Director of NSF NRT SUSMED and Harris Saunders Jr. Professor Georgia Tech Atlanta, Georgia, United States
Introduction: : Continuous intracranial pressure (ICP) monitoring is essential for the early diagnosis and management of critical neurological conditions, including traumatic brain injury, idiopathic intracranial hypertension, and hydrocephalus. Current gold-standard methods, such as intraparenchymal microsensors and intraventricular catheters, require surgery to access the skull. This increases the risk of infection, bleeding, and discomfort for the patient, and proves these methods to be less suitable for long-term use. A newer approach involves placing a sensor inside the dural venous sinus using a catheter. This method is referred to as an endovascular approach and is less invasive. Furthermore, some versions utilize wireless telemetry, which avoids the use of uncomfortable wires, but can lose signal strength as they penetrate tissue and bone [1]. Moreover, wireless devices have a short readout range and may not fit within the space constraints of small blood vessels [2]. However, wired systems have comparatively more signal integrity, but need sensing layers that are strong and do not disintegrate easily. In earlier designs, micropyramidal-structured capacitive layers were used to improve sensitivity [3]. However, these small pyramid-shaped features can lose effectiveness after prolonged pressure or even during handling. In this project, we developed porous polymer dielectrics to make miniaturized in-stent biosensors that measure capacitive pressure, targeting real-time ICP monitoring. In the materials study, by controlling polystyrene bead templating, we could improve both sensitivity and mechanical durability. These results played a critical role in developing very thin, stent-based ICP sensors that are smaller, safer, and longer-lasting for continuous brain pressure monitoring.
Materials and
Methods: : The device integrates a clinical-grade, self-expanding stent platform with a capacitive pressure sensor (CPS) containing a thin-film porous PDMS dielectric layer, sandwiched between two copper electrodes. To fabricate the porous dielectric, PDMS (Dow Sylgard) was mixed with polystyrene (PS) beads of different diameters and at varying weight ratios. This systematic variation allowed us to study the independent effects of pore size and porosity on CPS performance. The PDMS-PS bead mixture was degassed under vacuum for 30 min, cured at 100°C for 1 hour, and immersed in an organic solvent to dissolve the PS beads, leaving a uniform porous network. The porous PDMS films were characterized using optical microscopy and microcomputed tomography (micro-CT) to verify pore morphology and measure pore geometry. The films were first cut into 2x2 cm sections and assembled into CPS units for benchtop testing. Static pressure response was measured by applying incremental loads, while capacitance was recorded using an LCR meter connected to LabVIEW for real-time data acquisition. For physiological testing, CPS units were miniaturized into ultrathin films (hundreds of micrometer-thick, 0.7 mm wide, 7 mm long) and mounted inside a custom-built cranial phantom. The phantom was connected to a pulsatile pump operating at 60 rpm to replicate venous pulsations. CPS outputs were recorded alongside a commercial reference sensor for comparison. Long-term stability was evaluated under continuous pulsatile flow for 24h to assess drift, sensitivity retention, and structural durability.
Results, Conclusions, and Discussions:: To determine optimal template material for porous PDMS dielectrics, we compared: (1) solid PDMS (fully degassed), (2) PDMS mixed with sugar crystals, and (3) PDMS mixed with PS beads, all thermally cured under identical conditions. Sugar-template PDMS films were soaked in water (80°C, 24h) and PS-template films in THF (50°C, 24h), then assembled into 2x2 cm CPS units for pressure testing. Figure 1A shows the miniaturized thin-film CPS unit next to a U.S. quarter, highlighting its small size, while Figure 1B illustrates how the CPS can be integrated into a self-expanding stent for endovascular deployment in the dural venous sinus. Optical microscopy images (Figure 1C) revealed distinct differences in pore morphology across the three dielectric types. Solid PDMS exhibited no pores, sugar-templated PDMS showed irregular and incomplete pores, and PS bead-templated PDMS displayed uniform, well-defined pores. The poor morphology of sugar-templated PDMS is attributed to several factors: sugar particles were significantly larger than PS beads (tens to hundreds of micrometers), had a broad size distribution, and did not fully dissolve even after 24 h in heated water. Additionally, the heavier sugar particles settled during thermal curing, creating a non-uniform porous network. These structural irregularities likely reduced the material’s compressibility and thus limited its pressure responsiveness. Capacitance–pressure testing results are shown in Figure 1D. PS bead-templated PDMS achieved the highest sensitivity, with a slope of 0.00589 pF/kPa and a linear fit of R² = 0.995. Solid PDMS and sugar-templated PDMS exhibited lower sensitivities, with R² values of 0.981 and 0.979, respectively. The enhanced performance of PS bead-templated PDMS is attributed to its uniform pore distribution, which increases air fraction within the dielectric, thereby enhancing its effective dielectric constant and mechanical compliance under load. In conclusion, this study demonstrates that PS beads are the optimal template material for producing uniform porous dielectrics in capacitive pressure sensors. Compared to both solid and sugar-templated PDMS, PS bead-templated PDMS provides a superior combination of structural uniformity and sensitivity, making it a promising dielectric choice for miniaturized, stent-integrated ICP pressure sensors.
Acknowledgements and/or References (Optional): : This study was conducted and supported by Prof. Woon-Hong Yeo group (https://www.yeolabgatech.com/) at Georgia Tech. [1] H. M. Fernandes et al., Clinical Evaluation of the Codman Microsensor Intracranial Pressure Monitoring System. Acta Neurochirurgica Supplements, 71, Springer(1998). [2] R. Herbert et al., Fully implantable wireless batteryless vascular electronics with printed soft sensors for multiplex sensing of hemodynamics. Sci. Adv. 8, eabm1175(2022). [3] J. Lee et al., Non-Surgical, In-Stent Membrane Bioelectronics for Long-Term Intracranial Pressure Monitoring. Adv. Healthcare Mater. 14, 2404680(2025).
