A Bionic Hand that Can Grip Like a Human

Andrew Corselli

(Image: Sriramana Sankar/Johns Hopkins University)

Johns Hopkins University engineers have developed a pioneering prosthetic hand that can grip plush toys, water bottles, and other everyday objects like a human, carefully conforming and adjusting its grasp to avoid damaging or mishandling whatever it holds.

The system's hybrid design is a first for robotic hands, which have typically been too rigid or too soft to replicate a human's touch when handling objects of varying textures and materials. The innovation offers a promising solution for people with hand loss and could improve how robotic arms interact with their environment.

Details about the device recently appeared in Science Advances.

"The goal from the beginning has been to create a prosthetic hand that we model based on the human hand's physical and sensing capabilities — a more natural prosthetic that functions and feels like a lost limb," said Research Lead Sriramana Sankar, Johns Hopkins Ph.D. Student in Biomedical Engineering. "We want to give people with upper-limb loss the ability to safely and freely interact with their environment, to feel and hold their loved ones without concern of hurting them."

The device, developed by the same Neuroengineering and Biomedical Instrumentations Lab that in 2018 created the world's first electronic "skin" with a humanlike sense of pain, features a multifinger system with rubberlike polymers and a rigid 3D-printed internal skeleton. Its three layers of tactile sensors, inspired by the layers of human skin, allow it to grasp and distinguish objects of various shapes and surface textures, rather than just detect touch. Each of its soft air-filled finger joints can be controlled with the forearm's muscles, and machine learning algorithms focus the signals from the artificial touch receptors to create a realistic sense of touch, Sankar said.

Here is an exclusive Tech Briefs interview, edited for length and clarity, with Sankar.

Tech Briefs: What was the biggest technical challenge you faced while developing this bionic hand?

Sankar: There were a few different things. The first was COVID, which affected the early development, so for the first couple of years we had to focus on the design and virtual simulations before finally being able to start building the hands and doing testing.

The second challenge was that in a lot of ways this is at the forefront of research, so there weren’t very many guidelines we could follow for our planning. That's where a lot of mistakes can happen. We had to do many different iterations, keeping up with the initial goal, but then needing to pivot when things didn’t work properly.

Finally, part of this is working in a university setting — there's not a huge budget, not a ton of professional engineers working on it. So, it's a lot of undergraduate teams trying to make do with smaller budgets.

Tech Briefs: Can you please explain in simple terms how it works?

Sankar: Conceptually, we're trying to create a hybrid of soft and rigid robotics as the main control system. The actual joints, the points that you control, are soft robotic actuators, basically little silicone balloons. When you pump air into the joints they expand and, because of how the joints have been designed, as more air gets pumped into them, the higher pressure makes them bend. So, each of these joints is a whole pneumatic system. Whatever force and distance we want the hand to move toward, or whichever joints we want to move — whichever fingers; those specific joints get a certain amount of air pressure, which then leads to a certain level of flexion for each of those fingers and joints and all of them together create a full power grasp.

Then there's the sensing side of it, where the sensors are embedded in different layers. So, we're able to collect that data, and translate it to neuromorphic information: action potentials and spikes — the language of nerves.

Tech Briefs: Do you have any set plans for further research work, et cetera? And if not, what are your next steps?

Sankar: I just finished my Ph.D. in January, and this was my main project. So, I'm not going to be directly working on it anymore. However, the lab has plans to improve upon it. This was a very early stage, so there are a lot of issues with it, and it's in a very simple state. The goal would be trying to improve all the things we've built, while maintaining the core ideas.

One goal is redesigning the fingers to increase their strength, adding a lot more sensors. This is something that's already been worked on — temperature sensing, having more tactile sensors throughout the fingers — just making it a more cohesive, rounded-out system like the human hand.

Then, finally, right now all the robotics components were focused on the fingers. So, the next stage would be trying to also have a hybrid robotic palm.

Tech Briefs: Do you have any advice for researchers aiming to bring their ideas to fruition, broadly speaking?

Sankar: The main thing I would say is being ready for challenges, pushbacks, and rejections. If you truly believe in your idea and want to keep going forward with it, you have to learn from the critiques that people give you and use it to improve your ideas and persevere in that way. And some of it is just luck, sometimes it doesn't work the way you planned. If there is feedback, especially from papers and reviewers, or other critiques, try to use what they offer and don’t take a critique personally, try to learn from it.