Robot Body Building

Despite the impressive technology and complex mechanical design of today’s most advanced robots, their physical capabilities pale in comparison to what biological organisms are capable of. Most robots are very much purpose-built, and when conditions differ even a little bit from their expectations, they fail spectacularly. Contrast that with humans and animals, where adaptation to different environments comes naturally.

At least part of the reason for this shortcoming of robots stems from the way their actuators are designed. Whereas biological organisms rely on muscle tissue, bones, and tendons that can conform and adapt to the world around them for movement, the stuff of robots is completely different. Not only are their actuators typically made of rigid, non-compliant materials, but they also move in unnatural ways. And a spinning electric motor and set of gears is simply not as adaptable as muscle tissue.

The most obvious solution to this problem would be to mimic the function of biological musculoskeletal systems, and that is exactly what a group of researchers at Northwestern University is attempting to do. They have developed a system containing stretchy, muscle-like actuators, plastic bones, and elastic artificial tendons that are more flexible than traditional robotic actuation systems. Their approach also includes sensors to simulate the feedback provided by biological sensory receptors.

The artificial muscle is based on a 3D‑printed cylindrical structure called a “handed shearing auxetic” (HSA), which has a unique geometry that allows it to extend and expand when twisted. By coupling this HSA with a small, integrated servo motor, the researchers can convert rotational motion into linear extension and contraction, essentially mimicking how biological muscle fibers shorten and lengthen.

The HSA is encased within a Yoshimura origami-inspired bellows. This outer structure constrains unwanted rotation while still permitting the actuator to expand and contract. Both the HSA and the bellows are 3D‑printed from thermoplastic polyurethane, a lightweight, flexible rubber-like material commonly used in consumer products. The result is an actuator that is not only mechanically compliant but also strong. It was shown to be capable of lifting objects up to 17 times its own weight.

The actuators can be powered by portable batteries, avoiding the bulky compressors or external high‑voltage supplies required by many other soft actuator technologies. Tests revealed that the system can achieve actuation strokes of nearly 30% of its length and generate forces around 75 newtons (comparable to lifting several kilograms) while remaining lightweight and flexible.

To demonstrate the potential of their design, the team built a life‑size robotic leg with 3D-printed plastic bones, rubber tendons, and three artificial muscles functioning as a quadricep, hamstring, and calf. The leg was able to bend at the knee and ankle and even kick a volleyball off a pedestal. A flexible 3D‑printed sensor embedded in the leg allowed it to “feel” its own movements by changing electrical resistance as the muscle stretched or contracted.

The researchers believe this bioinspired approach could eventually lead to robots that walk, run, and interact with humans in safer and more adaptable ways. By combining soft, muscle‑like actuators with bone‑ and tendon‑like elements, robots may one day achieve the efficiency, resilience, and natural motion seen in animals, which is something machines have struggled with for decades.