A Highly Sensitive Flexible Capacitive Tactile Sensor with Sparse and High‐Aspect‐Ratio Microstructures

Abstract

Highly sensitive flexible tactile sensors that can be fabricated in a low cost and efficient way are in great demand for intelligent soft robotics and friendly human–machine interaction. Herein, a highly sensitive flexible tactile sensor is developed by using bionic micropatterned polydimethylsiloxane (m‐PDMS) replicated from lotus leaf. The m‐PDMS substrate consists of high‐aspect‐ratio and low‐density microtowers, and is covered by ultrathin silver nanowires as a bottom electrode. The capacitive sensing device is constructed by sandwiching the bottom electrode, a colorless polyimides dielectric layer, and a top electrode, and exhibits a high sensitivity of ≈1.2 k Pa−1, a ultralow limit of detection <0.8 Pa, and a fast response time of 36 ms. The finite‐elemental analysis indicates that the sparse and high‐aspect‐ratio microtowers are critical to achieve high sensitivity, low limit of detection, and fast response to external stimulus. The flexible tactile sensor also exhibits high robustness: it can be tested for at least 100 000 cycles without showing fatigue. More importantly, the flexible tactile sensors are potentially useful in intelligent soft robots, health monitoring, and motion detection. Besides, the fabrication strategy may offer a guideline to design other microstructures for improving the performance of flexible tactile sensors.

Piezocapacitive Flexible E‐Skin Pressure Sensors Having Magnetically Grown Microstructures

“…To determine the limit of detection (LOD), a water drop of ≈(13 ± 2) mg was dispensed on the top of sensor, exerting a pressure of (1.9 ± 0.2) Pa, as shown in Figure e. Successful detection of such a minute pressure confirms that our device exhibits ultralow LOD, which equals or surpasses the LOD of previously reported microstructured sensors, varying from 0.5–17.5 Pa . To determine the sensor's response time, we applied 2.5 kPa pressure on sensor by using Instron (2400 mm min −1 ).…”

“…Generally, microstructured pressure sensors manufactured by complex and expensive methods are only suitable for short pressure ranges. Working range of most of the reported sensors lies ≤10 kPa, some work up to 20 kPa and only few reaches up to ≤50 kPa pressure range . Also, their output saturates when applied pressure exceeds some critical value.…”

“…During compression, conical structures bear nonuniform stress distribution, in which stress is wholly concentrated at their pointed tips rather than the broader base. Thus, the tip of conical structures compresses more due to its smaller area which results in higher mechanical deformation at the top . Therefore, conical structures show large sensitivity initially due to small contact area, which drops at higher compressive pressures with the increase of contact area, and structure itself reverts to a more truncated cross‐sectional area .…”

“…Especially communication of home service robots and artificial limbs with human beings is carried out via friendly human–machine interaction . An ideal e‐skin is expected to be highly flexible, sensitive, lightweight, easy to fabricate, inexpensive, and should be capable of performing like the natural skin, which feels tactile pressures ranging from light touches (0–10 kPa) to object handling levels (10–100 kPa) . Generally, e‐skin pressure sensors use piezoresistive, piezocapacitive, piezoelectric, and triboelectric sensing mechanisms to transduce applied pressure into an electrical signal.…”

“…Polydimethylsiloxane (PDMS) is considered a suitable candidate for pressure‐sensitive dielectric thin films, but its high Young's modulus (≈2 MPa) makes it less deformable, toward minute applied pressures (e.g., <1 kPa) . Besides this, only a few micrometer (µm) thick flat PDMS dielectric films exhibit significant viscoelastic creep, which leads to lesser compressibility and increased relaxation times after removal of pressure . To cope with these problems, thin dielectric films are microstructured by using geometries like domes, pyramids, and pillars .…”