Anisotropic 3D-Knitted Sleeves as Inverse Pneumatic Artificial Muscles for Soft and Wearable Robotics

Inverse Pneumatic Artificial Muscles (IPAMs) have emerged as an alternative to traditional artificial muscle approaches due to their exhibited high deformations. However, existing IPAM designs often require complex fabrication processes involving fiber winding or embedding around inflatable components, limiting adaptability and scalability. In this study, we present a novel approach to IPAM design using digitally designed 3D-knitted sleeves as seamless external shells. These sleeves, made from a combination of Elastane and cotton threads, exhibit tailored anisotropic strain properties to achieve high axial strain of up to 117% and force output up to 3.2 N while minimizing radial expansion. Our method simplifies assembly by eliminating the need for bonding or mechanical tensioning between the sleeve and the inflatable bladder, enhancing modularity and ease of modification. We compare three sleeve designs created through digital knitting to demonstrate the versatility of this technique. The optimal design is further characterized for repeatable strain and blocked force performance, and we show how minor structural modifications enable programmable shape formation. This work establishes a foundation for scalable, cost-effective production of adaptable IPAMs, suitable for soft robotics and wearable technology applications.