Three-dimensional mesostructures as high-temperature growth templates, electronic cellular scaffolds, and self-propelled microrobots

Conducting polymer scaffolds: a new frontier in bioelectronics and bioengineering

Conducting polymer (CP) scaffolds have emerged as a transformative tool in bioelectronics and bioengineering, advancing the ability to interface with biological systems. Their unique combination of electrical conductivity, tailorability, and biocompatibility surpasses the capabilities of traditional nonconducting scaffolds while granting them access to the realm of bioelectronics. This review examines recent developments in CP scaffolds, focusing on material and device advancements, as well as their interplay with biological systems. We highlight applications for monitoring, tissue stimulation, and drug delivery and discuss perspectives and challenges currently faced for their ultimate translation and clinical implementation.

Highly Ordered Eutectic Mesostructures via Template-Directed Solidification within Thermally Engineered Templates

Template-directed self-assembly of solidifying eutectics results in emergence of unique microstructures due to diffusion constraints and thermal gradients imposed by the template. Here, the importance of selecting the template material based on its conductivity to control heat transfer between the template and the solidifying eutectic, and thus the thermal gradients near the solidification front, is demonstrated. Simulations elucidate the relationship between the thermal properties of the eutectic and template and the resultant microstructure. The overarching finding is that templates with low thermal conductivities are generally advantageous for forming highly organized microstructures. When electrochemically porosified silicon pillars (thermal conductivity < 0.3 Wm-1K-1) are used as the template into which an AgCl-KCl eutectic is solidified, 99% of the unit cells in the solidified structure exhibit the same pattern. In contrast, when higher thermal conductivity crystalline silicon pillars (≈100 Wm-1K-1) are utilized, the expected pattern is only present in 50% of the unit cells. The thermally engineered template results in mesostructures with tunable optical properties and reflectances nearly identical to the simulated reflectances of perfect structures, indicating highly ordered patterns are formed over large areas. This work highlights the importance of controlling heat flows in template-directed self-assembly of eutectics.

Structural and Material-Based Approaches for the Fabrication of Stretchable Light-Emitting Diodes

Stretchable displays, capable of freely transforming their shapes, have received significant attention as alternatives to conventional rigid displays, and they are anticipated to provide new opportunities in various human-friendly electronics applications. As a core component of stretchable displays, high-performance stretchable light-emitting diodes (LEDs) have recently emerged. The approaches to fabricate stretchable LEDs are broadly categorized into two groups, namely "structural" and "material-based" approaches, based on the mechanisms to tolerate strain. While structural approaches rely on specially designed geometries to dissipate applied strain, material-based approaches mainly focus on replacing conventional rigid components of LEDs to soft and stretchable materials. Here, we review the latest studies on the fabrication of stretchable LEDs, which is accomplished through these distinctive strategies. First, we introduce representative device designs for efficient strain distribution, encompassing island-bridge structures, wavy buckling, and kirigami-/origami-based structures. For the material-based approaches, we discuss the latest studies for intrinsically stretchable (is-) electronic/optoelectronic materials, including the formation of conductive nanocomposite and polymeric blending with various additives. The review also provides examples of is-LEDs, focusing on their luminous performance and stretchability. We conclude this review with a brief outlook on future technologies.

Scalable Electrophysiology of Millimeter-Scale Animals with Electrode Devices

Millimeter-scale animals such as Caenorhabditis elegans, Drosophila larvae, zebrafish, and bees serve as powerful model organisms in the fields of neurobiology and neuroethology. Various methods exist for recording large-scale electrophysiological signals from these animals. Existing approaches often lack, however, real-time, uninterrupted investigations due to their rigid constructs, geometric constraints, and mechanical mismatch in integration with soft organisms. The recent research establishes the foundations for 3-dimensional flexible bioelectronic interfaces that incorporate microfabricated components and nanoelectronic function with adjustable mechanical properties and multidimensional variability, offering unique capabilities for chronic, stable interrogation and stimulation of millimeter-scale animals and miniature tissue constructs. This review summarizes the most advanced technologies for electrophysiological studies, based on methods of 3-dimensional flexible bioelectronics. A concluding section addresses the challenges of these devices in achieving freestanding, robust, and multifunctional biointerfaces.

Roadmap on Label-Free Super-Resolution Imaging

Label-free super-resolution (LFSR) imaging relies on light-scattering processes in nanoscale objects without a need for fluorescent (FL) staining required in super-resolved FL microscopy. The objectives of this Roadmap are to present a comprehensive vision of the developments, the state-of-the-art in this field, and to discuss the resolution boundaries and hurdles which need to be overcome to break the classical diffraction limit of the LFSR imaging. The scope of this Roadmap spans from the advanced interference detection techniques, where the diffraction-limited lateral resolution is combined with unsurpassed axial and temporal resolution, to techniques with true lateral super-resolution capability which are based on understanding resolution as an information science problem, on using novel structured illumination, near-field scanning, and nonlinear optics approaches, and on designing superlenses based on nanoplasmonics, metamaterials, transformation optics, and microsphere-assisted approaches. To this end, this Roadmap brings under the same umbrella researchers from the physics and biomedical optics communities in which such studies have often been developing separately. The ultimate intent of this paper is to create a vision for the current and future developments of LFSR imaging based on its physical mechanisms and to create a great opening for the series of articles in this field.

Mechanically-Guided 3D Assembly for Architected Flexible Electronics

Architected flexible electronic devices with rationally designed 3D geometries have found essential applications in biology, medicine, therapeutics, sensing/imaging, energy, robotics, and daily healthcare. Mechanically-guided 3D assembly methods, exploiting mechanics principles of materials and structures to transform planar electronic devices fabricated using mature semiconductor techniques into 3D architected ones, are promising routes to such architected flexible electronic devices. Here, we comprehensively review mechanically-guided 3D assembly methods for architected flexible electronics. Mainstream methods of mechanically-guided 3D assembly are classified and discussed on the basis of their fundamental deformation modes (i.e., rolling, folding, curving, and buckling). Diverse 3D interconnects and device forms are then summarized, which correspond to the two key components of an architected flexible electronic device. Afterward, structure-induced functionalities are highlighted to provide guidelines for function-driven structural designs of flexible electronics, followed by a collective summary of their resulting applications. Finally, conclusions and outlooks are given, covering routes to achieve extreme deformations and dimensions, inverse design methods, and encapsulation strategies of architected 3D flexible electronics, as well as perspectives on future applications.

