Stretchable Bioelectronics for Medical Devices and Systems

Liquid metals are the softest and most deformable class of electrical conductors. They are intrinsically stretchable and can be embedded in elastomeric or gel matrices without altering the mechanical properties of the resulting composite. These composites can maintain metallic electrical conductivity at extreme strains and can form soft, conformal contacts with surfaces. Gallium and several of its alloys, which are liquid metals at or near room temperature, offer a low toxicity alternative to mercury. These metals have negligible vapor pressure (so they do not evaporate) and low viscosity. The surface of these metals reacts rapidly with air to form a thin surface oxide ‘skin’ that allows these liquids to be patterned despite their large surface tension. For example, liquid metal can be 3D printed, molded, or injected into microchannels. This chapter summarizes the properties, patterning methods, and applications of these remarkable materials to form devices with extremely soft mechanical properties. Liquid metals may be used, for example, as conductors for hyper-elastic wires, stretchable antennas, optical structures, conformal electrodes, deformable interconnects, self-healing wires, components in microsystems, reconfigurable circuit elements, and soft circuit boards. They can also be integrated as functional components in circuits composed entirely of soft materials such as sensors, capacitors, memory devices, and diodes. Research is just beginning to explore ways to utilize these ‘softer than skin’ materials for biolectronic applications. This chapter summarizes the properties, patterning methods, and applications of liquid metals and concludes with an outlook and future challenges of these materials within this context.

Epidermal sensors and electronics represent a class of artificial devices whose thickness, mass density, and mechanical stiffness are well-matched with human epidermis. They can be applied as temporary transfer tattoos on the surface of any part of human body for physiological measurements, electrical or thermal stimulation, as well as wireless communications. Except for comfort and wearability, epidermal sensors can offer unprecedented signal quality even under severe skin deformation. This chapter tries to address two fundamental mechanics challenges for epidermal sensors: first, how to predict and improve the stretchability and compliance when epidermal devices are made out of intrinsically brittle and rigid inorganic electronic materials; and second, when laminating on human skin, how to predict and improve the conformability between epidermal devices and the microscopically rough skin surfaces. Since the ideal use of epidermal devices would be one-time, disposable patches, a low cost, high throughput manufacture process called the “cut-and-paste” method is introduced at the end of this chapter.

This chapter reviews mechanics-guided designs that enable highly deformable forms of bioelectronics, through soft, conformal integration of hard functional components with soft elastomeric substrates. Three representative strategies, including wavy, wrinkled design, island-bridge design, and Origami/Kirigami-inspired design, are summarized, highlighting the key design concepts, unique mechanical behaviors, and analytical/computational mechanics models that guide the design optimization. Finally, some perspectives are provided on the remaining challenges and opportunities.

The development of ultra-compliant power sources is prerequisite to the realization of imperceptible biomedical systems destined to be worn or implanted in the human body. This chapter assesses the viability of conformal piezo and triboelectric, thermoelectric, and photovoltaic technologies as power sources for biomedical applications. It begins by identifying the amount of energy available to each of these modes of power conversion and then gives a brief overview on the methods of fabricating stretchable electronic devices using deterministic structures, random composites, or molecularly stretchable electronic materials. It then provides a detailed description of innovations in “soft power,” where the mentioned design techniques have been employed to develop mechanically compliant power scavengers amenable to integration with stretchable medical devices. The chapter concludes with an analysis of system level power requirements and application specific compatibility, the result of which identifies piezoelectrics and triboelectrics as well suited for intermittent and implantable devices, such as low-power pacemakers for piezoelectrics or higher power wearables and neural stimulators for triboelectrics. Thermoelectrics are highly compatible with epidermal and wearable applications, and can be used as a consistent source of power for tattoo chemical or heat sensors, and photovoltaics can generate large amounts of power in full sun, for high power applications like cochlear implants, or less energy in diffuse or ambient light, for powering hearing aids.

