Preparation of stretchable AuNW-PDMS composite conductors
During the project, soft, stretchable and highly conducting composites of gold nanowires (AuNW) and a silicone rubber (PDMS, Sylgard 184) were prepared. The process involves filtering a suspension of AuNWs in water through a filter membrane forming a mesh of AuNWs. The mesh was then transferred onto a semi-cured PDMS substrate and adhered by being partially embedded in PDMS as it cured. These conductors maintain electrical conductivity even under very high deformations because the individual nanowires can mechanically slide past each other while still maintaining electrical contact. Owing to high conductivity and inertness of gold, these conductors can provide biocompatible, highly conductivite and stretchable conductors for neural interfaces.
Development of laser micromachining based process for fabrication of soft OECTs and optimization of laser micromachining parameters
A femtosecond laser-based fabrication process was designed and optimized to enable fast turn around, fewer process steps, and minimal contamination compared to conventional photolithography-based methods. The laser system fires highly focussed, ultrashort laser pulses to ablate material from a small spot with minimal damage to surrounding areas. Laser parameters such as power, scan speed, lens type, and scan patterns were optimized for selective and highly controlled ablation of different types of materials in the OECT material stack. Optimization enabled achieving machining with high spatial resolution (down to 1um) in all three axes.
The OECT fabrication process developed is as follows: Conducting lines were formed by patterning AuNW meshes according to CAD designs using a femtosecond laser to selectively remove AuNWs leaving the underlying PDMS substrate unaffected. The conducting lines were then encapsulated using PDMS. Next, the femtosecond laser was used to selectively ablate the PDMS encapsulation and combined with reactive ion etching (RIE) to form the OECT active area. The OECT active material, PEDOT:PSS was then deposited inside the OECT active area using spin coating.
Development of experimental setup and OECT electrical characterization
To measure the electrical characteristics of the OECT, devices were submerged in an electrolyte solution while a gate voltage was applied thought the electrolyte and a bias voltage was applied across the channel. The current flowing through the channel is modulated by the voltage applied on the gate electrode. The magnitude of variation in the current for a given change in gate voltage is called transconductance, which is a key figure of merit characterizing the ability of the OECT to amplify signals.
Another figure of merit is the bandwidth, which characterizes the capability of the device to transduce fast-changing signals. For efficient neural signal recording, OECT devices need to capture signals varying as fast as 1kHz. A custom setup was built to characterize bandwidth, consisting of two potentiostats interfaced with a computer and controlled via Python scripts. Signals of different frequencies were applied on the gate electrode and the corresponding changes in drain current were measured. The amplification factor is obtained for different frequencies and bandwidth is calculated.
The transconductance and bandwidth of the OECTs are inversely related and depend on the device geometry. Different sized OECTs were fabricated and electrically characterized to obtain the optimal geometry for neural transduction.
Down scaling of OECTs
OECTs as small as 10x10 um with channel width of the order of 1 um were fabricated and tested using the process described above.
Integration of OECTs into neural probes and development of measurement setup
Multiple OECTs were farbicated and integrated into a common source circuit. This configuration reduces the number of contact lines required – for, n OECTs, the number of lines are reduced from 2n to n+1. This is important for scaling the device to a large number of transducers. Neural probes are cut out using the femtosecond laser and released from the substrate. The probes are bonded to flexible flat cables, which were then connected to a biasing resistor circuit designed to apply voltage bias to the devices and measure the recorded neural signals.
The interface was compatible with multi-channel neural signal recording systems such as Intan, enabling simultaneous recording from multiple OECT sites. Neurals probe were prepared for the planned in vivo testing in the sciatic nerve of a rat.
Preparation for animal experiments
Device designs have been developed together with a neuroscientist to fit implantation in rat sciatic nerve. Preparatory surgeries have been performed to develop the implantation procedure, tactile stimulation protocol and recording settings. Implantation of the OECT probes will happen during the spring of 2026.
