Fabrication approaches to biodegradable and biocompatible microelectromechanical (MEMS) devices suitable for implantation in the body will be discussed. Three topical areas will be touched upon: biodegradable wireless pressure sensors; biodegradable batteries; and MEMS fabricated from protein materials.
Biodegradable Wireless Pressure Sensors: A wireless pressure sensor made entirely of biodegradable materials will be discussed. Such biodegradable sensors may be appropriate for short-term, acute medical implantation applications as they potentially eliminate the need for implant extraction when sensing is no longer required. Metallic bilayers with controllable corrosion characteristics are utilized as conductors, and biodegradable polymers poly-L-lactide (PLLA) and polycaprolactone are used as dielectric and structural materials. The fabricated sensor is based on a resonant variable capacitance technique and was wirelessly tested in both air and 0.9% saline. The sensor exhibited a linear resonant frequency response to external applied pressure, over the physiologically-relevant 0-20 kPa applied pressure range. Sensors remained stable and functional for approximately 86 hours in saline solution.
Biodegradable Batteries: Implantable and biodegradable batteries play an important role in the fully degradable medical systems; however, in such systems compactness and energy density are of great importance. Harnessing liquid from the body to serve as the battery electrolyte may, therefore, be desirable; however, for stable operation, maintaining a constant environment inside the electrochemical cell is required even in the presence of changing body conditions. A biodegradable battery featuring a solid electrolyte of sodium chloride and polycaprolactone will be discussed. This approach harnesses the body fluid that diffuses into the cell as an element of the electrolyte; however, the large excess of sodium chloride suspended in the polycaprolactone holds ionic conditions inside the battery relatively constant. A constant discharge profile can then be achieved even in the presence of varying external aqueous conditions, enabling compact, stable-performing cells.
Protein MEMS: The use of extracellular matrix (ECM) protein materials in the fabrication of MEMS offer the possibility of MEMS-based devices that are more easily integrated with the body, are better tolerated by the immune system, and/or which have mechanical properties that are well-matched to potential biological applications. However, the incorporation of these relatively delicate materials into standard MEMS processes can result in significant fabrication challenges. Suitable fabrication approaches to Protein MEMS will be discussed, and their use will be illustrated in a collagen-based cortical neural electrode, which has demonstrated reduced inflammation and superior persistence of functionality in in vivo models when compared to similarly-sized MEMS electrodes fabricated from conventional materials.