Wireless and battery-free technologies for neuroengineering

Tethered and battery-powered devices that interface with neural tissues can restrict natural motions and prevent social interactions in animal models, thereby limiting the utility of these devices in behavioural neuroscience research. In this Review Article, we discuss recent progress in the development of miniaturized and ultralightweight devices as neuroengineering platforms that are wireless, battery-free and fully implantable, with capabilities that match or exceed those of wired or battery-powered alternatives. Such classes of advanced neural interfaces with optical, electrical or fluidic functionality can also combine recording and stimulation modalities for closed-loop applications in basic studies or in the practical treatment of abnormal physiological processes.

Assembly of Fillable Microrobotic Systems by Microfluidic Loading with Dip Sealing

Microrobots can provide spatiotemporally well-controlled cargo delivery that can improve therapeutic efficiency compared to conventional drug delivery strategies. Robust microfabrication methods to expand the variety of materials or cargoes that can be incorporated into microrobots can greatly broaden the scope of their functions. However, current surface coating or direct blending techniques used for cargo loading result in inefficient loading and poor cargo protection during transportation, which leads to cargo waste, degradation and non-specific release. Herein, a versatile platform to fabricate fillable microrobots using microfluidic loading and dip sealing (MLDS) is presented. MLDS enables the encapsulation of different types of cargoes within hollow microrobots and protection of cargo integrity. The technique is supported by high-resolution 3D printing with an integrated microfluidic loading system, which realizes a highly precise loading process and improves cargo loading capacity. A corresponding dip sealing strategy is developed to encase and protect the loaded cargo whilst maintaining the geometric and structural integrity of the loaded microrobots. This dip sealing technique is suitable for different materials, including thermal and light-responsive materials. The MLDS platform provides new opportunities for microrobotic systems in targeted drug delivery, environmental sensing, and chemically powered micromotor applications.

3D morphable systems via deterministic microfolding for vibrational sensing, robotic implants, and reconfigurable telecommunication

DNA and proteins fold in three dimensions (3D) to enable functions that sustain life. Emulation of such folding schemes for functional materials can unleash enormous potential in advancing a wide range of technologies, especially in robotics, medicine, and telecommunication. Here, we report a microfolding strategy that enables formation of 3D morphable microelectronic systems integrated with various functional materials, including monocrystalline silicon, metallic nanomembranes, and polymers. By predesigning folding hosts and configuring folding pathways, 3D microelectronic systems in freestanding forms can transform across various complex configurations with modulated functionalities. Nearly all transitional states of 3D microelectronic systems achieved via the microfolding assembly can be easily accessed and modulated in situ, offering functional versatility and adaptability. Advanced morphable microelectronic systems including a reconfigurable microantenna for customizable telecommunication, a 3D vibration sensor for hand-tremor monitoring, and a bloomable robot for cardiac mapping demonstrate broad utility of these assembly schemes to realize advanced functionalities.

Submillimeter-scale multimaterial terrestrial robots

Robots with submillimeter dimensions are of interest for applications that range from tools for minimally invasive surgical procedures in clinical medicine to vehicles for manipulating cells/tissues in biology research. The limited classes of structures and materials that can be used in such robots, however, create challenges in achieving desired performance parameters and modes of operation. Here, we introduce approaches in manufacturing and actuation that address these constraints to enable untethered, terrestrial robots with complex, three-dimensional (3D) geometries and heterogeneous material construction. The manufacturing procedure exploits controlled mechanical buckling to create 3D multimaterial structures in layouts that range from arrays of filaments and origami constructs to biomimetic configurations and others. A balance of forces associated with a one-way shape memory alloy and the elastic resilience of an encapsulating shell provides the basis for reversible deformations of these structures. Modes of locomotion and manipulation span from bending, twisting, and expansion upon global heating to linear/curvilinear crawling, walking, turning, and jumping upon laser-induced local thermal actuation. Photonic structures such as retroreflectors and colorimetric sensing materials support simple forms of wireless monitoring and localization. These collective advances in materials, manufacturing, actuation, and sensing add to a growing body of capabilities in this emerging field of technology.

Kirigami-Inspired Biodesign for Applications in Healthcare

Mechanically flexible and conformable materials and integrated devices have found diverse applications in personalized healthcare as diagnostics and therapeutics, tissue engineering and regenerative medicine constructs, surgical tools, secure systems, and assistive technologies. In order to impart optimal mechanical properties to the (bio)materials used in these applications, various strategies have been explored-from composites to structural engineering. In recent years, geometric cuts inspired by the art of paper-cutting, referred to as kirigami, have provided innovative opportunities for conferring precise mechanical properties via material removal. Kirigami-based approaches have been used for device design in areas ranging from soft bioelectronics to energy storage. In this review, the principles of kirigami-inspired engineering specifically for biomedical applications are discussed. Factors pertinent to their design, including cut geometry, materials, and fabrication, and the effect these parameters have on their properties and configurations are covered. Examples of kirigami designs in healthcare are presented, such as, various form factors of sensors (on skin, wearable), implantable devices, therapeutics, surgical procedures, and cellular scaffolds for regenerative medicine. Finally, the challenges and future scope for the successful translation of these biodesign concepts to broader deployment are discussed.

Biomimetic and Biologically Compliant Soft Architectures via 3D and 4D Assembly Methods: A Perspective

Recent progress in soft material chemistry and enabling methods of 3D and 4D fabrication-emerging programmable material designs and associated assembly methods for the construction of complex functional structures-is highlighted. The underlying advances in this science allow the creation of soft material architectures with properties and shapes that programmably vary with time. The ability to control composition from the molecular to the macroscale is highlighted-most notably through examples that focus on biomimetic and biologically compliant soft materials. Such advances, when coupled with the ability to program material structure and properties across multiple scales via microfabrication, 3D printing, or other assembly techniques, give rise to responsive (4D) architectures. The challenges and prospects for progress in this emerging field in terms of its capacities for integrating chemistry, form, and function are described in the context of exemplary soft material systems demonstrating important but heretofore difficult-to-realize biomimetic and biologically compliant behaviors.

