Tailoring Cellular Auxetics for Wearable Applications With Multimaterial 3D Printing

Multimaterial (MM) 3D printing shows great potential for application in metamaterials, flexible electronics, biomedical devices and robots, since it can seamlessly integrate distinctive materials into one printed structure. Among numerous MM 3D printing technologies, digital light processing (DLP) MM 3D printing is compatible with a wide range of materials from hydrogels to ceramics, and can print MM 3D structures with high resolution, high complexity and fast speed. This paper introduces the fundamental mechanisms of DLP 3D printing, and reviews the recent advances of DLP MM 3D printing technologies with emphasis on material switching methods and material contamination issues. It also summarizes a number of typical examples of DLP MM 3D printing systems developed in the past decade, and introduces their system structures, working principles, material switching methods, residual resin removal methods, printing steps, as well as the representative structures and applications. Finally, we provide perspectives on the directions of the further development of DLP MM 3D printing technology.

Multiscale and multimaterial three-dimensional (3D) printing is new frontier in additive manufacturing (AM). It has shown great potential to implement the simultaneous and full control for fabricated object including external geometry, internal architecture, functional surface, material composition and ratio as well as gradient distribution, feature size ranging from nano-, micro-, to macro-scale, embedded components and electrocircuit, etc. Furthermore, it has the ability to construct the heterogeneous and hierarchical structured object with tailored properties and multiple functionalities which cannot be achieved through the existing technologies. That paves the way and may result in great breakthrough in various applications, e.g., functional tissue and organ, functionally graded (FG) material/structure, wearable devices, soft robot, functionally embedded electronics, metamaterial, multifunctionality product, etc. However, very few of the established AM processes have now the capability to implement the multimaterial and multiscale 3D printing. This paper presented a single nozzle-based multiscale and multimaterial 3D printing process by integrating the electrohydrodynamic jet printing and the active mixing multimaterial nozzle. The proposed AM technology has the capability to create multifunctional heterogeneously structured objects with control of the macroscale external geometry and microscale internal structures as well as functional surface features, particularly, the potential to dynamically mix, grade, and vary the ratios of different materials. An active mixing nozzle, as a core functional component of the 3D printer, is systematically investigated by combining with the theoretical analysis, numerical simulation, and experimental verification. The study aims at exploring a feasible solution to implement the multiscale and multimaterial 3D printing at low cost.

The purpose of this study is to explore the main research hotspots in the design of wearable interactive devices and to explore the new trend of future development in light of the current research hotspots. The method is to use the information and literature on wearable interactive device design collected by Web of Science as the main data source, and use VOSviewer and CiteSpace to map the scientific knowledge of the literature, including the distribution of years and outputs, countries, research institutions, and keyword clustering, and then visualize and analyze them. The results show that the number of literature within the search range is on the rise overall, with China, the United States, and other countries in the leading position of research. The research hotspots mainly focus on the research of intelligent medical and human-computer interaction experience in wearable interaction systems, the research of intelligent device performance and core components, the research of material properties applicable to wearable devices, and the research of mechanical devices and functions in wearable device design. It is concluded that wearable interactive device design research has a strong interdisciplinary nature, and the future research focus will concentrate on the direction of intelligent interaction, user experience, sustainable material application, and device performance improvement of wearable devices. The future industry trend may be toward the direction of intelligent health detection, multi-device interconnection and interactive experience, sustainable and ecosystem integration, artificial intelligence analysis, cross-border integration, and innovation. Summarizing the current status and trends of research, we discuss the future development trend of wearable interactive device design from the strategy of natural interaction attributes of human-machine and human-ecology.

3D printing technologies for electrochemical energy storage

Direct-ink-writing 3D-printed bioelectronics

Ni–P and TiO2 codeposition on silk textile via supercritical CO2 promoted electroless plating for flexible and wearable photocatalytic devices

The application of three-dimensional (3D) printing in medicine has increased rapidly in the last few years. Various new commercial 3D printers and 3D bioprinters are introduced into the current market. 3D printing scientific community has witnessed a rapid growth in the application of 3D printing for various medical needs. There has been a research advancement in 4D printing with a widened scope of 3D bioprinting. The growing demand for 3D printing and its use for commercial and industrial purposes in medical technology have facilitated the need for introduction of new international standards and regulations. The need for organ replacement has generated curiosity and competition among researchers to race for organ 3D printing, which can be identified with increased publications and patents that are filed in recent years. Hence, there is a need to update the readers with the latest information. The second edition of this chapter updates the contents of first section to fourth section with current information and some modified pictorial representations. New section describes the relevance of the new chapters added to this edition and updates the readers with insights on advancement of 3D printing in the last decade. Briefly, this chapter introduces the reader with a brief history of 3D printing in medical technology, components of 3D printing, 3D bioprinting, organ 3D bioprinting, advantages of 3D printing with current challenges and commercially available 3D printers in the current market. New sections introduced in this edition detail about the ASTM and ISO standards that are used for 3D printing in medicine, the regulatory framework with current challenges, ethical and social concerns with the current 3D printing scientific community, and the importance of intellectual property in safeguarding the interests of the innovators.

