Chronically implanted microelectrodes are crucial for neuroscience research, but their stiffness, compared to brain tissue, causes damage and rejection. This mismatch in Young's modulus (silicon-based probes ≈150 GPa vs. brain tissue ≈10 kPa) leads to glial scar formation and device failure within weeks or months. To address this, flexible probes are being developed using soft polymeric materials (parylene, polyimide, PDMS, hydrogels) or thinner stiff materials. However, the softness of these probes makes insertion challenging. Strategies like using a stiff shuttle removed post-implantation or integrating a bioresorbable coating have been explored. Bioresorbable coatings, offering better mechanical and biological outcomes, include polymers like PEG, PLA, chitosan, and silk fibroin. Existing methods often involve manual handling, limiting downscaling and scalability. This paper proposes a new fabrication framework integrating a bioresorbable silk fibroin layer into a microfabrication process to create ultrathin parylene-based penetrating probes, addressing the limitations of current approaches.

The literature extensively covers the challenges associated with rigid neural probes and the development of flexible alternatives. Studies highlight the mechanical mismatch between traditional electrodes and brain tissue, leading to inflammation and device failure. Numerous investigations explore the use of various biocompatible and biodegradable polymers to enhance the biointegration of flexible probes, while methods for improving insertion and minimizing tissue trauma have also been explored. However, a significant gap existed in scalable batch fabrication techniques that seamlessly integrate bioresorbable coatings without compromising the miniaturization and precision of the probes. This paper addresses this gap.

The fabrication process uses standard microsystem techniques to create a silk-parylene bilayer probe. First, a cellulose acetate-coated glass substrate is coated with silk fibroin. Then, an ultrathin parylene-based structure with four gold microelectrodes coated with PEDOT:PSS is microfabricated on top. Finally, reactive ion etching (RIE) shapes the probe before release from the substrate. Silk fibroin, chosen for its biocompatibility, tunable biodegradability, and high mechanical strength, allows for reduced parylene thickness. Methanol treatment controls silk degradation time. The parylene layer provides biocompatibility, chemical inertness, and electrical insulation. The fabrication is scalable, producing 80 probes on a single 4-inch substrate. Insertion testing was performed in a 1 wt.% agarose gel brain phantom and ex vivo in a mouse brain. Mechanical properties were evaluated using compression tests, analyzing buckling force and resistance to cyclic loading. Electrical and electrochemical characteristics were also assessed.

The study successfully demonstrated a scalable batch fabrication method for ultrathin (4 µm thick) flexible neural probes. The silk-parylene bilayer design, with a bioresorbable silk layer acting as a temporary stiffener, allowed for successful probe insertion into both brain phantoms and ex vivo mouse brain tissue without buckling. The silk layer's degradation time was shown to be controllable through methanol treatment. Compression tests revealed high mechanical strength (average buckling strength of 10.9 ± 1.3 mN), 15 times greater than the insertion force. The probes showed minimal performance degradation (10% decrease) after 1000 bending cycles. Ex vivo experiments in mouse brain slices demonstrated high-fidelity recordings of epileptic seizures and single-unit action potentials, confirming the functionality of the probes.

The results address the significant challenge of combining the flexibility required for minimal tissue damage with the stiffness needed for successful probe insertion. The novel fabrication method successfully integrates a bioresorbable stiffener into a scalable batch process, avoiding the limitations of manual handling and enabling the creation of ultrathin, high-fidelity recording probes. The controllable degradation time of the silk layer offers flexibility in implantation strategies. The high mechanical strength and durability demonstrated by the probes suggest high potential for long-term chronic implantation. The successful ex vivo recordings validate the functionality of the probes, paving the way for in vivo studies.

This study presents a significant advancement in neural probe technology, offering a scalable and efficient method for fabricating ultrathin, flexible probes with high mechanical strength and excellent recording capabilities. The use of a bioresorbable silk layer as a temporary stiffener effectively addresses the challenges of probe insertion while minimizing tissue trauma. Future work will focus on in vivo chronic implantation studies to assess long-term biocompatibility and recording stability, along with exploring different materials and probe designs to further optimize performance.

The study's ex vivo experiments, while promising, need to be complemented with in vivo long-term studies to fully assess the biocompatibility and efficacy of the probes. The brain phantom used, while a common model, may not perfectly represent all aspects of brain tissue mechanics. Further optimization of the probe design and fabrication process may be necessary to address specific challenges associated with different brain regions or implantation techniques.