Conformal bioelectronics in flexible or stretchable format that make direct contact to the skin or tissues have contributed extensively to diverse clinical applications. Wireless modules in such minimally invasive forms have developed in parallel to extend the capabilities and to improve the quality of such bioelectronics, in assurances to offer safer and more convenient clinical practice. Such remote capabilities are facilitating significant advances in clinical medicine, by removing bulky energy storage devices and tangled electrical wires, and by offering cost-effective and continuous monitoring of the patients. This chapter provides a snapshot of current developments and challenges of wireless conformal bioelectronics with various examples of applications utilizing either wireless powering or communication system. The chapter begins with near-field wirelessly powered therapeutic devices owing to the simplicity of power transfer mechanism followed by far-field powering systems which require integration of numerous electrical components. In the later sections of the chapter, sensors in conformal format that transfer clinical data wirelessly are discussed and ends by reviewing the developments of wireless bioelectronics that utilize integrated circuits for advanced capabilities in clinical applications.

Precision thermal measurements of skin and soft tissue can provide clinically relevant information about cardiovascular health, cognitive state, hydration levels, heterogeneousvasculature changes, and many other important aspects of human physiology. In this chapter we discuss recent advances in ultrathin, compliant skin-like sensor/actuator technologies that enable forms of continuous thermal mapping, of temperature as well as transport properties, that are unavailable with other methods. We review the key mechanical and thermal properties that are fundamental to the operation of this class of devices. Further discussion of devices configured for mapping temperature, monitoring local thermal transport and skin hydration, and mapping thermal transport for blood flow analysis provides a few examples of the types of capabilities that are enabled with these technologies.

We will review the recent progresses of large-area, ultraflexible, and ultrasoft electronic sensors. This chapter focuses on integration technologies of thin-film, ultraflexible electronics comprising ultrasoft gel electrodes, thin-film amplifier, Si-LSI wireless platform, thin-film battery, and information engineering, which are imperceptible active sensors. Here we would like to demonstrate the applications of the patch-type wearable biosignal sensors including brain wave (Electroencephalogram: EEG) monitoring from a forehead. Furthermore, this chapter will review the technologies for realizing the new era of electronic system, that is, Internet of Things (IoT.)

As nanotechnology has advanced, deformable nanoscale materials with superb electrical, chemical, and optical properties have made possible the development of high-performance multifunctional electronic devices with flexible and stretchable form factors. Deformability in electronics is achieved mainly by replacing rigid bulk materials (e.g., a silicon wafer) with various promising nanomaterials (e.g., silicon/oxide nanomembranes, carbon nanotubes, graphene, and metal nanoparticles/nanowires). These ultrathin, lightweight, and deformable electronics have attracted widespread interest and offer new opportunities in personalized healthcare, such as wearable bioelectronics. Their deformability, in particular, helps overcome the mechanical mismatch between the conventional bioelectronics, which are flat and rigid, and the soft, curvilinear human skin and internal organs. It resolves prevalent problems in conventional biomedical devices, such as inaccurate biosignal sensing, low signal-to-noise ratio, and user discomfort. Here, we provide an overview of recent developments in wearable bioelectronics integrated with functional nanomaterials with a focus on mobile personal healthcare technologies. The devices introduced in this chapter include wearable sensors, actuators, memory units, and nanogenerators dedicated to healthcare applications. Detailed descriptions of such integrated systems and their uses in clinical medicine are also presented.

Sensor skins can be broadly defined as distributed sensors over a surface to provide proprioceptive, tactile, and environmental feedback. This chapter focuses on sensors and sensor networks that can achieve strains on the same order as elastomers and human skin, which makes these sensors compatible with emerging wearable technologies. A combination of material choices, processing limitations, and design must be considered in order to achieve multimodal, biocompatible sensor skins capable of operating on objects and bodies with complex geometries and dynamic functionalities. This chapter overviews the commonly used materials, fabrication techniques, structures and designs of stretchable sensor skins, and also highlights the current challenges and future opportunities of such sensors.

Wearable sensors have the potential to enable longitudinal, objective health monitoring in patients with chronic diseases, including cardiac rhythm disorders, neurological and movement disorders, diabetes, and pain. However, conventional wearable devices are typically comprised of rigid, packaged electronics, which may compromise overall signal fidelity and wearer comfort during activities of daily living and sleep. In this chapter, we present recent advances in the development of thin and stretchable epidermal systems for biometric data measurements. These non-invasive epidermal systems are fully integrated with multiple sensors, an analog front end module, a radio for wireless communication, onboard flash memory, a rechargeable battery all encapsulated in a soft, stretchable and water-resistant silicone, and with an air permeable adhesive layer that interfaces with the human skin.  The encapsulated system intimately couples with the skin at multiple locations on the body. We present results showing the potential of this technology to quantitatively assess bio-kinematics and electrophysiological signals. Finally, we provide perspectives on remaining challenges and opportunities to achieve clinical validation and commercial adoption of these technologies.

