effects. 5) Additionally, at any stage of nanomaterial entry into the body, there is a potential for the activation of the complement system. This activation can trigger the degranulation of mast cells and basophils, leading to the release of histamine and other bioactive substances. Consequently, this may result in vasodilation, a decrease in blood pressure, bronchospasm, and other allergic reactions, all of which contribute to toxic effects. 6) Furthermore, autoimmunity may also arise; the properties of nanomaterials can disrupt the immune system's self-tolerance, prompting the body to produce autoantibodies and self-reactive T lymphocytes that target its own tissues and cells [89]. This can damage the body's tissues and cause many toxic symptoms. Therefore, the immunogenicity of nanomaterials should be given special consideration based on their antigenic properties, adjuvant efficacy, inflammatory effects, and capacity to activate the complement system. Although the details of toxicological /biocompatibility assessment routes for nanomaterials vary among regulatory authorities, there is a consensus that the proposed tests should be viewed as evaluation endpoints rather than essential test items. These endpoints must be considered and interpreted in conjunction with other tests or evidence. It is crucial to emphasize that, in such studies, the form of the nanoparticles used must be equivalent to those present in biological systems, whether released or created.

Preclinical research has shown differences in the ability of hepatic phagocytes to uptake and remove nanoparticles between old and young mice [76]. While multiple variables can influence biological evaluation results, these are often overlooked in medical device evaluations. Regulatory science needs to be grounded in basic research, incorporating multifactorial studies in preclinical models and human cohorts [90,91].

3.3. Animal tests might provide the strategies to explore the life path and development law of nanomaterials instead of humans

Animal testing plays a significant role in supporting product feasibility, safety, and effectiveness. Due to the limitations of current regulatory frameworks and validation technologies and the unique structures of nanomaterials that can bypass traditional absorption pathways, animal testing is crucial for simulating and validating medical devices using nanomaterials. Currently, animal testing for medical devices adheres to the 3R principle (Replacement, Reduction, and Refinement), which aligns with animal welfare requirements. With advancements in regulatory science, the DQ principle (Design and Quality) has been added, forming the 3R + DQ principle. This approach ensures scientifically sound, validated, objective, and credible animal research results [92].

However, regulatory requirements and considerations specific to animal testing of medical devices using nanomaterials remain inadequate. The CMDE, NMPA have provided an overview in their guidance document, "The Safety and Effectiveness Evaluation of Medical Devices Using Nanomaterials and Nanotechnology-Part 1: System Framework," with more specific guidance expected in Part 4 in the future.

Animal testing plays a crucial role in assessing the feasibility, safety, and effectiveness of medical devices using nanomaterials. First, the new functions, mechanisms, and unique properties of these devices may not be fully captured by physicochemical characterization and biological evaluations alone. Animal testing provides strong evidence of the feasibility of product design. Secondly, while toxicokinetics and toxicological assessments have been conducted on medical devices using nanomaterials, some evaluation methods rely on extracts or exposure techniques that may not accurately reflect clinical conditions. Therefore, animal testing offers more precise evidence for these assessments. Finally, effectiveness evaluation is a key focus of animal testing for medical devices using nanomaterials, as many of these devices are still in the pre-clinical research phase. The challenge lies in designing animal tests that balance efficiency and risk and accurately reflect the advantages of nanomaterial-based devices compared to those devices without nanomaterials.

Overall, whether evaluating traditional medical devices or those incorporating nanomaterials and nanotechnology, adherence to the 3R + DQ principles is essential. Currently, regulatory science has not provided specific guidance for animal testing of medical devices using nanomaterials. Future focus may include implementing and assessing special post-operative evaluation programs in animal testing, considering the unique aspects of nanoparticle exposure.

