Bristle Skin for Soft Robots

Our “bristle” structure is formed the same way as the spacer fabric, but one face is unraveled after knitting. The resulting structure is very similar to a “plush” or “terry” knit, but with much longer loops than are typically produced in these processes. Knitting a sacrificial face helps ensure that the bristles are formed properly; without it, the monofilament may not drop cleanly off the needles after each row. The most predictable results are achieved if the “sacrificial” unraveled face was knit in the same kind of yarn as the remaining one.

Inspired by kirigami-clad pneumatic mechanisms, [51], we applied a biased bristle skin to a fiber-reinforced elastomeric extension actuator [16] Figure 12(a). When the actuator is pressurized, it stretches uniformly; when it is allowed to relax, the bristle structure acts as a ratchet and keeps the front part of the actuator in place while the back slides forward. This “caterpillar” robot is soft and lightweight.

Haptic feedback, sensing, and body support

By combining capacitive sensing with tunable thickness, a button pad can be made which highlights interface areas with visible, tactile thickness differences. These differentially raised areas can be used to form custom collections of soft controls–in this case the geometry of a soft button pad has been formed directly within the fabric of an armband, Figure 7 (and Figure 1). For this application as a button pad, we used simple capacitive touch sensing on the top surface only. The conductive yarn was plated alongside the main face yarn in the contact areas

Shearing bias for actuation

Spacer fabrics can have a slight but noticeable mechanical bias in the vertical direction: under pressure, the fabric will tend to shear to the same direction.

Integration into knitted objects

The flexibility of v-bed knitting as a process enables combining several knitting techniques within a knit object. Tendons can be incorporated within a face of knit fabric to transmit mechanical forces; we combine tendon knitting with spacer fabric to produce a “pre-stuffed” knit tendon assembly suitable for soft gripping. The full assembly has a “palm” with multiple fingers, each with rounded fingertips. The assembly is knit sideways, in the direction parallel to the fingers, and uses a horizontal inlay for the tendons. After knitting, the structure is fully bound-off and stable and it ready to attach to a 3D-printed snap-together mount.

The knit and tuck operations: The fabric faces are formed using the knit operation, which pulls a loop of yarn through a previous loop[s] of yarn (used with permission from). The filler yarn is attached with the tuck operation, which does not form a new row of face fabric.