Silk proteins, particularly fibroin and sericin, have emerged as promising materials for developing biodegradable batteries. These naturally derived polymers offer unique electrochemical properties, environmental compatibility, and the ability to degrade harmlessly, making them suitable for sustainable energy storage solutions. Their application spans electrolytes, separators, and substrates, enabling the fabrication of fully degradable battery systems for eco-sensitive applications.

Fibroin, the structural protein of silk, exhibits excellent film-forming ability, mechanical flexibility, and tunable ionic conductivity. When processed into thin films, it can function as a solid electrolyte or separator due to its porous structure and ability to host ion-conducting species. Sericin, the glue-like protein coating silk fibers, possesses hydrophilic properties and can be modified to enhance ionic transport. Both proteins are biocompatible and degrade into non-toxic byproducts under physiological conditions, a critical requirement for medical implants.

The electrochemical performance of silk-based batteries depends on the protein processing method. Fibroin electrolytes are typically prepared by dissolving silk cocoons in lithium bromide solution, followed by dialysis to remove salts. The resulting aqueous fibroin solution can be cast into films and doped with lithium salts to achieve ionic conductivities in the range of 10^-4 to 10^-3 S/cm at room temperature. Higher conductivities are achievable by incorporating plasticizers or creating composite materials with conductive polymers. Sericin-based electrolytes often require crosslinking to improve mechanical stability while maintaining ionic pathways. Chemical modification with carboxyl or sulfonate groups enhances anion mobility, enabling its use in both lithium and sodium-ion systems.

As a separator, fibroin's nano-porous structure facilitates ion transport while preventing electrode shorting. Its thermal stability up to 250°C surpasses many conventional polymer separators, improving battery safety. The protein's inherent mechanical strength allows for thin separators below 20 micrometers, reducing internal resistance. Sericin separators exhibit superior wettability with aqueous electrolytes, promoting uniform current distribution. Both materials show low interfacial resistance with common electrode materials, a key advantage over synthetic biodegradable polymers.

Silk substrates serve as biodegradable current collectors or electrode supports. Fibroin can be patterned into conductive scaffolds by coating with thin layers of biodegradable metals like magnesium or zinc. These substrates maintain adhesion during cycling while degrading predictably in physiological environments. Electrodes deposited on silk substrates demonstrate comparable performance to conventional systems, with specific capacities reaching 150-200 mAh/g for lithium-ion configurations. The flexibility of silk substrates enables conformal batteries for wearable applications.

Processing techniques for silk-based batteries emphasize sustainability. Water-based processing avoids toxic solvents, and mild temperature requirements reduce energy consumption compared to synthetic polymer processing. Electrode slurry casting, spin coating, and electrospinning are compatible with silk proteins. Dry electrode processing has also been demonstrated, where fibroin powders are compacted into dense electrolyte layers without solvents. These methods enable scalable manufacturing while maintaining the material's biodegradability.

Performance metrics of prototype silk batteries show promise for low-power applications. Lithium-ion cells with fibroin electrolytes achieve stable cycling over 100-200 cycles with capacity retention above 80%. Operating voltages range from 2.5-3.7V depending on electrode materials. Aqueous zinc-ion batteries using sericin electrolytes demonstrate higher cycle counts (300+) due to reduced dendrite formation. Energy densities typically range from 50-120 Wh/kg, suitable for applications where environmental impact outweighs the need for high energy density.

Medical implants represent a primary application for silk protein batteries. Their complete biodegradation eliminates the need for surgical removal, reducing patient risk. Examples include temporary power sources for drug delivery devices or biosensors, where battery lifespan matches the implant's operational period. In vivo studies show fibroin batteries maintaining functionality for 2-4 weeks before gradual resorption, with no inflammatory response. The proteins' natural origin minimizes immune reactions compared to synthetic biodegradable polymers.

Eco-friendly electronics benefit from silk batteries' environmental profile. Disposable sensors, agricultural monitors, or biodegradable packaging with integrated power can decompose after use without harmful residues. The moisture sensitivity of silk proteins becomes an advantage in these applications, as exposure to environmental water triggers controlled degradation. Encapsulation with thin wax layers extends operational life while maintaining overall biodegradability.

Moisture sensitivity remains a key challenge for silk-based batteries. Ambient humidity can plasticize the proteins, altering ionic conductivity and mechanical properties. Strategies to address this include hydrophobic coatings that degrade only under specific conditions, or chemical crosslinking to reduce water absorption. Another limitation is the moderate ionic conductivity compared to liquid electrolytes, restricting high-rate applications. Hybrid systems combining silk with conductive nanocellulose show potential for improvement.

Long-term degradation kinetics require precise matching to application needs. In medical use, degradation must align with the therapeutic timeline, while environmental applications need predictable breakdown under composting conditions. Protein engineering techniques allow tuning degradation rates from days to months by adjusting molecular weight or crystallinity. The breakdown products—amino acids and peptides—have been verified as non-toxic in soil and aquatic environments.

Future development focuses on enhancing performance while maintaining biodegradability. Incorporating nanostructured electrodes improves energy density, and protein engineering creates optimized ion transport pathways. Multi-functional designs integrate energy storage with structural support in biodegradable devices. Standardized testing protocols are needed to assess environmental impact across different disposal scenarios.

Silk protein batteries represent a convergence of sustainable materials science and energy storage technology. Their unique properties address growing demands for temporary power solutions that leave no persistent waste. While performance metrics currently lag behind conventional systems, their environmental advantages create irreplaceable niches in medical and ecological applications. Continued research into protein modification and device integration will expand their capabilities while preserving the core benefits of biodegradability and biocompatibility.