3.4. Clinical evaluation might provide direct evidence for the safety and effectiveness of medical devices using nanomaterials

Clinical evaluation provides direct evidence to validate the safety and efficacy of medical devices. It offers the public, clinical community, and manufacturers a clearer understanding of how specific therapeutic devices should be used [93]. From a regulatory perspective, systematic clinical evaluation is essential for assessing the safety, clinical performance, and effectiveness of medical devices using nanomaterials. This is particularly important given the unknown risks associated with these devices, and many do not qualify for clinical exemption. Regarding the scientific accuracy of evaluation methods, differences between animals and humans-such as variations in blood proteomes-result in significant discrepancies in nanoparticle aggregation behavior and body distribution [94]. Consequently, it is challenging to accurately assess the safety and efficacy of nanomaterial-based medical devices through preclinical studies alone, which include physicochemical characterization, biological evaluation, and animal testing.

Similarly, the specific regulatory requirements and considerations for medical devices using nanomaterials, as outlined in the CMDE, NMPA's guidance, "The Safety and Effectiveness Evaluation of Medical Devices Using Nanomaterials and Nanotechnology - Part 1: System Framework," are currently broad. More specific guidance will be provided in Part 6 in the future. The general requirements for clinical evaluation include adhering to standard clinical evaluation practices while paying particular attention to the potential exposure risks of nanomaterials. This includes analyzing the quantity and form of nanomaterials in biological samples such as blood and urine and conducting metabolic and performance studies of the devices. This presents a significant challenge in developing accurate and useable assessment techniques for human application. Additionally, the guidance highlights the importance of designing targeted observational indicators for long-term safety evaluations, considering possible contact routes and target organs/tissues affected by the nanomaterials.

Advancing the regulation of clinical evaluation for medical devices using nanomaterials is critical. Clinical evaluation poses significant Challenges for translating these devices into clinical practice, largely due to the lack of substantial equivalents and the uncertainty surrounding the new biological effects of nanomaterials when interacting with the human body. This uncertainty has made the industry hesitate to conduct clinical evaluations. Therefore, to mitigate the risks associated with nanomaterials and promote the regulatory process, it is essential to develop clear guidance, accelerate international cooperation and information sharing, and provide professional advice [95].

4. Future of regulatory science in medical device using nanomaterials: convergence with In vitro alternative methods

The clinical translation of medical devices using nanomaterials lags behind basic research primarily because the interactions between nanomaterials and the human body, along with potential risks, are not well understood. More rigorous and extensive preclinical evaluations are required compared to devices without nanomaterials to assess their safety and efficacy. These evaluations, such as toxicological testing, often depend on animal studies, which are time-consuming and resource-intensive. To address this, it is crucial to minimize animal use While ensuring accurate assessments by developing emerging in vitro alternative technologies in line with the 3Rs principle (Replacement, Reduction, Refinement). Modern methods, including personalized design approaches, are expected to bridge gaps in data, analysis methods, and practices related to medical devices using nanomaterials, thereby advancing regulatory efforts. Fig. 5 illustrates the application of in vitro alternative approaches in the regulatory process for these devices.

The progress in in vitro alternatives is already yielding results. Various regulatory authorities, including the FDA, European Medicines Agency (EMA), and NMPA, are actively promoting the development and application of these methods [96]. The FDA defines alternative methods as a testing strategy that reduces or replaces animal testing to support benefit-risk assessment for FDA-regulated products. Examples include systems biology, engineered tissue, artificial intelligence, and microphysiological systems (MPS) [97]. A significant milestone was the passage of the FDA Modernization Act 2.0 in 2022, which removed mandatory federal animal testing requirements for new and generic drugs. The EU has introduced several OECD (Organisation for Economic Co-operation and Development) in vitro testing methods, primarily focused on cosmetics [98]. CMDE, NMPA are also working on new guidance for the in vitro evaluation of medical devices using nanomaterials and nanotechnology.