Island Effect in Stretchable Inorganic Electronics

Island-bridge architectures represent a widely used structural design in stretchable inorganic electronics, where deformable interconnects that form the bridge provide system stretchability, and functional components that reside on the islands undergo negligible deformations. These device systems usually experience a common strain concentration phenomenon, i.e., "island effect", because of the modulus mismatch between the soft elastomer substrate and its on-top rigid components. Such an island effect can significantly raise the surrounding local strain, therefore increasing the risk of material failure for the interconnects in the vicinity of the islands. In this work, a systematic study of such an island effect through combined theoretical analysis, numerical simulations and experimental measurements is presented. To relieve the island effect, a buffer layer strategy is proposed as a generic route to enhanced stretchabilities of deformable interconnects. Both experimental and numerical results illustrate the applicability of this strategy to 2D serpentine and 3D helical interconnects, as evidenced by the increased stretchabilities (e.g., by 1.5 times with a simple buffer layer, and 2 times with a ring buffer layer, both for serpentine interconnects). The application of the patterned buffer layer strategy in a stretchable light emitting diodes system suggests promising potentials for uses in other functional device systems.

Mechanically Guided Hierarchical Assembly of 3D Mesostructures

3D, hierarchical micro/nanostructures formed with advanced functional materials are of growing interest due to their broad potential utility in electronics, robotics, battery technology, and biomedical engineering. Among various strategies in 3D micro/nanofabrication, a set of methods based on compressive buckling offers wide-ranging material compatibility, fabrication scalability, and precise process control. Previously reports on this type of approach rely on a single, planar prestretched elastomeric platform to transform thin-film precursors with 2D layouts into 3D architectures. The simple planar configuration of bonding sites between these precursors and their assembly substrates prevents the realization of certain types of complex 3D geometries. In this paper, a set of hierarchical assembly concepts is reported that leverage multiple layers of prestretched elastomeric substrates to induce not only compressive buckling of 2D precursors bonded to them but also of themselves, thereby creating 3D mesostructures mounted at multiple levels of 3D frameworks with complex, elaborate configurations. Control over strains used in these processes provides reversible access to multiple different 3D layouts in a given structure. Examples to demonstrate these ideas through both experimental and computational results span vertically aligned helices to closed 3D cages, selected for their relevance to 3D conformal bio-interfaces and multifunctional microsystems.

Research Progress on the Flexibility of an Implantable Neural Microelectrode

Neural microelectrode is the important bridge of information exchange between the human body and machines. By recording and transmitting nerve signals with electrodes, people can control the external machines. At the same time, using electrodes to electrically stimulate nerve tissue, people with long-term brain diseases will be safely and reliably treated. Young’s modulus of the traditional rigid electrode probe is not matched well with that of biological tissue, and tissue immune rejection is easy to generate, resulting in the electrode not being able to achieve long-term safety and reliable working. In recent years, the choice of flexible materials and design of electrode structures can achieve modulus matching between electrode and biological tissue, and tissue damage is decreased. This review discusses nerve microelectrodes based on flexible electrode materials and substrate materials. Simultaneously, different structural designs of neural microelectrodes are reviewed. However, flexible electrode probes are difficult to implant into the brain. Only with the aid of certain auxiliary devices, can the implant be safe and reliable. The implantation method of the nerve microelectrode is also reviewed.

Bioinspired elastomer composites with programmed mechanical and electrical anisotropies

Concepts that draw inspiration from soft biological tissues have enabled significant advances in creating artificial materials for a range of applications, such as dry adhesives, tissue engineering, biointegrated electronics, artificial muscles, and soft robots. Many biological tissues, represented by muscles, exhibit directionally dependent mechanical and electrical properties. However, equipping synthetic materials with tissue-like mechanical and electrical anisotropies remains challenging. Here, we present the bioinspired concepts, design principles, numerical modeling, and experimental demonstrations of soft elastomer composites with programmed mechanical and electrical anisotropies, as well as their integrations with active functionalities. Mechanically assembled, 3D structures of polyimide serve as skeletons to offer anisotropic, nonlinear mechanical properties, and crumpled conductive surfaces provide anisotropic electrical properties, which can be used to construct bioelectronic devices. Finite element analyses quantitatively capture the key aspects that govern mechanical anisotropies of elastomer composites, providing a powerful design tool. Incorporation of 3D skeletons of thermally responsive polycaprolactone into elastomer composites allows development of an active artificial material that can mimic adaptive mechanical behaviors of skeleton muscles at relaxation and contraction states. Furthermore, the fabrication process of anisotropic elastomer composites is compatible with dielectric elastomer actuators, indicating potential applications in humanoid artificial muscles and soft robots.

Engineering Stress in Thin Films: An Innovative Pathway Toward 3D Micro and Nanosystems

Transformation of conventional 2D platforms into unusual 3D configurations provides exciting opportunities for sensors, electronics, optical devices, and biological systems. Engineering material properties or controlling and modulating stresses in thin films to pop-up 3D structures out of standard planar surfaces has been a highly active research topic over the last decade. Implementation of 3D micro and nanoarchitectures enables unprecedented functionalities including multiplexed, monolithic mechanical sensors, vertical integration of electronics components, and recording of neuron activities in 3D organoids. This paper provides an overview on stress engineering approaches to developing 3D functional microsystems. The paper systematically presents the origin of stresses generated in thin films and methods to transform a 2D design into an out-of-plane configuration. Different types of 3D micro and nanostructures, along with their applications in several areas are discussed. The paper concludes with current technical challenges and potential approaches and applications of this fast-growing research direction.

Organic Bioelectronics for In Vitro Systems

Bioelectronics have made strides in improving clinical diagnostics and precision medicine. The potential of bioelectronics for bidirectional interfacing with biology through continuous, label-free monitoring on one side and precise control of biological activity on the other has extended their application scope to in vitro systems. The advent of microfluidics and the considerable advances in reliability and complexity of in vitro models promise to eventually significantly reduce or replace animal studies, currently the gold standard in drug discovery and toxicology testing. Bioelectronics are anticipated to play a major role in this transition offering a much needed technology to push forward the drug discovery paradigm. Organic electronic materials, notably conjugated polymers, having demonstrated technological maturity in fields such as solar cells and light emitting diodes given their outstanding characteristics and versatility in processing, are the obvious route forward for bioelectronics due to their biomimetic nature, among other merits. This review highlights the advances in conjugated polymers for interfacing with biological tissue in vitro, aiming ultimately to develop next generation in vitro systems. We showcase in vitro interfacing across multiple length scales, involving biological models of varying complexity, from cell components to complex 3D cell cultures. The state of the art, the possibilities, and the challenges of conjugated polymers toward clinical translation of in vitro systems are also discussed throughout.

