Fellow Baltimore-area business – Infinite Biomedical Technologies (IBT) – partnered with our team at Root3 Labs to design a multi-electrode armband sensor for wearers impacted by limb loss. IBT is over 25 years old and has its origins in the research labs of Johns Hopkins. They specialize in gesture-based control strategies for prosthetic limbs through electrode-informed pattern recognition.
“Electrode-informed pattern recognition”? What does that mean, exactly?
An amputee wearing IBT electrodes will activate any remaining limb musculature through a full range of different perceived grips or gesticulations. Usually this is done in a clinical setting with the help of a physician. The electrodes then record and pass Electromyographic (EMG) data to a learning algorithm which creates a unique signature for each unique grip. From there, the logic looks for the same EMG signatures in real-time to produce the amputee’s desired grip in their own electromechanical prosthetic limb.
To accomplish this biomedical feat, IBT needed eight of their custom electrodes arrayed uniformly around the armband. The band was sized nominally for the smallest dimension measured on an adult upper arm, which was also adjusted down for potential atrophy. It would also be able to stretch to accommodate larger arms. So, the band needed to be able to stretch equally between each electrode position, rather than adjust from the end like a wristwatch or belt. IBT offers the lowest-profile electrode on the market for this application, so maintaining a small size and weight was important.
Additionally, the armband would make direct skin contact, so material selection was important for compatibility, comfort, and wipe-down sterilization between users. Most critically, the expandable armband itself encapsulates all the electrical wiring between the eight electrodes. Electrical wires do not elongate, so we had to design a system that could expand like mechanical joints rather than stretchable rubber while avoiding fatigue in the thin wires.
Materials and the Development Process for Biomedical Devices
Silicone and urethane were two of our material selections. Silicone was an obvious choice due to its excellent percentage of elongation, ubiquitous use in biomedical applications, excellent chemical compatibility, moldability, and low cost. Urethane was a good alternative choice with its percent elongation, resistance to tear propagation, availability in medical grades, good chemical compatibility, moldability, and low cost.
At Root3 Labs, we have extensive experience laser cutting and 3D printing custom low-cost molds for rubber casting prototype parts in low quantities. In this case, several different styles of silicone and urethane rubber interconnects were cast in custom molds. From there, the different solutions were tested by hand for their ability to perform as a flexible housing for electrical interconnects, as well as a viable rubber armband. To save our partner the cost of injection mold tooling, we 3D printed components in-house and laser-cut our own acrylic. We then went through several rounds of prototype iterations made from real silicone and urethane in different durometers and grades.
- User Experience and 99% Percentile Considerations
- Reduced Size and Complexity
- Patterned Subcomponent Design for Cost Savings
- Embedded Flexible Electronics
- Design for Manufacturing
- Material Selection
- Rubber Molding
- Laser Cut Jigs and Mold Patterns
- Potted Flexible Conductors