4.1. Organoid and organs-on-chips: a more ethical innovation that responds to the 3R principle

Organoids and Organ-on-a-Chip (OOAC) are revolutionary technologies for in vitro testing and are considered among the most promising strategies for animal reduction and replacement. Organoids were recognized as one of Science's top 10 breakthroughs of 2013 and were named Method of the Year 2017 in Nature Methods. They enable the study of biological processes, such as cell behavior, tissue repair, and responses to drugs or mutations [99,100]. OOAC ranked among the top 10 emerging technologies in 2016, effectively replicates the biomechanical and biological interfaces of soft tissues, allowing for the simulation of organ functionality and responses to various stimuli [101]. Reported initially as environmental simulation technologies for simple cells or organs, organoids and OOAC have been employed in disease modeling and precision medicine research, with advancements such as three-dimensional culture and gene editing [102-105]. In August 2022, the FDA approved the first new drug (NCT04658472) to enter clinical trials based solely on preclinical data from "organ-on-a-chip" studies. This milestone marks the first time that "organ-on-a-chip" experiments have replaced traditional animal tests and have been officially recognized [106]. Since then, alternative animal tests using organoids and ООАС have been widely developed in drug screening (efficacy and toxicity prediction), disease modeling, toxicity testing [107], and regenerative medicine [108].

Organoids and OOAC strategies are expected to accelerate the clinical translation of medical devices using nanomaterials. They provide methods to understand better and address the unacceptable side effects of nanomaterials that may not be predicted in simpler cell systems or animal tests. These technologies enable animal replacement by simulating the human tissue/organ environment [109,110]. Thus far, the adoption of organoids and OOAC in the field of nanomaterials has been primarily for preclinical safety evaluations and has shown high agreement with animal models in metabolic, toxicological, and other tests [111]. For instance, in vitro multi-organ-on-a-chip (MOC) systems have recreated the therapeutic efficacy and biodistribution of small extracellular vesicles in kidney and liver injuries, highlighting their potential for in-depth analysis of modes of action and identification of potential side effects [112]. Additionally, 3-D in vitro organoids that support kidney toxicity assessments of soluble small-molecule and specific nanomaterial agents provide highly predictive in vivo-in vitro comparisons based on extrapolated dosing regimens. They also facilitate other predictive analyses for nanoparticle assessments [113]. Moreover, various organoids, including brain and hepatocyte organoids, have been used to research how factors such as concentration and shape of nanomaterials affect exposure levels, offering practical approaches for assessing nanotoxicity from a 3D perspective in vitro [114-116]. Overall, organoids and OOAC technology have been steadily adopted as alternatives to animal use, with liver chips being pushed toward commercialization [117]. Their integration with regulatory frameworks is expected to gradually take shape [118,119].

4.2. Computational modeling and simulation (M&S): an irresistible trend of the information age

Artificial intelligence (AI) technology has spurred innovation across many fields, including toxicology. AI offers alternative approaches for risk assessment in support of the 3Rs (replacement, reduction, and refinement) [120,121]. Modeling and simulation (M&S) combine algorithms with databases to create computational models that can display the metabolism and action of materials in the human body, the etiology and development of diseases, therapeutic effects, and delivery targeting processes. For instance, AMPSphere, based on machine learning (ML) predictions and cataloging, is used to predict antimicrobial peptides (AMPs) within the global microbiome [122]. Quantitative structure-activity relationship (QSAR) modeling, a well-established АТ model, has become an essential tool in virtual screening for pharmaceutical discovery [123,124]. PandaOmics™, an Al biotarget-discovery platform, has identified several previously unreported potential therapeutic targets for amyotrophic lateral sclerosis (ALS) [125,126]. Additionally, the development of Generative Adversarial Networks (GANs), such as AnimcalGAN, which simulate clinical pathology measurements in rats, represents a significant advancement in AT's contribution to the global effort in the 3Rs [120]. Consequently, AI has gained broader attention as a tool for predicting adverse toxicity and therapeutic effects of products before applying nanomaterials to patients [127-129].