Materials Chemistry of Neural Interface Technologies and Recent Advances in Three-Dimensional Systems

Advances in materials chemistry and engineering serve as the basis for multifunctional neural interfaces that span length scales from individual neurons to neural networks, neural tissues, and complete neural systems. Such technologies exploit electrical, electrochemical, optical, and/or pharmacological modalities in sensing and neuromodulation for fundamental studies in neuroscience research, with additional potential to serve as routes for monitoring and treating neurodegenerative diseases and for rehabilitating patients. This review summarizes the essential role of chemistry in this field of research, with an emphasis on recently published results and developing trends. The focus is on enabling materials in diverse device constructs, including their latest utilization in 3D bioelectronic frameworks formed by 3D printing, self-folding, and mechanically guided assembly. A concluding section highlights key challenges and future directions.

Research Progress of Microtransfer Printing Technology for Flexible Electronic Integrated Manufacturing

In recent years, with the rapid development of the flexible electronics industry, there is an urgent need for a large-area, multilayer, and high-production integrated manufacturing technology for scalable and flexible electronic products. To solve this technical demand, researchers have proposed and developed microtransfer printing technology, which picks up and prints inks in various material forms from the donor substrate to the target substrate, successfully realizing the integrated manufacturing of flexible electronic products. This review retrospects the representative research progress of microtransfer printing technology for the production of flexible electronic products and emphasizes the summary of seal materials, the basic principles of various transfer technology and fracture mechanics models, and the influence of different factors on the transfer effect. In the end, the unique functions, technical features, and related printing examples of each technology are concluded and compared, and the prospects of further research work on microtransfer printing technology is finally presented.

An Anti-Fatigue Design Strategy for 3D Ribbon-Shaped Flexible Electronics

Three-dimensional (3D) flexible electronics represent an emerging area of intensive attention in recent years, owing to their broad-ranging applications in wearable electronics, flexible robots, tissue/cell scaffolds, among others. The widely adopted 3D conductive mesostructures in the functional device systems would inevitably undergo repetitive out-of-plane compressions during practical operations, and thus, anti-fatigue design strategies are of great significance to improve the reliability of 3D flexible electronics. Previous studies mainly focused on the fatigue failure behavior of planar ribbon-shaped geometries, while anti-fatigue design strategies and predictive failure criteria addressing 3D ribbon-shaped mesostructures are still lacking. This work demonstrates an anti-fatigue strategy to significantly prolong the fatigue life of 3D ribbon-shaped flexible electronics by switching the metal-dominated failure to desired polymer-dominated failure. Combined in situ measurements and computational studies allow the establishment of a failure criterion capable of accurately predicting fatigue lives under out-of-plane compressions, thereby providing useful guidelines for the design of anti-fatigue mesostructures with diverse 3D geometries. Two mechanically reliable 3D devices, including a resistance-type vibration sensor and a janus sensor capable of decoupled temperature measurements, serve as two demonstrative examples to highlight potential applications in long-term health monitoring and human-like robotic perception, respectively.

Three dimensional bioelectronic interfaces to small-scale biological systems

Recent advances in bio-interface technologies establish a rich range of electronic, optoelectronic, thermal, and chemical options for probing and modulating the behaviors of small-scale three dimensional (3D) biological constructs (e.g. organoids, spheroids, and assembloids). These approaches represent qualitative advances over traditional alternatives due to their ability to extend broadly into volumetric spaces and/or to wrap tightly curved surfaces of natural or artificial tissues. Thin deformable sheets, filamentary penetrating pins, open mesh structures and 3D interconnected networks represent some of the most effective design strategies in this emerging field of bioelectronics. This review focuses on recent developments, with an emphasis on multimodal interfaces in the form of tissue-embedding scaffolds and tissue-surrounding frameworks.

Nano- and Microscale Optical and Electrical Biointerfaces and Their Relevance to Energy Research

Different research fields in energy sciences, such as photovoltaics for solar energy conversion, supercapacitors for energy storage, electrocatalysis for clean energy conversion technologies, and materials-bacterial hybrid for CO₂ fixation have been under intense investigations over the past decade. In recent years, new platforms for biointerface designs have emerged from the energy conversion and storage principles. This paper reviews recent advances in nano- and microscale materials/devices for optical and electrical biointerfaces. First, a connection is drawn between biointerfaces and energy science, and how these two distinct research fields can be connected is summarized. Then, a brief overview of current available tools for biointerface studies is presented. Third, three representative biointerfaces are reviewed, including neural, cardiac, and bacterial biointerfaces, to show how to apply these tools and principles to biointerface design and research. Finally, two possible future research directions for nano- and microscale biointerfaces are proposed.

Spatial light interference microscopy: principle and applications to biomedicine

In this paper, we review spatial light interference microscopy (SLIM), a common-path, phase-shifting interferometer, built onto a phase-contrast microscope, with white-light illumination. As one of the most sensitive quantitative phase imaging (QPI) methods, SLIM allows for speckle-free phase reconstruction with sub-nanometer path-length stability. We first review image formation in QPI, scattering, and full-field methods. Then, we outline SLIM imaging from theory and instrumentation to diffraction tomography. Zernike's phase-contrast microscopy, phase retrieval in SLIM, and halo removal algorithms are discussed. Next, we discuss the requirements for operation, with a focus on software developed in-house for SLIM that enables high-throughput acquisition, whole slide scanning, mosaic tile registration, and imaging with a color camera. We introduce two methods for solving the inverse problem using SLIM, white-light tomography, and Wolf phase tomography. Lastly, we review the applications of SLIM in basic science and clinical studies. SLIM can study cell dynamics, cell growth and proliferation, cell migration, mass transport, etc. In clinical settings, SLIM can assist with cancer studies, reproductive technology, blood testing, etc. Finally, we review an emerging trend, where SLIM imaging in conjunction with artificial intelligence brings computational specificity and, in turn, offers new solutions to outstanding challenges in cell biology and pathology.

