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AMS Spring 2023

4D Printed Splint Self-Tightens on Wearer’s Wrist

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The University of Stuttgart and the University of Freiburg in Germany have developed a new design method for 4D printing and have created an orthotic that contracts and forms to the wearer’s biology. The design is inspired by nature and paves the way for future pre-programmable devices that reshape and adjust to a patient’s unique anatomy. 4D printing, a form of 3D printing objects that are pre-programmed to change shape or change behavior over time, has come a long way, but how far away is this technology from broader use?

The design has been inspired by the Air Potato (Dioscorea bulbifera), an invasive vine that lightly wraps around the trunk of its host plant and then releases “stipules” on its inner surface, creating enough pressure to climb up the tree. The movement is structurally predetermined and requires little to no internal metabolic energy from the plant. This occurs through deformation at the cellular level, when individual cells and tissues absorb water molecules from the environment, and, depending on their orientation, will cause a swelling or shrinking. These passive-nastic movements in nature inspire promising designs in 4D printing and can be emulated through functional additive bilayers.

As outlined in the journal Advanced Science, these scientists were able to transfer the mechanical movements of D.Bulbifera into a computational design for a common orthotic forearm-wrist splint. Like the plant, the orthotic slowly adapts, forms, and tightens around the individual’s unique anatomy. The splint is printed using a combination of wood filament and stimuli-responsive actuating material and features a complex structure of pocket mechanisms which apply pressure and tighten in desired areas.

Air Potato (D. Bulbifera)

To pre-program a self-adjusting splint, a medical specialist first designs the splint on a patient’s arm, which is 3D scanned and digitally constructed. The splint design now becomes a workable surface and is unrolled. Each motion mechanism on the unrolled splint is programmed with the target curvature information, bending directions, orientations, and magnitudes. Finally, the toolpaths are generated and the piece is fabricated.

The journal publication speculates that moisture-stimulated adaptive tensioning orthotic and prosthetic devices have a promising future healing and improving the quality of life of the patients they serve. A wearable device potentially can be designed to release pressure and allow ventilation in response to a change in moisture, say for example during exercise or during a shower. Long-term devices can be programmed to tighten over time in order to compensate for muscle atrophy, relieving the need for readjustments throughout the lifetime of the device.

“Programming a self-adjusting orthotic splint, demonstrated through a top-down, inverse modeling approach: a) the envisaged process begins with the physical modeling of a splint design on the patient’s arm by a medical care specialist, which is then 3D-scanned, allowing both the splint and arm geometries to be digitally reconstructed; b) the splint design is processed as a surface and analyzed, then unrolled with the target curvature information (bending directions, orientations, and magnitudes) assigned to each motion mechanism in the flat assembly, and finally the toolpaths are generated for fabrication; c) the adaptive 4D-printed proof of concept is worn by a user.” Image/text courtesy of Advance Science.

Conventional orthoses and prostheses are often mass manufactured or custom-made with plaster castings and moldings. These devices are notorious for their discomfort and their need to be revised, adjusted, or redone as time passes and as the body changes. Additive manufacturing solves these issues by being completely customizable to a patient’s unique anatomy and pathology. The first working prototype of the 4D printed forearm-wrist splint shows a future where medical practitioners without any specialized knowledge in digital modeling will be able to create custom-fit self-adjusting orthotic devices.

Industry applications for 4D printing are nearly endless. Whether it be bioprinting human organs or producing running shoes, even in its early stages 4D printing already offers a wide range of opportunity. 4D printing up until this point has often depended on specialized printers. The combination filament for the forearm-wrist orthotic can be printed using any standard fused filament fabrication 3D printer, signifying a step forward towards mass adoption.  The universities’ 4D orthotic is one of the first examples of a pre-programmed bio-inspired adaptive system, opening up a design space for a multitude of on-demand printable applications.

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