Miniature battery-free bioelectronics

Miniature wireless bioelectronic implants that can operate for extended periods of time can transform how we treat disorders by acting rapidly on precise nerves and organs in a way that drugs cannot. To reach this goal, materials and methods are needed to wirelessly transfer energy through the body or harvest energy from the body itself. We review some of the capabilities of emerging energy transfer methods to identify the performance envelope for existing technology and discover where opportunities lie to improve how much—and how efficiently—we can deliver energy to the tiny bioelectronic implants that can support emerging medical technologies. Description Editor’s summary Bioelectronic devices that can sense or manipulate biological signals, such as pacemakers that can detect and regulate irregular cardiac cycles, can dramatically improve the health and lifestyle of the patients who use them. However, these devices are often limited by the storage capacity of an onboard battery or are tethered to wires that can cause infection. Nair et al. reviewed advances in developing alternate methods to generate, transmit, and store electrical charge that enable wireless power transfer and energy harvesting safely through and within the human body. In addition to removing power limitations, these advances often also enable communication from a device or possibly even between devices. —Marc S. Lavine A review describes methods to enable wireless power transfer and energy harvesting through and within the human body. BACKGROUND Bioelectronics, electronic devices that sense or manipulate biological signals, can substantially improve medical outcomes by modulating the activity of excitable cells and tissues in the nervous system, cardiovascular system, and beyond. This specific stimulation and monitoring can reduce side effects and adapt in real time to the needs of each patient. However, despite the potential for bioelectronics to treat numerous conditions, only few implanted bioelectronic devices are currently the standard of care. These include cardiac pacemakers that use electrical impulses to synchronize the contraction of heart cells and continuous glucose monitors that generate an electronic alert when a user needs insulin. Large battery packs, cumbersome tethers, and intricate packaging architectures complicate device design and present numerous possible failure modes, which limits the widespread deployment of therapeutic implantable bioelectronic devices. Wires can break and provide a pathway for infection. Furthermore, the need to route wires and leads limits the number and location of bioelectronic sensing and stimulating sites. Battery packs also create challenges for wearable bioelectronics such as heart rate monitors or pulse oximeters because the added size and weight of batteries limit the overall comfort, appearance, and ease of use. Bioelectronic devices that operate without batteries could be miniaturized to millimeter-scale dimensions and easily implanted or comfortably worn. This approach would allow distributed networks of sensors and actuators to measure and manipulate physiological activity throughout the body to enable precise and adaptive bioelectronic therapies with minimal risks or interference with daily activities. To reach this level of miniaturization, bioelectronics must receive power from an external transmitter or harvest energy from the body itself. Unfortunately, traditional methods of transmitting wireless power over long distances rely on electromagnetic waves that are absorbed or reflected by the body. To overcome this challenge, recent innovations in wireless power transfer and energy-harvesting technologies have been leveraged to support the power requirements of bioelectronic devices. These include ultrasound, electromagnetic, magnetic, and optical methods to deliver power and data safely through the body, as well as energy-harvesting methods that extract chemical, kinetic, and thermal energy from the body itself. These emerging materials and technologies for data and power transfer are allowing bioelectronic devices to be miniaturized with tiny rechargeable batteries or, in the extreme case, made entirely battery free. ADVANCES We review progress in the materials and methods that are enabling wireless power transfer and energy harvesting safely through and within the human body. These methods must contend with the fact that many forms of energy are absorbed or reflected by bone and soft tissues, placing limits on the safety and efficiency of traditional power and data transmission technologies. To overcome these challenges, piezoelectric, photovoltaic, and magnetoelectric materials are being evaluated for use in nontraditional energy transfer techniques because they have unique advantages in the context of power transfer through the human body. Similarly, materials such as triboelectrics, magnetoelastics, and thermoelectrics are being explored to harvest energy from the human body. Furthermore, the emerging methods for power transfer through the human body are also being explored as a means for efficient data communication channels that will support wireless networks or miniature implantable bioelectronics. OUTLOOK The extreme miniaturization and long functional lifetime that are enabled by battery-free technologies point toward a future in which networks of tiny bioelectronics could be distributed throughout the body to accurately sense physiological states and apply therapies when and where they are needed. Reaching this vision will require additional scientific advances in manufacturing and packaging of the materials that support this wireless bioelectronic network. For example, to reach submillimeter scale, alternatives to traditional hermetic seals must be explored to protect these electronics from the harsh environment of the human body. In addition to typical material biocompatibility, implantable bioelectronics will have a longer operating lifespan if there is little to no foreign body tissue response and scar formation to degrade the recording or stimulation efficacy. Advances in nanomaterials, soft electronics, and coatings with growth factors, anti-inflammatory drugs, and peptides are underway, but ultimately these technologies need more in vivo testing throughout different regions of the body. This is especially critical if we are to propel bioelectronic devices into true distributed networks inside and nearby different organs and tissues throughout the body, each with their own movements, vasculature, immune cells, and repair mechanisms. Furthermore, distributed implanted bioelectronics will require specialized data and power transfer modalities that will need systems and standards for these emerging communication and energy transfer methods. For continued long-term success, power and data transfer modalities must be efficient, safe, secure, reliable, and able to be updated as new implants and methods arise. Battery-free bioelectronics. Miniature bioelectronics could support myriad therapeutics.