Computational (in silico) modeling and simulation (M&S) are powerful tools that complement or replace traditional animal testing and are expected to provide evidence about nanomaterial exposure [130]. The high variability in the type and physicochemical properties of nanomaterials, the target cells, and the exposure route makes it challenging to assess risk using conventional animal toxicity data-based approaches. This complexity drives extensive research into computational models in nanotoxicology [131]. For example, in nanomedical drug development, Quantitative Systems Pharmacology (QSP) has been proposed to interpret, interrogate, and integrate drug effects from molecules to entire organisms through modeling and computation. This approach aims to predict therapeutic effects and improve drug development and is widely supported by the FDA [132]. Furthermore, recent Al-based models offer in silico quantitative safety assessments for nanomaterials, considering factors such as adsorption [133], structure [134], potential [135], and physiology [136]. These advancements are expected to reduce the workload and cost of nanomaterials discovery significantly. Overall, these studies highlight the potential of M&S for screening and developing safe and effective novel nanomaterials, replacing animal tests, and assisting in clinical trial design.

Although early M&S research focused on developing animal alternatives for preclinical safety assessments, newer models are now aimed at clinical simulation trials. For example, the performance assessment in-silico trial (FD-PASS) using a braided, self-expanding, stent-like flow diverter (FD) not only replicates findings from conventional clinical trials but also performs virtual experiments and subgroup analyses that are challenging or impossible in traditional trials. This approach provides new insights and generates hypotheses for medical clinical trials [137]. Similarly, based on bench and animal studies, confirmatory simulation modeling of the Medtronic Revo pacemaker system addressed critical safety and effectiveness questions, such as whether MRI can induce lead heating, that are difficult to answer through clinical trials alone. The results from this modeling were consistent with those of confirmatory unblinded clinical trials [138]. Clinical trials often involve numerous influencing factors, long durations, and high costs. Computational modeling combined with AI technology can optimize trial design, personalize clinical protocols, and advance materials with the highest clinical translation potential.

In summary, adhering to the 3R principle, which aims to reduce or replace animal testing, is an inevitable trend. However, advancements in in vitro alternative methods for medical devices using nanomaterials are still lagging. To address this, the next generation of regulatory science should focus on developing standardized in vitro methods through extensive fundamental research and verification and integrating these methods with regulatory science.

5. Perspective on future directions

We believe that advancing the regulatory science of medical devices using nanomaterials will have a significant impact by clarifying the exact path and methods for translating these innovations from the lab to clinical practice. Additionally, the development and application of modern science, technology, and tools throughout the life cycle of medical devices can reduce time, cost, and ethical concerns, thereby accelerating clinical translation. Encouraging collaboration among regulatory authorities, research institutions, medical centers, and manufacturers will help address the current imbalance between product innovation and clinical translation progress.

5.1. Challenges in the in vitro alternative method

It remains unclear whether current in vitro alternatives to animal testing, such as organoids, organ-on-a-chip (OOAC), and in silico models, will be reliable and relevant for all nanomaterials. Preparing microcopies of cellular and extracellular components of tissues or organs often results in approximations of tissue function. Given the diverse composition of nanoprotein coronas, the predictability and repeatability of in vitro alternative methods for these materials may be significantly reduced. Therefore, regulatory science must conduct standardized verification of reliability (including repeatability, transferability, and reproducibility) and relevance (predictive capacity and applicability domain) through extensive laboratory work during the development, validation, and adoption of in vitro alternative methods!%. Unfortunately, reliable tools and methods for validating modern in vitro alternatives are scarce. Building a knowledge base to properly validate the reliability and relevance of these complex emerging technologies is an essential goal for regulatory science.

5.2. Artificial intelligence and big data

Establishing a database based on artificial intelligence (AI) and deep learning can enhance regulatory efficiency and quality, facilitate comprehensive regulatory and risk prediction, and ensure the safety and quality of medical devices. According to the FDA's latest report on CDRH (Center for Devices and Radiological Health) regulatory science priorities, nearly all of its ten priorities involve Al-assisted research [139]. The regulatory process for medical devices, from application submission to certification, often spans several years. Medical devices using nanomaterials may incur additional time and costs due to safety risks, as they require stringent manual audits. Modern computer models and big data AI can automate data analysis and prediction by integrating information from the entire lifecycle of medical devices, thereby improving risk regulation and assessment. Achieving this requires a thorough understanding of the production, registration, sales, usage, and adverse events associated with various medical device products. Additionally, establishing a comprehensive database will necessitate enhancing the cybersecurity of digital health systems.