Wireless sensors for continuous, multimodal measurements at the skin interface with lower limb prostheses

Precise form-fitting of prosthetic sockets is important for the comfort and well-being of persons with limb amputations. Capabilities for continuous monitoring of pressure and temperature at the skin-prosthesis interface can be valuable in the fitting process and in monitoring for the development of dangerous regions of increased pressure and temperature as limb volume changes during daily activities. Conventional pressure transducers and temperature sensors cannot provide comfortable, irritation-free measurements because of their relatively rigid construction and requirements for wired interfaces to external data acquisition hardware. Here, we introduce a millimeter-scale pressure sensor that adopts a soft, three-dimensional design that integrates into a thin, flexible battery-free, wireless platform with a built-in temperature sensor to allow operation in a noninvasive, imperceptible fashion directly at the skin-prosthesis interface. The sensor system mounts on the surface of the skin of the residual limb, in single or multiple locations of interest. A wireless reader module attached to the outside of the prosthetic socket wirelessly provides power to the sensor and wirelessly receives data from it, for continuous long-range transmission to a standard consumer electronic device such as a smartphone or tablet computer. Characterization of both the sensor and the system, together with theoretical analysis of the key responses, illustrates linear, accurate responses and the ability to address the entire range of relevant pressures and to capture skin temperature accurately, both in a continuous mode. Clinical application in two prosthesis users demonstrates the functionality and feasibility of this soft, wireless system.

Wearable and Implantable Soft Bioelectronics: Device Designs and Material Strategies

High-performance wearable and implantable devices capable of recording physiological signals and delivering appropriate therapeutics in real time are playing a pivotal role in revolutionizing personalized healthcare. However, the mechanical and biochemical mismatches between rigid, inorganic devices and soft, organic human tissues cause significant trouble, including skin irritation, tissue damage, compromised signal-to-noise ratios, and limited service time. As a result, profuse research efforts have been devoted to overcoming these issues by using flexible and stretchable device designs and soft materials. Here, we summarize recent representative research and technological advances for soft bioelectronics, including conformable and stretchable device designs, various types of soft electronic materials, and surface coating and treatment methods. We also highlight applications of these strategies to emerging soft wearable and implantable devices. We conclude with some current limitations and offer future prospects of this booming field.

Compliant 3D frameworks instrumented with strain sensors for characterization of millimeter-scale engineered muscle tissues

Tissue-on-chip systems represent promising platforms for monitoring and controlling tissue functions in vitro for various purposes in biomedical research. The two-dimensional (2D) layouts of these constructs constrain the types of interactions that can be studied and limit their relevance to three-dimensional (3D) tissues. The development of 3D electronic scaffolds and microphysiological devices with geometries and functions tailored to realistic 3D tissues has the potential to create important possibilities in advanced sensing and control. This study presents classes of compliant 3D frameworks that incorporate microscale strain sensors for high-sensitivity measurements of contractile forces of engineered optogenetic muscle tissue rings, supported by quantitative simulations. Compared with traditional approaches based on optical microscopy, these 3D mechanical frameworks and sensing systems can measure not only motions but also contractile forces with high accuracy and high temporal resolution. Results of active tension force measurements of engineered muscle rings under different stimulation conditions in long-term monitoring settings for over 5 wk and in response to various chemical and drug doses demonstrate the utility of such platforms in sensing and modulation of muscle and other tissues. Possibilities for applications range from drug screening and disease modeling to biohybrid robotic engineering.

Implanted Flexible Electronics: Set Device Lifetime with Smart Nanomaterials

Flexible electronics is one of the most attractive and anticipated markets in the internet-of-things era, covering a broad range of practical and industrial applications from displays and energy harvesting to health care devices. The mechanical flexibility, combined with high performance electronics, and integrated on a soft substrate offer unprecedented functionality for biomedical applications. This paper presents a brief snapshot on the materials of choice for niche flexible bio-implanted devices that address the requirements for both biodegradable and long-term operational streams. The paper also discusses potential future research directions in this rapidly growing field.

Kirigami Engineering-Nanoscale Structures Exhibiting a Range of Controllable 3D Configurations

Kirigami structures provide a promising approach to transform flat films into 3D complex structures that are difficult to achieve by conventional fabrication approaches. By designing the cutting geometry, it is shown that distinct buckling-induced out-of-plane configurations can be obtained, separated by a sharp transition characterized by a critical geometric dimension of the structures. In situ electron microscopy experiments reveal the effect of the ratio between the in-plane cut size and film thickness on out-of-plane configurations. Moreover, geometrically nonlinear finite element analyses (FEA) accurately predict the out-of-plane modes measured experimentally, their transition as a function of cut geometry, and provide the stress-strain response of the kirigami structures. The combined computational-experimental approach and results reported here represent a step forward in the characterization of thin films experiencing buckling-induced out-of-plane shape transformations and provide a path to control 3D configurations of micro- and nanoscale buckling-induced kirigami structures. The out-of-plane configurations promise great utility in the creation of micro- and nanoscale systems that can harness such structural behavior, such as optical scanning micromirrors, novel actuators, and nanorobotics. This work is of particular significance as the kirigami dimensions approach the sub-micrometer scale which is challenging to achieve with conventional micro-electromechanical system technologies.

Wearable Sweat Biosensors Refresh Personalized Health/Medical Diagnostics

Sweat contains a broad range of critical biomarkers including ions, small molecules, and macromolecules that may indirectly or directly reflect the health status of the human body and thereby help track disease progression. Wearable sweat biosensors enable the collection and analysis of sweat in situ, achieving real-time, continuous, and noninvasive monitoring of human biochemical parameters at the molecular level. This review summarizes the physiological/pathological information of sweat and wearable sweat biosensors. First, the production of sweat pertaining to various electrolytes, metabolites, and proteins is described. Then, the compositions of the wearable sweat biosensors are summarized, and the design of each subsystem is introduced in detail. The latest applications of wearable sweat biosensors for outdoor, hospital, and family monitoring are highlighted. Finally, the review provides a summary and an outlook on the future developments and challenges of wearable sweat biosensors with the aim of advancing the field of wearable sweat monitoring technology.

