Recent applications of parylene and pyrolyzed parylene in sensors and devices: a review

Parylene is an insulating polymer that is coated in thin layers on sensors and devices. It exhibits excellent gas and moisture impermeability, enabling it to function as a biocompatible encapsulation layer. Parylene is commonly deposited by chemical vapor deposition (CVD) on substrates, with coating thicknesses ranging from nanoscale to microscale, depending on the amount of precursor loaded into the parylene coater. Parylene was deposited on cell culture devices made from 3D printing resins, and it prevents leachates that are toxic to cells, enabling long-term cultures. Pristine parylene is also used for sealing implantable sensors, the cantilever of atomic force microscopy (AFM), and microelectromechanical systems (MEMS), preventing damage in harsh environments. In addition to the intrinsic insulating properties of parylene, parylene is pyrolyzed at high temperatures, inducing structural reformation and converting it to a conductive material. Pyrolyzed parylene is utilized for electrochemical sensing of molecules, coupled with cyclic voltammetry (CV), fast scan cyclic voltammetry (FSCV), or immunoassays. For example, pyrolyzed parylene sensors can be used for in vivo tracking of neurotransmitters. A CO₂ infrared laser can be an alternative method to induce the graphitization of parylene films. Laser-induced graphene (LIG) is used to fabricate microsupercapacitors. A plasma deposition method for parylene has recently been developed, enabling the creation of a crosslinked structure that favors the immobilization of antigens for immunoassay fabrication. Overall, parylene is easy to coat on a variety of materials and provides biocompatible insulation of analytical devices, with easy pyrolysis to carbon used in sensors.