Our hands are works of art. A rigid skeleton provides structure. Muscles adjust to different weights. Our skin, embedded with touch, pressure, and temperature sensors, provides immediate feedback on what we’re touching. Flexible joints make it possible to type on a keyboard or use a video game controller without a thought.
Now, a team at Johns Hopkins University has recreated these perks in a life-like prosthetic robot hand. At its core is a 3D-printed skeleton. Each finger has three independently controlled joints made of silicone that are moved around with air pressure. A three-layer electronic skin covering the hand’s fingertips helps it gauge grip strength on the fly. The hand is controlled using electrical signals from muscles in the forearm alone.
In tests, able-bodied volunteers used the hand to pick up stuffed toys and dish sponges without excessive squeezing. It adjusted its grip when challenged with heavy metal water bottles and prickly pineapples—picking up items without dropping them or damaging the hand.
“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,” study author Sriramana Sankar said in a press release.
Softening Up
Prosthetic hands have come a long way. One of the first, crafted out of metal in the Middle Ages, had joints that could be moved passively using another hand.
Today, soft robotics have changed the game. Unlike rigid, unforgiving material, spongy hands can handle delicate objects without distorting or crushing them. Integrated sensors for pressure or temperature make them more life-like by providing sensory feedback.
But soft materials have a problem. They can’t consistently generate the same force to pick up heavy objects. Even with multiple joints and a dynamic palm, squishy robotic hands have a harder time detecting different textures compared to their rigid counterparts, wrote the team. They’re also weak. Existing soft robotic hands can only lift around 2.8 pounds.
In contrast, our hands have both a rigid skeleton and soft tissues—muscles and tendons—that stretch, twist, and contract. Pressure sensors in our skin provide instant feedback: Am I squeezing a plush toy, holding a slippery coffee mug, or manipulating my phone?
That why recent prosthetic designs incorporate both artificial skeletons and muscles.
For example, the commercially available LUKE arm has a metal and plastic skeleton for strength and stability. Its fingertips have soft materials for better dexterity. The prosthetic can grab objects using different inputs—for example, electrical signals from muscles or a foot peddle to switch between grasp strengths. But the hand is still mostly rigid and has limited mobility. The thumb and index finger can flex individually. All the other fingers move together.
Then there’s the problem of feedback. Our fingers use touch to calibrate our grip. Each of the skin’s three layers encodes slightly different sensations with a variety of receptors, or biological sensors. The outer layer feels light touch and slow vibration, like when hair lightly brushes your hand. Deeper layers detect pressure: the texture and weight of a heavy dumbbell, for example.
In 2018, the team behind the new study developed electronic skin inspired by human skin. The material, or E-dermis, sensed textures and transmitted them to surviving nerves in an amputee’s arm with small zaps of electricity. The skin used piezoresistive sensors, such that pressure would change how the sensors conducted electricity. Prosthetic fingertips coated in the sensors allowed an upper-limb amputee to detect a range of sensations, including pressure.
“If you’re holding a cup of coffee, how do you know you’re about to drop it? Your palm and fingertips send signals to your brain that the cup is slipping,” study author Nitish Thakor said in the recent study’s press release. “Our system is neurally inspired—it models the hand’s touch receptors to produce nerve-like messages so the prosthetics’ ‘brain,’ or its computer, understands if something is hot or cold, soft or hard, or slipping from the grip.”
Hands On
The new design incorporated E-dermis into a hybrid hand designed to mimic a human hand.
The thumb has two joints made of silicone and the fingers have three. Each joint can flex independently. These connect to a rigid 3D-printed skeleton and are moved about by air.
Compared to prosthetics with only soft components, the skeleton adds force and can support heavier weights. The prosthetic hand’s fingertips are covered in a patch of E-dermis the size of a fingernail. Each finger bends naturally, curling into the palm or stretching apart.
Electrical signals from a user’s forearm muscles control the hand. Such devices, dubbed myoelectric prostheses, tap into living nerve endings above the amputation site. When a person thinks of moving the hand, a microprocessor translates the nerve signals into motor commands.
Several studies with able-bodied volunteers showcased the hand’s dexterity. Participants wore a sheath over their forearms to capture the electrical signals in their upper arms—mimicking those used for amputees—and to send them along to the robotic hand.
With minimal training, the volunteers could grab a variety of objects of different sizes, weights, and textures. The hand gently picked up a sponge, without squishing it into oblivion, and a variety of produce—apple, orange, clementine—without bruising it. The prosthetic showed it could also lift heavier items, such as a small stone statue and a metal water bottle.
But the best example, according to the authors, was when it held a fragile plastic cup filled with water using only three fingers. The hand didn’t dent the cup or spill any water.
Overall, it had an impressive 99.7 percent accuracy rate handling 15 everyday items, rapidly adjusting its grip to avoid drops, spills, and other potential mishaps.
To be clear, the device hasn’t been tested on people who’ve lost a hand. And there’s more to improve. Adding a tendon of sorts between the artificial fingers could make them more stable. Mimicking how the palm moves could further boost flexibility. And adding sensors, such as those for temperature, could push the engineered hand even closer to a human’s.
Improving the dexterity of the hands isn’t only “essential for next-generation prostheses,” said Thakor. Future robotic hands will have to seamlessly integrate into everyday living, dealing with all the variety we do. “That’s why a hybrid robot, designed like the human hand, is so valuable—it combines soft and rigid structures, just like our skin, tissue, and bones.”
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