5.3. Real-world data/evidence (RWD/RWE) and evidence-based medicine

Monitoring and predicting the clinical performance of medical devices is essential to ensure their safety and effectiveness. Oversight by regulatory authorities is crucial for maintaining the reliability and scientific validity of clinical data. Recent advances in electronic health records (EHR), wearable technologies, data science, and machine learning are transforming evidence-based medicine, offering a glimpse into the future of next-generation "deep" medicine [140,141]. The REAL-WORLD EVIDENCE PROGRAM outlines an optimal framework for evaluating RWD/RWE for regulatory decisions. It demonstrates that modern technologies, such as the FDA Sentinel and Biologics Effectiveness and Safety (BEST) systems, can enhance the quality and effective use of RWE. However, their full potential requires ongoing learning and validation research [142,143]. Currently, due to rapid advances in genomics, regulatory authorities, and scientific institutions are focusing on RWE research in drug development and clinical research. There is an urgent need to direct similar attention to the field of medical devices. Therefore, building a comprehensive medical device EHR library and employing modern technologies like machine learning, deep neural networks, and multi-modal biomedical artificial intelligence to develop a computer model system for assessing infection risk and biofilm formation after medical device use is essential. The RDE model for medical devices and its application in clinical studies can benefit from the FDA's RWE Program, which emphasizes data source diversity, data quality and stability, clinical evaluation validity, and regulatory decision support.

In summary, the behavior and impact of nanomaterials in biological systems and their application in medical devices have garnered extensive research and attention, offering new possibilities to address growing medical needs and reduce risks. However, the journey from bench to clinical application for medical devices is rigorous and lengthy, with emerging technologies introducing additional unknown risks. Regulatory science plays a crucial role in this translation process. Integrating robust basic research with rigorous regulatory science is vital for advancing the application and clinical translation of emerging technologies.

CRediT authorship contribution statement

Chubing Lin: Writing - review & editing, Writing - original draft, Investigation, Conceptualization. Xin Huang: Writing - review & editing, Writing - original draft, Investigation, Conceptualization. Yueguang Xue: Writing - review & editing, Methodology. Shasha Jiang: Writing - review & editing. Chunying Chen: Writing - review & editing, Funding acquisition. Ying Liu: Writing - review & editing, Funding acquisition, Conceptualization. Kuan Chen: Writing - original draft, Funding acquisition, Conceptualization.

Ethics approval and consent to participate

Not applicable.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work was supported by National Key R&D Program of China of Ministry of Science and Technology of the People's Republic of China (Grant No. 2022YFC2409700, 2021YFA1200900), National Natural Science Foundation of China (Grant No. 22476031, 22027810).

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ARTICLE INFO

Received 26 October 2024; Received in revised form 29 January 2025; Accepted 10 February 2025

Available online 20 February 2025

Chubing Lin - School of Biomedical Sciences and Engineering, South China University of Technology, Guangzhou, 511442 China; CAS Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety & CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology of China, Beijing 100190, China

Xin Huang - School of Biomedical Sciences and Engineering, South China University of Technology, Guangzhou, 511442 China; CAS Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety & CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology of China, Beijing 100190, China

Yueguang Xue - School of Biomedical Sciences and Engineering, South China University of Technology, Guangzhou, 511442 China; CAS Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety & CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology of China, Beijing 100190, China

Shasha Jiang The 990th Hospital of the Jointservice Support Force of the PLA, Zhu-madian, Henan province 463000, China

Chunying Chen - CAS Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety & CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology of China, Beijing 100190, China

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