  Granular jamming is a popular soft actuation mechanism that provides high
stiffness variability with minimum volume variation. Jamming is particularly
interesting from a design perspective, as a myriad of design parameters can
potentially be exploited to induce a diverse variety of useful behaviours. To
date, grain shape has been largely ignored. Here, we focus on the use of 3D
printing to expose design variables related to grain shape and size. Grains are
represented by parameterised superquadrics (superellipsoids); four diverse
shapes are investigated along with three size variations. Grains are 3D printed
at high resolution and performance is assessed in experimental pull-off testing
on a variety of benchmark test objects. We show that grain shape and size are
key determinants in granular gripping performance. Moreover, there is no
universally-optimal grain shape for gripping. Optical imaging assesses the
accuracy of printed shapes compared to their ideal models. Results suggest that
optimisation of grain shape is a key enabler for high-performance, bespoke,
actuation behaviour and can be exploited to expand the range and performance of
granular grippers across a range of diverse usage scenarios.