3D Interfacing between Soft Electronic Tools and Complex Biological Tissues

Recent developments in soft functional materials have created opportunities for building bioelectronic devices with tissue-like mechanical properties. Their integration with the human body could enable advanced sensing and stimulation for medical diagnosis and therapies. However, most of the available soft electronics are constructed as planar sheets, which are difficult to interface with the target organs and tissues that have complex 3D structures. Here, the recent approaches are highlighted to building 3D interfaces between soft electronic tools and complex biological organs and tissues. Examples involve mesh devices for conformal contact, imaging-guided fabrication of organ-specific electronics, miniaturized probes for neurointerfaces, instrumented scaffold for tissue engineering, and many other soft 3D systems. They represent diverse routes for reconciling the interfacial mismatches between electronic tools and biological tissues. The remaining challenges include device scaling to approach the complexity of target organs, biological data acquisition and processing, 3D manufacturing techniques, etc., providing a range of opportunities for scientific research and technological innovation.

Bioinspired flexible electronics for seamless neural interfacing and chronic recording

Implantable neural probes are among the most widely applied tools for the understanding of neural circuit functions and the treatment of neurological disorders. Despite remarkable progress in recent years, it is still challenging for conventional rigid probes to achieve stable neural recording over long periods of time. Recently, flexible electronics with biomimetic structures and mechanical properties have been demonstrated for the formation of seamless probe-neural interfaces, enabling long-term recording stability. In this review, we provide an overview of bioinspired flexible electronics, from their structural design to probe-brain interfaces and chronic neural recording applications. Opportunities of bioinspired flexible electronics in fundamental neuroscience and clinical studies are also discussed.

Advances in Sweat Wearables: Sample Extraction, Real-Time Biosensing, and Flexible Platforms

Wearable biosensors for sweat-based analysis are gaining wide attention due to their potential use in personal health monitoring. Flexible wearable devices enable sweat analysis at the molecular level, facilitating noninvasive monitoring of physiological states via real-time monitoring of chemical biomarkers. Advances in sweat extraction technology, real-time biosensors, stretchable materials, device integration, and wireless digital technologies have led to the development of wearable sweat-biosensing devices that are light, flexible, comfortable, aesthetic, affordable, and informative. Herein, we summarize recent advances of sweat wearables from the aspects of sweat extraction, fabrication of stretchable biomaterials, and design of biosensing modules to enable continuous biochemical monitoring, which are essential for a biosensing device. Key chemical components of sweat, sweat capture methodologies, and considerations of flexible substrates for integrating real-time biosensors with electronics to bring innovations in the art of wearables are elaborated. The strategies and challenges involved in improving the wearable biosensing performance and the perspectives for designing sweat-based wearable biosensing devices are discussed.

Graphene Hybrid Structures for Integrated and Flexible Optoelectronics

Graphene (Gr) has many unique properties including gapless band structure, ultrafast carrier dynamics, high carrier mobility, and flexibility, making it appealing for ultrafast, broadband, and flexible optoelectronics. To overcome its intrinsic limit of low absorption, hybrid structures are exploited to improve the device performance. Particularly, van der Waals heterostructures with different photosensitive materials and photonic structures are very effective for improving photodetection and modulation efficiency. With such hybrid structures, Gr hybrid photodetectors can operate from ultraviolet to terahertz, with significantly improved R (up to 10⁹ A W^-1 ) and bandwidth (up to 128 GHz). Furthermore, integration of Gr with silicon (Si) complementary metal-oxide-semiconductor (CMOS) circuits, the human body, and soft tissues is successfully demonstrated, opening promising opportunities for wearable sensors and biomedical electronics. Here, the recent progress in using Gr hybrid structures toward high-performance photodetectors and integrated optoelectronic applications is reviewed.

Materials for flexible bioelectronic systems as chronic neural interfaces

Engineered systems that can serve as chronically stable, high-performance electronic recording and stimulation interfaces to the brain and other parts of the nervous system, with cellular-level resolution across macroscopic areas, are of broad interest to the neuroscience and biomedical communities. Challenges remain in the development of biocompatible materials and the design of flexible implants for these purposes, where ulimate goals are for performance attributes approaching those of conventional wafer-based technologies and for operational timescales reaching the human lifespan. This Review summarizes recent advances in this field, with emphasis on active and passive constituent materials, design architectures and integration methods that support necessary levels of biocompatibility, electronic functionality, long-term stable operation in biofluids and reliability for use in vivo. Bioelectronic systems that enable multiplexed electrophysiological mapping across large areas at high spatiotemporal resolution are surveyed, with a particular focus on those with proven chronic stability in live animal models and scalability to thousands of channels over human-brain-scale dimensions. Research in materials science will continue to underpin progress in this field of study.

3D Self-Assembled Microelectronic Devices: Concepts, Materials, Applications

Modern microelectronic systems and their components are essentially 3D devices that have become smaller and lighter in order to improve performance and reduce costs. To maintain this trend, novel materials and technologies are required that provide more structural freedom in 3D over conventional microelectronics, as well as easier parallel fabrication routes while maintaining compatability with existing manufacturing methods. Self-assembly of initially planar membranes into complex 3D architectures offers a wealth of opportunities to accommodate thin-film microelectronic functionalities in devices and systems possessing improved performance and higher integration density. Existing work in this field, with a focus on components constructed from 3D self-assembly, is reviewed, and an outlook on their application potential in tomorrow's microelectronics world is provided.

Harmonically decoupled gradient light interference microscopy (HD-GLIM)

Differential phase sensitive methods, such as Nomarski microscopy, play an important role in quantitative phase imaging due to their compatibility with partially coherent illumination and excellent optical sectioning ability. In this Letter, we propose a new system, to the best of our knowledge, to retrieve differential phase information from transparent samples. It is based on a 4f optical system with an amplitude-type spatial light modulator (SLM), which removes the need for traditional differential interference contrast (DIC) optics and specialized phase-only SLMs. We demonstrate the principle of harmonically decoupled gradient light interference microscopy using standard samples, as well as static and dynamic biospecimens.

