Stretchable Capacitive and Resistive Strain Sensors: Accessible Manufacturing Using Direct Ink Writing

TL;DR

This work addresses the need for highly stretchable, conformal tactile and proprioceptive sensors for soft robotics and wearables by developing a scalable, accessible DIW fabrication platform. The authors integrate a custom DIW printhead with a standard 3D printer to deposit liquid conductive ink onto cured silicone in a layer-by-layer process, enabling both capacitive and resistive transducers within a silicone matrix. They report capacitive sensors with high linearity ($R^2 = 0.99$) and near-ideal sensitivity ($GF \approx 0.95$) with low hysteresis ($DH \approx 1.36\%$) and stretchability up to $550\%$, and resistive sensors with higher hysteresis ($DH \approx 21.88\%$) and a larger $GF$ ($GF \approx 16.83$) and stretchability up to $600\%$. The results indicate the method offers substantial design flexibility at low cost, suitable for soft robotics, wearables, and health monitoring, with future work focusing on printhead reliability, alternative inks, and scalable, multi-material printing.

Abstract

As robotics advances toward integrating soft structures, anthropomorphic shapes, and complex tasks, soft and highly stretchable mechanotransducers are becoming essential. To reliably measure tactile and proprioceptive data while ensuring shape conformability, stretchability, and adaptability, researchers have explored diverse transduction principles alongside scalable and versatile manufacturing techniques. Nonetheless, many current methods for stretchable sensors are designed to produce a single sensor configuration, thereby limiting design flexibility. Here, we present an accessible, flexible, printing-based fabrication approach for customizable, stretchable sensors. Our method employs a custom-built printhead integrated with a commercial 3D printer to enable direct ink writing (DIW) of conductive ink onto cured silicone substrates. A layer-wise fabrication process, facilitated by stackable trays, allows for the deposition of multiple liquid conductive ink layers within a silicone matrix. To demonstrate the method's capacity for high design flexibility, we fabricate and evaluate both capacitive and resistive strain sensor morphologies. Experimental characterization showed that the capacitive strain sensor possesses high linearity (R^2 = 0.99), high sensitivity near the 1.0 theoretical limit (GF = 0.95), minimal hysteresis (DH = 1.36%), and large stretchability (550%), comparable to state-of-the-art stretchable strain sensors reported in the literature.

Figures (8)

• Figure 1: (a) Printing of the conductive pattern. (b) The stretchable capacitive (top) and resistive (bottom) strain sensors.
• Figure 2: (a) Capacitive strain sensor cross-section with two conductive ink layers shown in photo and schematic. (b) Resistive strain sensor cross-section with a single conductive ink layer in photo and schematic.(c) Front and back side of a capacitive strain sensor, showing the electrical interface utilising conductive thread.
• Figure 3: Printer setup. (a) The printer setup with the modified printhead. (b) The modified printhead.
• Figure 4: Additive manufacturing fabrication process for capacitive and resistive sensors, consisting of three main steps: pouring silicone, curing silicone, and printing conductive ink. The process used a stackable tray system to facilitate layer-wise assembly, allowing for control over silicone layer thickness and the integration of conductive ink. For capacitive sensors, a dielectric silicone layer was added between conductive layers, while resistive sensors required a single conductive layer encapsulated in silicone.
• Figure 5: Experimental setup for strain testing. (a) Capacitive strain sensor under strain. The sensor in its unstrained state is shown on the right. (b) Resistive strain sensor under strain. The sensor in its unstrained state is shown on the right.