Abstract
This paper investigates the effects of annealing of the electrodes based on parylene-caulked polydimethylsiloxane (pc-PDMS) in terms of mechanical strength and long-term electrical property. Previously, the electrodes based on pc-PDMS showed a better ability to withstand in vivo environments because of the low water absorption and beneficial mechanical properties of the substrate, compared to native PDMS. Moreover, annealing is expected to even strengthen the mechanical strength and lower the water absorption of the pc-PDMS substrate. To characterize the mechanical strength and water absorption of the annealed pc-PDMS, tensile tests were carried out and infrared (IR) spectra were measured using Fourier transform infrared spectroscopy over a month. The results showed that annealed pc-PDMS had higher mechanical strength and lower water absorption than non-annealed pc-PDMS. Then, electrochemical impedance spectroscopy was measured to evaluate the electrical stability of the electrodes based on annealed pc-PDMS in phosphate-buffered saline solution at 36.5 °C. The impedance magnitude of the electrodes on annealed pc-PDMS was twice higher than that of the electrodes on non-annealed pc-PDMS in the initial days, but the impedance magnitude of the electrodes based on two different substrates converged to a similar value after eight months, indicating that the annealing effects disappear after a certain period of time in a physiological environment.
Highlights
In neural signal recording using implantable electrodes based on polymers, the durability and low water absorption of the electrode substrate have a major impact on the recording performance
Parylene in a thickness of 400 nm was deposited on the PDMS substrate using low-pressure chemical vapor deposition (LPCVD) (PDS 2010, Specialty Coating Systems, Indianapolis, IN, USA)
Electrochemical impedance spectroscopy (EIS) was performed by using an electrochemical impedance measurement system (Reference 600, Gamry, Warminster, PA, USA) to monitor the long-term electrical stability of the electrodes based on annealed parylene-caulked polydimethylsiloxane (pc-PDMS) in 0.9% phosphate buffered saline (PBS) solution at 36.5 ◦ C
Summary
In neural signal recording using implantable electrodes based on polymers, the durability and low water absorption of the electrode substrate have a major impact on the recording performance. Annealing has been commonly used to optimize polymers because it increases the mechanical strength and adhesion strength between the metal and polymer, which leads to good durability of the electrodes based on a polymer, but it increases the impedance magnitude [11,12]. Pc-PDMS is proved to preserve the inherent beneficial mechanical properties of PDMS, such as elasticity and flexibility, and to lower the water absorption better than native PDMS [15] The latter is due to the fact that parylene has a lower water absorption than native PDMS, being suitable as a substrate for implantable electrodes [13,14,15,17]. The electrodes based on annealed pc-PDMS are expected to have beneficial inherent properties of pc-PDMS with improved durability and low water absorption in comparison with pc-PDMS that is not annealed. To evaluate the impedance characteristics, the electrochemical impedance spectroscopy (EIS) was measured for the electrodes based on annealed and non-annealed pc-PDMS in a physiological environment for eight months
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