Lifetime engineering of bioelectronic implants with mechanically reliable thin film encapsulations

Abstract

While the importance of thin form factor and mechanical tissue biocompatibility has been made clear for next generation bioelectronic implants, material systems meeting these criteria still have not demonstrated sufficient long-term durability. This review provides an update on the materials used in modern bioelectronic implants as substrates and protective encapsulations, with a particular focus on flexible and conformable devices. We review how thin film encapsulations are known to fail due to mechanical stresses and environmental surroundings under processing and operating conditions. This information is then reflected in recommending state-of-the-art encapsulation strategies for designing mechanically reliable thin film bioelectronic interfaces. Finally, we assess the methods used to evaluate novel bioelectronic implant devices and the current state of their longevity based on encapsulation and substrate materials. We also provide insights for future testing to engineer long-lived bioelectronic implants more effectively and to make implantable bioelectronics a viable option for chronic diseases in accordance with each patient’s therapeutical timescale.

Polyisobutylene Encapsulated PEDOT: Pss Electrode Honeycomb Skeleton Electrolyte for Moisture Resistant Electro‐Ionic Soft Artificial Muscles

“…[41][42][43] The inorganic encapsulation films present excellent hermeticity, and their limited elasticity (≈1% of crack onset strain) and brittleness (elongation limit of a few percent with Young's modulus on the order of 100 GPa) still can offer impressive bendability onto compliant polymer substrates (e.g., polyethylene terephthalate (PET), polyimide, on the order of 1 GPa [39,44] ) as long as the film thickness is maintained thin. For example, bending 100μm-thick 3 cm-long substrate to a complete circle radius of 5 mm requires only 1% of applied strain based on applied bending strain equation 𝜖app = t/D, comprised of thickness t and diameter of the circle D. [44][45][46] However, the inorganic encapsulation onto elastomer or any soft polymer substrates (<100 MPa [11,47] ) creates extreme elastic mismatch at the interface and can generate mechanical failures (e.g., cohesive cracking [48,65] , substrate cracking, [49] and interfacial delamination [50] ) even at the negligible applied strain (≈0.1%). [51] In the meantime, most organic encapsulation films present insufficient hermeticity where barrier improvement factor is unlikely over 1.…”

“…For example, bending 100µm‐thick 3 cm‐long substrate to a complete circle radius of 5 mm requires only 1% of applied strain based on applied bending strain equation ε app = t/D , comprised of thickness t and diameter of the circle D . [ 44‐46 ] However, the inorganic encapsulation onto elastomer or any soft polymer substrates (<100 MPa [ 11,47 ] ) creates extreme elastic mismatch at the interface and can generate mechanical failures (e.g., cohesive cracking [ 48,65 ] , substrate cracking, [ 49 ] and interfacial delamination [ 50 ] ) even at the negligible applied strain (≈0.1%). [ 51 ] In the meantime, most organic encapsulation films present insufficient hermeticity where barrier improvement factor is unlikely over 1.…”