3D Printing Exoskeletons, As Unique As Every Hand

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Fraunhofer-Institut für Werkzeugmaschinen und Umformtechnik IWU

FAULHABER Drive Systems

Wuxi InfiMotion Technology Co., Ltd.

Fraunhofer IWU utilizes innovative shape memory alloys, stepper motors, and 3D printing technologies for hand exoskeletons that train and support hand mobility after accidents or strokes.

Especially after accidents with tendon injuries or as a consequence of strokes, the mobility of the hands can be impaired. As a therapeutic aid, exoskeletons are increasingly being used, which are placed over the hand like a second skeleton and can specifically move the wrist and fingers. However, only a product that is fully tailored to the patient's hand and meets important requirements such as low weight and compact dimensions provides good conditions for therapeutic success. Because a support and movement structure that hinders its wearer more than it supports often ends up unused on the shelf.

Why Customize Exoskeletons?

The Fraunhofer IWU is now focusing on innovative shape-memory alloys, stepper motors, and 3D printing technologies to provide custom-fit and gently supportive exoskeletons. The idea: the exoskeleton itself must first be perfectly adapted to the anatomy of the hand. Human hands differ not only significantly in size but also in proportions. There are even differences between the left and right hand of the same person.

The production of the exoskeleton via 3D printing using the selective laser sintering (SLS) process offers virtually unlimited geometric freedom. In this process, three-dimensional shapes are built layer by layer from plastic powder. The manufacturing process can be made particularly efficient if a parametric CAD model serves as the basis. Ideally, a digital 3D scan of the patient's hand is available, so that the geometric shape also maps the relationships (parameters) between the individual elements of the hand. In this way, the printing creates not only a particularly precise "negative" of the hand, but any necessary adjustments for future reproductions are also easier to implement with a CAD model as the foundation.

Exoskeleton Requires Compact And Precise Drive Technology

But not only is the anatomy of each hand unique. Hand strength also varies significantly. For most patients, it makes sense to customize the strength and stroke of a hand exoskeleton. At the same time, a high weight of the aid would be very burdensome.

In addition to the fit of the shell, the drive mechanism is particularly important. Alina Carabello, a doctoral student at Fraunhofer IWU and a research associate at Chemnitz University of Technology, relies on a combination of a bidirectional stepper motor and actuator wires made of shape-memory alloys. This concept is based on just two artificial tendons, which the stepper motor precisely controls for flexion and extension movements of the hand and fingers. Various positions can be maintained and readjusted as needed. For instance, readjustment is required when handling a plastic mineral water bottle that gives way as the cap is opened and carbon dioxide escapes.

This is How the Drive System Works

The step drive is based on the elements of a gear, lever, pawl, and shape-memory alloy (SMA) wire. When the wire is heated, it contracts and moves the lever by a defined angle. This effectively moves the gear, driving it in the desired direction; the cable of the extension or flexion tendon is wound onto or unwound from a spool. This tendon movement causes the finger to flex or extend.

In Carabello's step drive, shape-memory alloys (SMAs) act as actuators: their ability to change shape controls the movement of the gear. When an SMA wire is selectively heated by electrical current from a microcontroller, it contracts, and as it cools down again, it extends due to a counterforce, such as a spring or another SMA wire. This mechanism controls the artificial limbs of the exoskeleton. Overall, a very precise and repeatable movement can be achieved; the gentle response behavior of the SMA wires ensures smooth movement steps.

Tailor-Made Exoskeletons are Just the Beginning

The Fraunhofer IWU sees a need for further individualization in many medical technology aids to better address the needs of patients and provide more targeted support for therapeutic personnel. Many aids are based on standard specifications and are rarely adapted or even adaptable to the age, gender, size, weight, or actual limitations of patients.

With its hand exoskeleton, Fraunhofer IWU primarily targets tendon injuries following accidents. Even after strokes or with (congenital) paralysis, it is important to move the affected hand during the recovery process—often in closer intervals than the time budget of therapists allows. Here, the exoskeleton developed at Fraunhofer IWU can provide relief, as the continuous presence of a caregiver is not necessarily required.

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