The development of ultrathin flexible batteries has enabled significant advancements in epidermal electronics and medical skin patches. These power sources must conform to the mechanical and biological constraints of human skin while delivering sufficient energy for applications such as transdermal drug delivery and biosensing. Unlike traditional rigid medical batteries, flexible batteries integrate seamlessly with wearable systems, offering comfort and continuous functionality.

Substrate materials for flexible batteries prioritize mechanical flexibility, durability, and biocompatibility. Polymer films such as polyimide (PI), polyethylene terephthalate (PET), and polydimethylsiloxane (PDMS) are commonly used due to their thinness, bendability, and chemical stability. These materials exhibit thicknesses ranging from 10 to 100 micrometers, ensuring minimal interference with natural skin movement. Some designs incorporate stretchable substrates to accommodate dynamic motion, such as joint flexion.

Electrode designs for ultrathin batteries must maintain conductivity while enduring repeated mechanical stress. Thin-film lithium-based electrodes, carbon-based materials, and conductive polymers are frequently employed. For example, lithium cobalt oxide (LCO) cathodes and graphite anodes can be deposited in layers as thin as 5 to 20 micrometers. Alternative materials like silver nanowires or graphene offer enhanced flexibility and conductivity. Current collectors, typically made of aluminum or copper foils, are also thinned or patterned to prevent cracking under strain.

Biocompatibility is critical for batteries in direct contact with skin or implanted in epidermal layers. Materials must be non-toxic, non-irritating, and resistant to corrosion from bodily fluids. Encapsulation layers, often made of parylene or silicone, prevent leakage of battery components while allowing flexibility. Regulatory standards such as ISO 10993 assess biocompatibility through cytotoxicity, sensitization, and irritation tests.

Power demands for epidermal applications vary depending on function. Transdermal drug delivery systems typically require 0.1 to 10 milliwatts to operate micropumps or iontophoresis circuits. Biosensing applications, such as glucose monitoring or electrophysiological recording, consume 0.01 to 1 milliwatt due to low-power sensor electronics. Energy density targets for these batteries range from 5 to 50 milliwatt-hours per square centimeter to sustain operation for days or weeks without replacement.

In contrast, rigid medical batteries, such as those in implantable pacemakers or hearing aids, prioritize higher energy density over flexibility. These devices use thicker lithium-ion or zinc-air cells, which provide 200 to 500 milliwatt-hours per gram but cannot conform to skin. Their rigid enclosures also limit placement options, often requiring surgical implantation or bulky external housings.

Flexible batteries face challenges in scaling energy capacity without compromising thinness. Innovations such as stacked cell architectures or hybrid solid-state electrolytes aim to improve energy density while maintaining flexibility. Another consideration is rechargeability—some epidermal systems use wireless charging to avoid frequent battery replacement.

Environmental stability is another key factor. Flexible batteries must operate under varying temperatures, humidity levels, and mechanical deformations without performance degradation. Accelerated aging tests simulate long-term wear to ensure reliability over the intended lifespan.

The future of ultrathin flexible batteries lies in material advancements and integration with other wearable technologies. Research into biodegradable batteries could further enhance safety and sustainability for medical applications. As epidermal electronics evolve, these power sources will play a pivotal role in enabling seamless, long-term health monitoring and treatment.

In summary, ultrathin flexible batteries represent a critical enabler for next-generation epidermal devices. Their design prioritizes mechanical compliance, biocompatibility, and sufficient energy delivery for medical applications. While rigid batteries still dominate high-power implantable devices, flexible alternatives offer unmatched versatility for wearable healthcare solutions. Continued innovation in materials and miniaturization will expand their potential in transdermal therapy and biosensing.