Engineering Smart Hybrid Tissues with Built-In Electronics

One of the major hurdles faced in tissue engineering is the inability to monitor and control the function of an engineered tissue following transplantation. Recent years have seen major developments in the field by integrating electronics within engineered tissues. Previously, the most common types of devices integrated into the body used to be pacemakers and deep brain stimulation electrodes that are stiff and non-compliant; the advent of ultra-thin and flexible electronics has brought forth a significant expansion of the field. Recent developments have enabled interfacing electronics onto, into, and within all tissues and organs with minimal adverse reactions. These have introduced the ability to engineer tissues with built-in electronics that allow for remote monitoring and regulation of tissue function. In this review, we discuss the development of technologies that allowed for the formation of tissue-electronics hybrids and give an overview of the existing examples of these hybrid "cyborg" tissues.

Reprogrammable 3D Shaping from Phase Change Microstructures in Elastic Composites

Herein, we demonstrate reprogrammable 3D structures that are assembled from elastic composite sheets made from elastic materials and phase change microparticles. By controlling the phase change of the microparticles by localized thermal patterning, anisotropic residual strain is generated in the pre-stretched composite sheets and then triggers 3D structure assembly when the composite sheets are released from the external stress. Modulation of the geometries and location of the thermal patterns leads to complex 2D-3D shaping behaviors such as bending, folding, buckling, and wrinkling. Because of the reversible phase change of the microparticles, these programmed 3D structures can later be recovered to 2D sheets once they are heated for reprogramming different 3D structures. To predict the 3D structures assembled from the 2D composite sheets, finite element modeling was employed, which showed reasonable agreement with the experiments. The demonstrated strategy of reversibly programming 3D shapes by controlling the phase change microstructures in the elastic composites offers unique capabilities in fabricating functional devices such as a rewritable "paper" and a shape reconfigurable pneumatic actuator.

Archimedean lattices emerge in template-directed eutectic solidification

Template-directed assembly has been shown to yield a broad diversity of highly ordered mesostructures^1,2, which in a few cases exhibit symmetries not present in the native material^3-5. However, this technique has not yet been applied to eutectic materials, which underpin many modern technologies ranging from high-performance turbine blades to solder alloys. Here we use directional solidification of a simple AgCl-KCl lamellar eutectic material within a pillar template to show that interactions of the material with the template lead to the emergence of a set of microstructures that are distinct from the eutectic's native lamellar structure and the template's hexagonal lattice structure. By modifying the solidification rate of this material-template system, trefoil, quatrefoil, cinquefoil and hexafoil mesostructures with submicrometre-size features are realized. Phase-field simulations suggest that these mesostructures appear owing to constraints imposed on diffusion by the hexagonally arrayed pillar template. We note that the trefoil and hexafoil patterns resemble Archimedean honeycomb and square-hexagonal-dodecagonal lattices⁶, respectively. We also find that by using monolayer colloidal crystals as templates, a variety of eutectic mesostructures including trefoil and hexafoil are observed, the former resembling the Archimedean kagome lattice. Potential emerging applications for the structures provided by templated eutectics include non-reciprocal metasurfaces⁷, magnetic spin-ice systems^8,9, and micro- and nano-lattices with enhanced mechanical properties^10,11.

Stimulus Responsive 3D Assembly for Spatially Resolved Bifunctional Sensors

3D electronic/optoelectronic devices have shown great potentials for various applications due to their unique properties inherited not only from functional materials, but also from 3D architectures. Although a variety of fabrication methods including mechanically guided assembly have been reported, the resulting 3D devices show no stimuli-responsive functions or are not free standing, thereby limiting their applications. Herein, the stimulus responsive assembly of complex 3D structures driven by temperature-responsive hydrogels is demonstrated for applications in 3D multifunctional sensors. The assembly driving force, compressive buckling, arises from the volume shrinkage of the responsive hydrogel substrates when they are heated above the lower critical solution temperature. Driven by the compressive buckling force, the 2D-formed membrane materials, which are pre-defined and selectively bonded to the substrates, are then assembled to 3D structures. They include "tent," "tower," "two-floor pavilion," "dome," "basket," and "nested-cages" with delicate geometries. Moreover, the demonstrated 3D bifunctional sensors based on laser induced graphene show capability of spatially resolved tactile sensing and temperature sensing. These multifunctional 3D sensors would open new applications in soft robotics, bioelectronics, micro-electromechanical systems, and others.

3D-printed automation for optimized PET radiochemistry

Reproducible batch synthesis of radioligands for imaging by positron emission tomography (PET) in a manner that maximizes ligand yield, purity, and molar activity, and minimizes cost and exposure to radiation, remains a challenge, as new and synthetically complex radioligands become available. Commercially available automated synthesis units (ASUs) solve many of these challenges but are costly to install and cannot always accommodate diverse chemistries. Through a reiterative design process, we exploit the proliferation of three-dimensional (3D) printing technologies to translate optimized reaction conditions into ASUs composed of 3D-printed, electronic, and robotic parts. Our units are portable and robust and reduce radiation exposure, shorten synthesis time, and improve the yield of the final radiopharmaceutical for a fraction of the cost of a commercial ASU. These 3D-printed ASUs highlight the gains that can be made by designing a fit-for-purpose ASU to accommodate a synthesis over accommodating a synthesis to an unfit ASU.

Integration of biological systems with electronic-mechanical assemblies

Biological systems continuously interact with the surrounding environment because they are dynamically evolving. The interaction is achieved through mechanical, electrical, chemical, biological, thermal, optical, or a synergistic combination of these cues. To provide a fundamental understanding of the interaction, recent efforts that integrate biological systems with the electronic-mechanical assemblies create unique opportunities for simultaneous monitoring and eliciting the responses to the biological system. Recent innovations in materials, fabrication processes, and device integration approaches have created the enablers to yield bio-integrated devices to interface with the biological system, ranging from cells and tissues to organs and living individual. In this short review, we will provide a brief overview of the recent development on the integration of the biological systems with electronic-mechanical assemblies across multiple scales, with applications ranging from healthcare monitoring to therapeutic options such as drug delivery and rehabilitation therapies. STATEMENT OF SIGNIFICANCE: An overview of the recent progress on the integration of the biological system with both electronic and mechanical assemblies is discussed. The integration creates the unique opportunity to simultaneously monitor and elicit the responses to the biological system, which provides a fundamental understanding of the interaction between the biological system and the electronic-mechanical assemblies. Recent innovations in materials, fabrication processes, and device integration approaches have created the enablers to yield bio-integrated devices to interface with the biological system, ranging from cells and tissues to organs and living individual.