3D printing has become a powerful technique in electrochemistry for fabricating electrodes, thanks to readily available conductive nanocomposite filaments, such as those based on carbon fillers (i.e., carbon nanotubes (CNTs) or carbon black (CB)) within an insulating polymeric matrix like polylactic acid (PLA). Inspired by inorganic heterostructures that enhance the functional characteristics of nanomaterials, we fabricated hetero-layered 3D printed devices based on carbon allotropes using a layer-by-layer assembly approach. The heterolayers were customised through the alternate integration of different carbon allotrope filaments via a multi-material 3D printing technique, allowing for a time-effective method to enhance electrochemical performance. As a first demonstration of applicability, CNT/PLA and CB/PLA filaments were utilised to construct ordered hetero-layered carbon-based electrodes. This contrasts with conventional methods where various carbon species are mixed in the same composite-based filament used for building electrochemical devices. Multi-material 3D-printed carbon electrodes exhibit improved electrochemical performance in energy conversion (e.g., hydrogen evolution reaction or HER) and sensing applications (e.g., ascorbic acid detection) compared to single-material electrodes. This work paves the way for manufacturing advanced 3D-printed heterolayered electrodes with enhanced electrochemical activity through multi-material 3D printing technology.

Bio-inspired electrochromic skin based on tungsten oxide

Auxetic materials, known as materials with negative Poisson’s ratio (NPR), have many promising application areas. However, there are only few natural and man-made materials such as certain living bone tissues, certain rocks and minerals, polymeric honeycombs, microporous polytetrafluoroethylene (PTFE), foams, and carbon-fiber-reinforced epoxy composite laminate panels that possess this property. In recent years, various auxetic material structures have been designed and fabricated for diverse applications that utilized normal materials which follow Hooke’s law but still show the NPR properties. One of the applications is body protection pads that are comfortable to wear and effective in protecting body parts by reducing impact force and preventing injuries in high-risk individuals such as elderly people, industry workers, law enforcement and military personnel, and sports players. It is important to develop new body protectors that best combine each individual’s requirements for wearing comfort (flexible, light-weight), ease of fitting (customized), ensured protection, and cost-effectiveness. The protection pad would be made from multilayer materials and adaptive structures to achieve unique multifunctional properties such as high hardness, impact toughness, light weight, and excellent shock absorption suitable for the needs. This paper reports an integrated theoretical, computational (finite element analysis), and experimental investigation conducted for typical auxetic polymeric materials that exhibit negative Poisson’s ratio (NPR) effect. Parametric 3D CAD models of auxetic polymeric structures such as re-entrant hexagonal cells and arrowhead were developed. Then, key structural characteristics of protectors were evaluated through static analyses of FEA models. In addition, impact/shock analyses were conducted through dynamic analyses of FEA models to validate the results obtained from the static analyses. Particularly, an advanced additive manufacturing (3D printing) technique was used to build prototypes of the auxetic polymeric structures. Specifically, three different materials typically used for FDM (Fused Deposition Modeling) technology such as Polylactic acid (PLA) and thermoplastic polyurethane (TPU) material (NinjaFlex® and SemiFlex®) were used for different stiffness and shock-absorption performances. The 3D printed prototypes were then tested and the results were compared with the computational prediction. The results showed that the auxetic material can be effective for body protection pads. Each structure and material had unique structural properties such as stiffness, Poisson’s ratio, and efficiency in shock absorption. Particularly, auxtetic structures showed better shock absorption performance than non-auxetic ones. The mechanism for ideal input force distribution or shunting could be suggested for designing protectors using various shapes, thicknesses, and materials of auxetic materials to reduce the risk of injury.

