Our hands are artworks. 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 with out 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 from silicone which can be moved around with air pressure. A 3-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 within the forearm alone.
In tests, able-bodied volunteers used the hand to select 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 start 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 looks like a lost limb,” study writer Sriramana Sankar said in a press release.
Softening Up
Prosthetic hands have come a great distance. One among the primary, crafted out of metal within the Middle Ages, had joints that could possibly be moved passively using one other hand.
Today, soft robotics have modified the sport. 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 an issue. They’ll’t consistently generate the identical force to select up heavy objects. Even with multiple joints and a dynamic palm, squishy robotic hands have a harder time detecting different textures in comparison with their rigid counterparts, wrote the team. They’re also weak. Existing soft robotic hands can only lift around 2.8 kilos.
In contrast, our hands have each a rigid skeleton and soft tissues—muscles and tendons—that stretch, twist, and contract. Pressure sensors in our skin provide quick feedback: Am I squeezing a luxurious toy, holding a slippery coffee mug, or manipulating my phone?
That why recent prosthetic designs incorporate each artificial skeletons and muscles.
For instance, the commercially available LUKE arm has a metal and plastic skeleton for strength and stability. Its fingertips have soft materials for higher dexterity. The prosthetic can grab objects using different inputs—for instance, electrical signals from muscles or a foot peddle to change between grasp strengths. However the hand continues to be mostly rigid and has limited mobility. The thumb and index finger can flex individually. All the opposite fingers move together.
Then there’s the issue of feedback. Our fingers use touch to calibrate our grip. Each of the skin’s three layers encodes barely different sensations with quite a lot of receptors, or biological sensors. The outer layer feels light touch and slow vibration, like when hair calmly brushes your hand. Deeper layers detect pressure: the feel and weight of a heavy dumbbell, for instance.
In 2018, the team behind the brand new study developed electronic skin inspired by human skin. The fabric, 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 within the sensors allowed an upper-limb amputee to detect a spread of sensations, including pressure.
“In case you’re holding a cup of coffee, how do you already know you are about to drop it? Your palm and fingertips send signals to your brain that the cup is slipping,” study writer Nitish Thakor said within the recent study’s press release. “Our system is neurally inspired—it models the hand’s touch receptors to supply 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 brand new design incorporated E-dermis right into a hybrid hand designed to mimic a human hand.
The thumb has two joints made from silicone and the fingers have three. Each joint can flex independently. These hook up with a rigid 3D-printed skeleton and are moved about by air.
In comparison with prosthetics with only soft components, the skeleton adds force and may support heavier weights. The prosthetic hand’s fingertips are covered in a patch of E-dermis the dimensions 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 an individual 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 of their upper arms—mimicking those used for amputees—and to send them along to the robotic hand.
With minimal training, the volunteers could grab quite a lot of objects of various sizes, weights, and textures. The hand gently picked up a sponge, without squishing it into oblivion, and quite a lot of produce—apple, orange, clementine—without bruising it. The prosthetic showed it could also lift heavier items, corresponding to a small stone statue and a metal water bottle.
But the very best example, in keeping with the authors, was when it held a fragile plastic cup crammed with water using only three fingers. The hand didn’t dent the cup or spill any water.
Overall, it had a formidable 99.7 percent accuracy rate handling 15 on a regular basis items, rapidly adjusting its grip to avoid drops, spills, and other potential mishaps.
To be clear, the device hasn’t been tested on individuals who’ve lost a hand. And there’s more to enhance. Adding a tendon of sorts between the substitute fingers could make them more stable. Mimicking how the palm moves could further boost flexibility. And adding sensors, corresponding to 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 could have to seamlessly integrate into on a regular basis living, coping with all the variability we do. “That is why a hybrid robot, designed just like the human hand, is so useful—it combines soft and rigid structures, similar to our skin, tissue, and bones.”
