Radiopaque coating for improved implantability and in vivo imaging of neural probes

The invasive nature of brain implants remains a major limitation in neuromodulation strategies, often leading to chronic inflammation. To address this, soft coatings are applied on rigid probes to reduce the mechanical mismatch at the interface, or flexible probes are implemented accompanied by temporary stiffeners. This study presents a hybrid strategy integrating both approaches by applying a permanent hydrogel coating onto flexible neural probes. Moreover, we utilise the applied coatings as tool for post-operative non-invasive imaging via functionalisation of the hydrogel with 5-acrylamido-2,4,6-triiodoisophthalic acid (AATIPA), a monomer that increases radiodensity. Rheological measurements confirmed that AATIPA incorporation did not significantly alter the hydrogels' mechanical properties (storage moduli ranging from 139 ± 33.5 to 186 ± 55.5 kPa). Subsequently, we show that coated flexible probes exhibited a two-fold increase in critical buckling force compared to uncoated counterparts, indicating improved mechanical robustness evidenced through enhanced insertion performance in agarose brain phantoms. The mechanical contrast supports the dual purpose of the material in our application: the coatings provide stiffness to facilitate probe insertion in the dry state, while transitioning to a compliant, soft interface upon swelling, post-implantation. Finally, the radiodense coating enabled successful visualization of the probes in the hippocampus of a mouse model using μ-CT imaging. This approach offers a promising route for improving the mechanical and imaging performance of neural implants, potentially reducing reliance on post-mortem histology and enhancing real-time feedback in neuromodulation research.