In recent years, various auxetic material structures have been designed and fabricated for diverse applications that utilize normal materials that follow Hooke’s law but still show the properties of negative Poisson’s ratios (NPR). One potential application is body protection pads that are comfortable to wear and effective in protecting body parts by reducing impact force and preventing injuries in high-risk individuals such as elderly people, industrial workers, law enforcement and military personnel, and athletes. This paper reports an integrated theoretical, computational, and experimental investigation conducted for typical auxetic materials that exhibit NPR properties. Parametric 3D CAD models of auxetic structures such as re-entrant hexagonal cells and arrowheads were developed. Then, key structural characteristics of protection pads were evaluated through static analyses of FEA models. Finally, impact analyses were conducted through dynamic simulations of FEA models to validate the results obtained from the static analyses. Efforts were also made to relate the individual and/or combined effect of auxetic structures and materials to the overall stiffness and shock-absorption performance of the protection pads. An advanced additive manufacturing (3D printing) technique was used to build prototypes of the auxetic structures. Three different materials typically used for fused deposition modeling technology, namely polylactic acid (PLA) and thermoplastic polyurethane material (NinjaFlex® and SemiFlex®), were used for different stiffness and shock-absorption properties. The 3D printed prototypes were then tested and the results were compared to the computational predictions. The results showed that the auxetic material could be effective in reducing the shock forces. Each structure and material combination demonstrated unique structural properties such as stiffness, Poisson’s ratio, and efficiency in shock absorption. Auxetic structures showed better shock absorption performance than non-auxetic ones. The mechanism for ideal input force distribution or shunting could be suggested for designing protectors using various shapes, thicknesses, and materials of auxetic materials to reduce the risk of injury.

Magnetic materials are of increasing importance for many essential applications due to their unique magnetic properties. However, due to the limited fabrication ability, magnetic materials are restricted by simple geometric shapes. Three-dimensional (3D) printing is a highly versatile technique that can be utilized for constructing magnetic materials. The shape flexibility of magnets unleashes opportunities for magnetic composites with reducing post-manufacturing costs, motivating the review on 3D printing of magnetic materials. This paper focuses on recent achievements of magnetic materials using 3D printing technologies, followed by the characterization of their magnetic properties, which are further enhanced by modification. Interestingly, the corresponding properties depend on the intrinsic nature of starting materials, 3D printing processing parameters, and the optimized structural design. More emphasis is placed on the functional applications of 3D-printed magnetic materials in different fields. Lastly, the current challenges and future opportunities are also addressed.

Micro‐electrochemical energy storage devices (MEESDs) including micro‐supercapacitors (MSCs), micro‐batteries (MBs), and metal‐ion hybrid micro‐supercapacitors (MIHMSCs) are critical components of electronic systems, especially in the expanding field of the Internet of Things (IoT). In recent years, three‐dimensional (3D) printing techniques also known as additive manufacturing (AM) techniques have emerged as promising approaches to overcome the limitations of conventional fabrication procedures. Employing advanced manufacturing techniques to fabricate MEESDs offers potential benefits, including mass production and programmable prototyping. In this review, we provide a comprehensive overview of the recent advancements in the applications of 3D printing techniques for MEESDs such as MBs, MSCs, and MIHMSCs. First, this review discusses the fundamental of micro/nano energy storage devices by 3D printing technology. Further, we examine the critical properties of the printable inks used in these processes. We also highlighted the current developments in 3D printing‐based MEESDs including various types of MBs, pseudocapacitive and electrochemical double‐layer‐based MSCs, and a range of MIHMSCs. Additionally, this review addresses the challenges and future prospects of 3D printing based MEESDs, including material limitations, printing resolution, manufacturing scalability, mechanical properties, and cost. Overall, this review presents a unique and valuable contribution by providing a comprehensive overview of the recent advancements in 3D printing based MEESDs.

In recent years, wearable devices have rapidly emerged in the medical field, offering simplicity of operation, low cost, and freedom from spatial constraints, holding broad prospects in orthopedic rehabilitation.This article analyzes the application of wearable devices in the prevention, assessment, and training of knee joint injuries, summarizing their current status in the prevention and treatment of knee injuries, and points out the current shortcomings.It also proposes future research prospects for wearable devices in the medical field, aiming to provide a reference for further application and research of wearable devices in knee joint injuries.

Wearable devices are now recognized as a powerful tool to collect physiological and environmental information in a smart, noninvasive, and real-time manner. Despite the rapid progress of wearable devices especially wearable electronic devices, there are still several challenges that limit their further development, for example, a complicated electrical signal acquisition and processing process to eliminate the interference from the surrounding signals, bulky power supply, inevitable e-waste, and environmental pollution. Herein, we report a 3D-printed recyclable, flexible, and wearable device for visualized UV, temperature, and sweat pH sensing. Compared with wearable electronic devices, our visualized wearable device senses environmental (UV light, ambient temperature), biophysical (skin temperature), and biochemical (sweat pH) signals via stimuli-responsive color change, which does not require complicated electronic circuit design/assembly, time-consuming data processing and additional power source. In addition, this visualized wearable device is fabricated via a 3D support bath printing technology by printing UV-, temperature-, and sweat pH-sensing inks containing photochromic, thermochromic, and pH-chromic materials, respectively, into/onto sustainable starch solution, resulting in a multi-functional, recyclable, and flexible sensing device with high reproducibility. Our results reveal that UV light intensities under sunlight (0–2500 μW/cm2), ambient, and skin temperatures (0–38 °C) as well as sweat pH (4.0–7.0) can be successfully monitored.