Fiber Embroidery of Self-Sensing Soft Actuators

Abstract

Natural organisms use a combination of contracting muscles and inextensible fibers to transform into controllable shapes, camouflage into their surrounding environment, and catch prey. Replicating these capabilities with engineered materials is challenging because of the difficulty in manufacturing and controlling soft material actuators with embedded fibers. In addition, while linear and bending motions are common in soft actuators, rotary motions require three-dimensional fiber wrapping or multiple bending or linear elements working in coordination that are challenging to design and fabricate. In this work, an automatic embroidery machine patterned Kevlar™ fibers and stretchable optical fibers into inflatable silicone membranes to control their inflated shape and enable sensing. This embroidery-based fabrication technique is simple, low cost, and allows for precise and custom patterning of fibers in elastomers. Using this technique, we developed inflatable elastomeric actuators embedded with a planar spiral pattern of high-strength Kevlar™ fibers that inflate into radially symmetric shapes and achieve nearly 180° angular rotation and 10 cm linear displacement.

Keywords: elastomer; shape-changing; soft actuators; twisting.

Figure 1

A pattern of inextensible fibers made in a water-soluble plastic sheet using (a,b) an embroidery machine (c–f) is embedded in a silicone membrane (g,h) causing vertical and rotational displacement. PVA: Poly(vinyl alcohol).

Figure 2

Spherical cap coordinate system used in energy balance model for an inflating silicone membrane (yellow region). h: Center height; R: Radius of the spherical cap; r₀: Base radius of the membrane; α: Zenith angle.

Figure 3

Layout and implementation for three different spiral designs (a–c) Embroidery layout for spiral patterns with three different wrap numbers k and similar thread densities. (d–f) Uninflated top views. (g–i) Inflated top views near the maximum rotation value for each actuator. (j–l) Side views used for comparing inflated shapes to the spherical cap model. These three designs were fabricated and inflated. As pressures increased, the membranes approached a maximum rotation angle (Table 1) determined by the wrap number k.

Figure 4

Rotation vs. pressure for an individual test of each of the three spiral designs in Figure 3, along with the spherical cap energy–balance model using torsional spring constant $\kappa$ = 0.06 N m, strain energy coefficient $\gamma$ = 30 J m⁻², and base radius r₀ = 0.038 m. Dashed lines indicate where the model extends beyond measured inflation pressures.

Figure 5

Sphere similarity metric vs. pressure, and measured vs. ideal spherical cap shape for three cases. A metric of 1 means the shape has 100% overlap with the spherical cap.

Figure 6

Torsional spring constant measured over the rotational range of three membranes with k = 0.44, k = 0.88, and k = 1.32.

Figure 7

Optical signal as a function of rotation angle for a membrane with an embedded spiral pattern of n = 24 and k = 0.88.