Laser-enabled fabrication methods, in particular laser surface modification of low-cost materials such as paper, is an attractive technology for fabrication of flexible sensors and microsystems. Such devices are uniquely suited for cutaneous wound interfaces in which one has to sense multiple parameters and deliver drugs using a disposable low-cost platform. In this chapter, we discuss our recent efforts towards using laboratory scale CO₂ lasers to modify commercial hydrophobic papers (e.g., parchment paper, wax paper, palette paper, etc.) and thermoset polymers (e.g., polyimide) by controlled surface ablation. Such treatment imparts unique physical and chemical properties (hydrophilicity, extreme porosity, carbonization, etc.) to the material and allows for selective surface functionalization. Using this method, we fabricated a variety of sensors (pH, oxygen, silver, strain) and chemical delivery (oxygen) modules on low-cost commercial substrates for chronic wound management.

Long-term continuous monitoring of body condition from the skin has been one of the critical issues in the ubiquitous healthcare. For this purpose, skin-like stretchable and flexible electrodes have been highly required and diverse electrodes have been developed. However, these electrodes have limits such as lower electrical property, biocompatibility, and discomfort to patients. To address these challenges, nanomaterial-based electronic devices have been developed. In this chapter, current status of nanomaterial-based skin-like electronics with mechanical properties comparable to those of skin is reviewed, and their applications in biomedical fields are described. The types of clinically significant biosignals that can be measured from skin using soft electrodes are briefly summarized. The requirements of electrode for long-term, continuous, and unconscious measurement of these biosignals are also briefly described. Among several nanomaterials for soft electronics, carbon nanotube (CNT), graphene, and metallic nanowire are mainly commented and diverse flexible and stretchable electrodes using nanomaterials and their fabrication methods were described. For the biomedical applications, safety for the human use is a critical requirement, and their biocompatibility, future research directions, and possible additional applications in various fields are assessed.

Neural interfaces are engineered devices that aim at replacing, restoring, and rehabilitating the injured or damaged nervous system. One of the challenges to overcome to deploy therapeutic neural interfaces as clinical treatments lies in the physical mismatch between biological tissues and artificial engineered devices. This chapter details recent development in materials science and technology focused on reducing this physical mismatch thereby opening the path for long-term biointegrated neural interfaces.

Neural interface plays an important role in monitoring and modulating brain activity. In order to study the neural information processing in vitro, microelectrode array (MEA) platform is used with cell culture or brain slice. To measure neural signals simultaneously from multiple cells for long-term period, extracellular neural recording technique is preferred and subcellular-scale microelectrodes, dense array, and flexible substrates are ideal. In this chapter, we will introduce the state-of-the-art in vitro neural recording technology based on microfabricated electrodes or transistors. MEAs with metal-type microelectrodes are passive types, and MEAs with active electronic components (field-effect transistors or integrated circuits) are active types. The motivation, operation principles, fabrication processes and materials, and current trends are reviewed.

The use of electrophysiology (EP) signals is the most relevant way to reflect biological activities in cells and tissues. In neuroscience, EP signals are standard indicators enable to display neural activities as action potentials. The action potentials are typically measured by the change of voltage or current from ion channels in the neurons. Usually, conductive electrodes formed on injectable probes that can be penetrated into deep brain tissue for recording EP signals. Over the last few decades, neural probes have been developed using microfabrication technology. Many researchers have attempted to develop and optimize various materials and designs of electrodes and neural probes to effectively minimize their invasive geometry with biocompatible materials. Compared to the rigid and non-flexible neural probes presented in the late 1980s, the shape of deformable neural probes, reported in the late 1990s, has many advantages. A multimodal function (i.e. electric recording with light or drug delivery) for optogenetics technique has also recently been developed as the next generation flexible neural probe. In this chapter, we deal with several examples of flexible neural probes (FNP) in terms of their geometry, materials, and functions. This study will facilitate a new paradigm for less invasive and more flexible multimodal neural probes that can be utilized in many research fields such as materials science, electrical engineering, and fundamental neuroscience.