3D-printing-assisted flexible pressure sensor with a concentric circle pattern and high sensitivity for health monitoring

Flexible electronic and wearable devices capable of sensing pressure, strain, and temperature are increasingly important for applications in health monitoring, robotics, and human–machine interfaces. Among these, flexible pressure sensors are critical for detecting physiological signals such as blood pressure and heartbeat. Based on their working mechanisms, pressure sensors include piezoresistive, capacitive, piezoelectric, and triboelectric types; piezoresistive sensors are attractive due to their high sensitivity, simple structure, and easy read-out. Sensitivity and a wide linear pressure range are key performance parameters, and microstructured contact surfaces (e.g., pyramids, microdomes, micropillars) have been widely explored to enhance sensitivity. PDMS is commonly used to replicate such microstructures owing to its flexibility and biocompatibility, but conventional microfabrication methods (lithography/etching) are complex, costly, and require cleanroom environments. To address these issues, 3D printing has emerged as a simple, low-cost alternative for creating microstructures. Prior work using DIW to print PDMS-based concentric circle patterns (CCPs) with CNTs or graphene achieved sensitivities of 2.08 and 2.4 kPa−1 in very low pressure ranges, but DIW has scalability limitations and sensitivity/linearity need improvement for effective physiological sensing. FDM-type 3D printing offers simplicity, low cost, fast speed, and controllable parameters, but typically produces rough surfaces due to layer-by-layer deposition. This work leverages that roughness to create CCP microstructures via a novel compression method, enabling a simple, scalable fabrication of high-sensitivity piezoresistive PDMS/PEDOT:PSS flexible pressure sensors with tunable microstructure size via printing layer height (PLH).

Microstructured surfaces (pyramids, microdomes, micropillars) on PDMS have been shown to enhance piezoresistive pressure sensitivity, but traditional lithographic/etching fabrication is complex and costly. 3D printing has been explored to simplify microstructure fabrication. Prior DIW-printed CCP sensors used PDMS with conductive additives (CNTs, graphene) and reported sensitivities of 2.08 and 2.4 kPa−1 within low pressure windows (0.12 and 0.18 kPa), but DIW prints one device at a time and is less scalable than molding/casting. FDM 3D printing is low cost and fast but produces rough surfaces; this limitation has been exploited in applications like strain gauges, anti-adhesion surfaces, and micromixers. The present study adopts FDM roughness to engineer CCP microstructures with controllable dimensions via PLH to improve sensitivity and extend linear range while using simple, reusable molds and cost-effective conductive layers.

Design: A cone-shaped model was designed in CAD (NX, Siemens) with inner length 7 mm, inner height 4.04 mm, upper thickness 0.25 mm, and cone angle 30°. The working mechanism is piezoresistive: under applied pressure, contact area between a CCP electrode and a flat electrode increases, decreasing resistance and increasing current. CCP size is tuned via printing layer height (PLH). FDM printing and compression to planar CCP: PLA cones were printed using an FDM printer (GUIDER IIs, FlashForge). Fixed parameters: printing speed 60 mm/s; travel speed 80 mm/s; extruder temperature 220 °C; bed/platform 40 °C. PLH was varied from 0.10 to 0.16 mm in 0.02 mm steps (0.10, 0.12, 0.14, 0.16 mm). To transform the 3D cone surface into a planar CCP while preserving surface roughness, the printed cones were heated on a hotplate at 120 °C for 10 s (above PLA Tg ≈ 55–70 °C) and compressed with a 2-kg weight for 50 s, then cooled in air to fix the 2D CCP. The 2-kg weight and 50 s compression were identified as minimal/optimal to uniformly unfold cones across all PLHs; longer compression (≥2 min) damaged microstructures. Thickness uniformity of the unfolded PLA plane was verified (0.31 mm at multiple positions for PLH 0.10 mm). The method enables sharp, uniform 2D CCPs compared to direct 2D printing. PDMS replication and electrode preparation: PDMS (Sylgard 184) was mixed 10:1 (prepolymer:curing agent), degassed 1 h, poured to cover the PLA CCP plane, and cured at 50 °C for 12 h (below PLA Tg to avoid shape recovery). Resulting PDMS dimensions: thickness ~4 mm; width ~15 mm. The PLA mold was peeled off and is reusable. The PDMS CCP surface was treated by oxygen plasma (CUTE-1MPR, 2 min) to increase surface energy and enable wetting by PEDOT:PSS. PEDOT:PSS (1.1% in H2O, Sigma-Aldrich) was drop-cast using four droplets (80 µL each) to uniformly coat the CCP microstructures and dried at 100 °C for 1 h. Wires were attached with Ag paste. A flat PDMS/PEDOT:PSS electrode was paired with the CCP electrode to assemble the device. Microstructure characterization and theory: Optical and SEM imaging confirmed that CCP ridge width increases with PLH and that widths and tip spacings are uniform for each PLH. The CCP ridge width w relates to PLH h via w = √3·h (h, w in mm), as derived theoretically and validated experimentally by measuring width and total perimeter versus PLH.

- Tunable microstructures: CCP ridge width and total perimeter increase with PLH, consistent with theoretical relation w = √3·h. - Sensitivity and linear range: Sensitivity increases with PLH; the sensor with PLH = 0.16 mm achieves sensitivity of 160 kPa−1 within a linear pressure range of 0–0.577 kPa with R² = 0.978. - Stability and durability: Stable, repeatable responses under various pressures; durability demonstrated for 4000 loading/unloading cycles at 6.56 kPa. - Dynamic response: Response time 114 ms and recovery time 192 ms. - Practical demonstrations: Real-time monitoring of physiological signals including wrist pulse (rest and exercise), swallowing, and word pronunciation. - Process advantages: Simple, low-cost, reusable PLA molds; drop-cast PEDOT:PSS provides conductive, flexible active layer; compression method yields sharp, uniform 2D CCPs superior to direct 2D FDM printing.

Exploiting the intrinsic layer-induced roughness of FDM printing and converting a 3D cone surface into a 2D CCP via compression provides a simple and scalable route to engineer microstructured contact geometries that enhance piezoresistive sensitivity. By tuning PLH, the CCP dimensions (ridge width and perimeter) are controllably adjusted, which modulates initial contact area and its evolution under pressure, leading to improved sensitivity and a usable linear range at low pressures relevant to physiological signals. Compared to prior DIW-printed CCP sensors with sensitivities around 2–2.4 kPa−1 in narrow pressure windows, the presented PDMS/PEDOT:PSS CCP sensors achieve markedly higher sensitivity (160 kPa−1) and maintain linearity up to 0.577 kPa, while benefiting from faster, lower-cost fabrication and mold reusability. The robust cycling performance and sub-200 ms dynamic response support practical deployment in wearable health monitoring, as validated by pulse, swallowing, and speech detection.

This work introduces a facile FDM-assisted and compression-enabled method to fabricate concentric circle microstructures on PDMS and, combined with drop-cast PEDOT:PSS, yields highly sensitive, flexible piezoresistive pressure sensors. The key contributions include: (1) a novel compression process that transforms 3D-printed PLA cones into sharp, planar CCP molds with reusable capability; (2) tunable microstructure dimensions via printing layer height, with theoretical and experimental validation; and (3) high device performance—sensitivity of 160 kPa−1 with linearity up to 0.577 kPa (R² = 0.978), stable cycling (4000 cycles at 6.56 kPa), and rapid response—enabling effective monitoring of diverse physiological signals. The approach offers a simple, low-cost, and scalable pathway for microstructured flexible pressure sensors suitable for health monitoring applications.