Micro/Nanoscale 3D Assembly by Rolling, Folding, Curving, and Buckling Approaches

The miniaturization of electronics has been an important topic of study for several decades. The established roadmaps following Moore's Law have encountered bottlenecks in recent years, as planar processing techniques are already close to their physical limits. To bypass some of the intrinsic challenges of planar technologies, more and more efforts have been devoted to the development of 3D electronics, through either direct 3D fabrication or indirect 3D assembly. Recent research efforts into direct 3D fabrication have focused on the development of 3D transistor technologies and 3D heterogeneous integration schemes, but these technologies are typically constrained by the accessible range of sophisticated 3D geometries and the complexity of the fabrication processes. As an alternative route, 3D assembly methods make full use of mature planar technologies to form predefined 2D precursor structures in the desired materials and sizes, which are then transformed into targeted 3D mesostructures by mechanical deformation. The latest progress in the area of micro/nanoscale 3D assembly, covering the various classes of methods through rolling, folding, curving, and buckling assembly, is discussed, focusing on the design concepts, principles, and applications of different methods, followed by an outlook on the remaining challenges and open opportunities.

Deterministic Nanoassembly of Quasi-Three-Dimensional Plasmonic Nanoarrays with Arbitrary Substrate Materials and Structures

Guided manipulation of light through periodic nanoarrays of three-dimensional (3D) metal-dielectric patterns provides remarkable opportunities to harness light in a way that cannot be obtained with conventional optics yet its practical implementation remains hindered by a lack of effective methodology. Here we report a novel 3D nanoassembly method that enables deterministic integration of quasi-3D plasmonic nanoarrays with a foreign substrate composed of arbitrary materials and structures. This method is versatile to arrange a variety of types of metal-dielectric composite nanoarrays in lateral and vertical configurations, providing a route to generate heterogeneous material compositions, complex device layouts, and tailored functionalities. Experimental, computational, and theoretical studies reveal the essential design features of this approach and, taken together with implementation of automated equipment, provide a technical guidance for large-scale manufacturability. Pilot assembly of specifically engineered quasi-3D plasmonic nanoarrays with a model hybrid pixel detector for deterministic enhancement of the detection performances demonstrates the utility of this method.

Nanowired Bioelectric Interfaces

Biological systems have evolved biochemical, electrical, mechanical, and genetic networks to perform essential functions across various length and time scales. High-aspect-ratio biological nanowires, such as bacterial pili and neurites, mediate many of the interactions and homeostasis in and between these networks. Synthetic materials designed to mimic the structure of biological nanowires could also incorporate similar functional properties, and exploiting this structure-function relationship has already proved fruitful in designing biointerfaces. Semiconductor nanowires are a particularly promising class of synthetic nanowires for biointerfaces, given (1) their unique optical and electronic properties and (2) their high degree of synthetic control and versatility. These characteristics enable fabrication of a variety of electronic and photonic nanowire devices, allowing for the formation of well-defined, functional bioelectric interfaces at the biomolecular level to the whole-organ level. In this Focus Review, we first discuss the history of bioelectric interfaces with semiconductor nanowires. We next highlight several important, endogenous biological nanowires and use these as a framework to categorize semiconductor nanowire-based biointerfaces. Within this framework we then review the fundamentals of bioelectric interfaces with semiconductor nanowires and comment on both material choice and device design to form biointerfaces spanning multiple length scales. We conclude with a discussion of areas with the potential for greatest impact using semiconductor nanowire-enabled biointerfaces in the future.

Harnessing the interface mechanics of hard films and soft substrates for 3D assembly by controlled buckling

Techniques for forming sophisticated, 3D mesostructures in advanced, functional materials are of rapidly growing interest, owing to their potential uses across a broad range of fundamental and applied areas of application. Recently developed approaches to 3D assembly that rely on controlled buckling mechanics serve as versatile routes to 3D mesostructures in a diverse range of high-quality materials and length scales of relevance for 3D microsystems with unusual function and/or enhanced performance. Nonlinear buckling and delamination behaviors in materials that combine both weak and strong interfaces are foundational to the assembly process, but they can be difficult to control, especially for complex geometries. This paper presents theoretical and experimental studies of the fundamental aspects of adhesion and delamination in this context. By quantifying the effects of various essential parameters on these processes, we establish general design diagrams for different material systems, taking into account 4 dominant delamination states (wrinkling, partial delamination of the weak interface, full delamination of the weak interface, and partial delamination of the strong interface). These diagrams provide guidelines for the selection of engineering parameters that avoid interface-related failure, as demonstrated by a series of examples in 3D helical mesostructures and mesostructures that are reconfigurable based on the control of loading-path trajectories. Three-dimensional micromechanical resonators with frequencies that can be selected between 2 distinct values serve as demonstrative examples.

Flexible electronic/optoelectronic microsystems with scalable designs for chronic biointegration

Flexible biocompatible electronic systems that leverage key materials and manufacturing techniques associated with the consumer electronics industry have potential for broad applications in biomedicine and biological research. This study reports scalable approaches to technologies of this type, where thin microscale device components integrate onto flexible polymer substrates in interconnected arrays to provide multimodal, high performance operational capabilities as intimately coupled biointerfaces. Specificially, the material options and engineering schemes summarized here serve as foundations for diverse, heterogeneously integrated systems. Scaled examples incorporate >32,000 silicon microdie and inorganic microscale light-emitting diodes derived from wafer sources distributed at variable pitch spacings and fill factors across large areas on polymer films, at full organ-scale dimensions such as human brain, over ∼150 cm² In vitro studies and accelerated testing in simulated biofluids, together with theoretical simulations of underlying processes, yield quantitative insights into the key materials aspects. The results suggest an ability of these systems to operate in a biologically safe, stable fashion with projected lifetimes of several decades without leakage currents or reductions in performance. The versatility of these combined concepts suggests applicability to many classes of biointegrated semiconductor devices.