Micromotion Derived Fluid Shear Stress Mediates Peri‐Electrode Gliosis through Mechanosensitive Ion Channels

**Background:** Chronic neuroelectrode function is compromised by the foreign body response, leading to peri-electrode gliosis and encapsulation. While initial inflammation from implantation trauma resolves, relative micromotion at the electrode/tissue interface from respiration and blood flow exposes tissues to cyclic shear stresses that exacerbate gliosis. The mechanotransductive molecular pathways initiating this process are not fully understood. This study investigates the role of millipascal fluid shear stress and mechanosensitive ion channels in peri-electrode gliosis.

**Methods:** A finite volume model (FVM) simulated electrode micromotion (30 µm peak-to-peak, 0.5 Hz) within a fluid-filled peri-electrode cavity, calculating wall shear stress (WSS) on surrounding tissue. In vitro, primary ventral mesencephalic (VM) cells from E14 rat embryos were exposed to oscillatory fluid shear stress (0.1 Pa, 0.5 Hz) for 4 or 6 hours using a parallel-plate flow chamber. Astrocyte reactivity and neuronal viability were assessed via immunocytochemistry (GFAP, ß-tubulin III), western blot, and protein microarrays at 1, 7, and 14 days post-flow. Gene expression was analyzed using PCR arrays targeting neurogenesis and Hippo pathway genes. In vivo, Pt/Ir electrodes were implanted in the rat subthalamic nucleus and either immediately removed (stab wound) or retained for 8 weeks. Immunohistochemistry assessed GFAP, Iba1, NeuN, PIEZO1, and TRPA1 expression. Chemical modulation of PIEZO1 was performed using Yoda1 (agonist, 10 µM) and GsMTx4 (antagonist, 0.5 µM), and mitochondrial function was evaluated via Seahorse extracellular flux analysis.

**Key Results:** The FVM predicted WSS ranging from 0.0006 Pa (150 µm electrode-tissue distance, viscosity 1.2 mPa·s) to 2.0167 Pa (10 µm distance, viscosity 100 mPa·s). At a representative 0.1 Pa, in vitro shear stress for 6 hours significantly increased the astrocyte:neuron ratio by 98% at 14 days (control 26% vs treated 51%; p=0.0242). Astrocyte soma area increased by 83% (control 787 µm² vs treated 1441 µm²; p=0.0053) and GFAP intensity increased >100% (control 36 RFU/µm² vs treated 76 RFU/µm²; p=0.0203). Neuronal number decreased by 26% (control 67% vs treated 50%; p=0.0303) and ß-tubulin III protein expression decreased by 76% (control 0.83 vs treated 0.20; p=0.0082). Chondroitin sulfate expression increased by 70% (p=0.0276) and aquaporin-4 expression increased ≈2.5-fold (p=0.0292). Gene expression revealed upregulation of Casp3 (102.54-fold), CCNE2 (45.57-fold), and downregulation of HPRT1 (556.41-fold). In vivo, PIEZO1 staining area increased ≈91% (p<0.0001) and TRPA1 staining area increased ≈75% (p=0.0341) at 50 µm from the electrode. PIEZO1 inhibition with GsMTx4 increased astrocyte number by ≈45% (p=0.0003) and decreased neuronal number by ≈49% (p=0.0006) under static conditions, while PIEZO1 activation with Yoda1 reduced astrocyte density by ≈29% (p=0.0395) and increased neuronal density by ≈39% (p=0.0289) under flow. GsMTx4 also reduced basal respiration by ≈57% (p=0.0364) and ATP production by ≈59% (p=0.0324).

**Clinical Implications:** This study identifies micromotion-induced fluid shear stress as a key driver of peri-electrode gliosis, mediated by mechanosensitive ion channels PIEZO1 and TRPA1. The finding that PIEZO1 activation attenuates gliosis while inhibition exacerbates it suggests that targeting PIEZO1 could be a therapeutic strategy to improve long-term neuroelectrode integration and performance. The in vitro model recapitulating glial scarring provides a valuable tool for testing anti-gliosis therapies. Future work should explore electrode functionalization with PIEZO1 agonists to promote chronic electrode stability.