Let us imagine a future in which a prosthesis is no longer perceived as a technical replacement part, but as part of one’s own body. An artificial hand could grasp with softness and precision, an artificial arm could perform fluid movements, and an exoskeleton could help people with paralysis to move their arms or legs again. Such technologies also raise fundamental questions. What makes a movement feel natural? Where does the biological body end, and where does its technological extension begin? And how does our understanding of what it means to be human change when lost bodily functions can be redesigned?

The human musculoskeletal system consists of the skeleton, muscles, tendons, and joints. The skeleton provides the body’s stable structure. Bones bear loads, give the body its shape, and protect sensitive organs. Joints connect the bones and enable movements such as bending, extending, or rotating. Muscles generate the force required for these movements. They contract and transmit this force to the bones through tendons. Tendons act as resilient connecting elements between soft muscle tissue and hard bone structures.

This biological system is both strong and flexible. Muscles and tendons can store energy, absorb shocks, and adapt to changing loads. An arm movement is not generated by a single rigid drive, but by many muscles working together in precise coordination. This makes human movements fluid, flexible, and safe when interacting with our surroundings. When we pick up something fragile, we modulate the force we apply. When we encounter resistance, the body can yield.

Traditional robots are generally built differently. They often consist of rigid segments, joints, electric motors, gears, and bearings. A motor generates rotational motion, a gearbox converts this motion, and a joint moves a rigid component. This principle has been highly successful in industrial applications. Robots can work quickly and precisely, with a high degree of repeatability. For applications involving close interaction with the human body, however, this type of construction has disadvantages. Rigid drives are often heavy and mechanically complex, and can feel uncomfortable or unsafe when in direct contact with people.

The exhibit explores this difference. It consists of a 3D-printed skeletal arm that is moved or deformed by a HASEL actuator. HASEL stands for “Hydraulically Amplified Self-Healing Electrostatic”. The printed arm functions as a simplified bone structure. The artificial muscle acts as a soft, active actuator. This illustrates what a technical system might look like if it were modelled more closely on the human musculoskeletal system instead of being based solely on traditional robotic principles.

HASEL actuators are soft artificial muscles that convert electrical energy into mechanical movement. Their basic structure consists of a thin, flexible enclosure filled with an electrically insulating fluid. Flexible electrodes are located on the outside. When an electrical voltage is applied, electrostatic forces pull the electrodes closer together. This displaces the fluid inside, causing the actuator to change shape.

Depending on its design, this change in shape can be used in different ways. A HASEL actuator can contract, expand, bend, or generate a tensile force. The fluid inside acts as a hydraulic medium. It distributes and amplifies the local electrostatic force, resulting in a larger movement of the entire actuator. An electrical signal is thus converted into a controllable mechanical deformation. The term “self-healing” refers to the ability of HASEL actuators, under certain conditions, to repair damage caused by electrical failure and continue operating, which increases their robustness against local material defects.

For lifelike designs, it is particularly important that HASEL actuators are soft, lightweight, and deformable. Unlike a conventional electric motor, they do not need to be fitted into a joint as a rigid component. Instead, they can be positioned as muscle-like elements along a framework. This makes it possible to develop structures that more closely resemble biological limbs. The actuator can operate closer to the point where movement is required. The system’s flexibility allows it to interact more safely with people and its surroundings.

In the long term, this research aims to help make prostheses and exoskeletons more natural, lighter, and more capable. To achieve this, many technical questions still need to be addressed. Which film materials are suitable? Which fluids offer good electrical and mechanical properties? How should the electrodes be shaped to produce the desired movement? How can the actuator be controlled safely? How long can it withstand repeated loads? How can force, speed, and position be reliably measured and controlled?

A future prosthesis could use artificial muscles to perform movements that are softer and more natural than those of today’s motorised systems. A future exoskeleton could support paralysed or weakened limbs without appearing like a heavy external machine. Instead, it could behave more like an additional layer of active muscle. The aim is not merely to make movement technically possible, but to create functional movement suitable for everyday life. People should be able to grasp, walk, stand, or stabilise themselves without having to constantly think about the technology. In the context of the posthuman, the project demonstrates how the human body can increasingly be technologically extended, supported, and reconstructed. This development begins with concrete research into materials, actuator technology, and mechanical design.

Text by Philipp Rothemund
Philipp Rothemund is an engineer