Researchers are trying to make it possible to sense textures on a computer screen and with artificial limbs.

Hand: Image Source/ Getty Images Plus; Sweaters: CerebroCreative/iStock/Getty Images Plus; Phone: Issarawat Tattong/Getty Images Plus; T. Tibbitts

On most mornings, Jeremy D. Brown eats an avocado. But first, he gives it a little squeeze. A ripe avocado will yield to that pressure, but not too much. Brown also weighs the fruit in his hand. He feels the waxy skin’s bumps and ridges.

“I can’t imagine not having the sense of touch to be able to do something as simple as judging the ripeness of that avocado,” says Brown. He’s a mechanical engineer at Johns Hopkins University. That’s in Baltimore, Md. Brown studies haptic feedback. That’s information conveyed through touch.

Many of us have thought about touch more than usual during the COVID-19 pandemic. Hugs and high fives have been rare. More online shopping has meant fewer chances to touch things before buying. People have missed out on trips to the beach where they might have sifted sand through their fingers. A lot goes into each of those sensory acts.

Our sense of touch is very complex. Every sensation arises from thousands of nerve fibers and millions of brain cells, explains Sliman Bensmaia. He’s a neuroscientist at the University of Chicago in Illinois. Nerve receptors detect cues about pressure, shape, motion, texture, temperature and more. Those cues activate nerve cells, or neurons. The central nervous system interprets those patterns of activity. It tells you if something is smooth or rough, wet or dry, moving or still.

Neuroscience is at the heart of research on touch. But Brown and other engineers study touch, too. So do experts in math and materials science. They want to translate the science of touch into helpful applications. Their work may lead to new technologies that mimic tactile sensations.

Some scientists are learning more about how our nervous system responds to touch. Others are studying how our skin interacts with different materials. Still others want to know how to produce and send simulated touch sensations.

All these efforts present challenges. But progress is underway. And the potential impacts are broad. Virtual reality may get more realistic. Online shoppers might someday “touch” products before buying them. Doctors could give physical exams online. And people who have lost limbs might regain some sensation through prostheses.

Virtual reality is already pretty immersive. Users can wander through the International Space Station. Or, they can visit Antarctica. These artificial worlds often have realistic sights and sounds. What they lack is realistic touch. That would require reproducing the signals that trigger haptic — or touch — sensations.

Our bodies are covered in nerve endings that respond to touch. Some receptors track where our body parts are. Others sense pain and temperature. One goal for researchers is to mimic sensations that arise from force and movement. Those include pressure, sliding and rubbing.

A few different types of receptors respond to force and movement. One is the Pacinian corpuscles that lie deep within the skin. They are especially good at picking up vibrations caused by touching different textures. When stimulated, these receptors send signals to the brain. The brain interprets those signals as a texture. Bensmaia compares this to hearing a series of notes and recognizing a tune.

Four main types of touch receptors respond to a mechanical stimulation of the skin. They are known as Meissner corpuscles, Merkel cells, Ruffini endings and Pacinian corpuscles. Some respond better than others to certain types of stimuli. Recent studies have focused on the deep-skin Pacinian corpuscles. Those receptors respond to vibrations created as fingers rub against textured materials.

“Corduroy will produce one set of vibrations,” Bensmaia says. Types of silk produce other sets. Scientists can measure those sets of vibrations. That work is a first step toward reproducing the feel of different textures.

But creating the right vibration pattern is not enough. Any stimulation meant to mimic a texture must be strong enough to trigger the skin’s touch receptors. And researchers are still figuring out how strong is strong enough.

For instance, vibrations caused by textures create different types of wave energy. One team found that rolling-type waves called Rayleigh (RAY-lee) waves go deep enough to reach Pacinian receptors. (Much larger versions of those waves ripple through Earth during earthquakes.) The team shared this finding last October in Science Advances.

This graph depicts a “universal scaling law.” It show how long the wavelength of Rayleigh waves need to be to trigger Pacinian touch receptors in a mammal’s skin. The thicker a species’ skin, the longer those waves must be for certain “touches” to be felt. The relatively shallow receptors in human skin, for instance, respond to shorter waves than the deeper ones in sperm-whale skin. This trend holds true for many mammals — but not very small ones, such as mice.

The size of Rayleigh waves also matters. For the most part, those waves must be at least 2.5 times as long as the depth of those Pacinian receptors in the skin. That’s enough for a person — and most other mammals — to feel a sense of touch through those receptors, explains James Andrews. He’s a mathematician at the University of Birmingham in England. His team discovered this rule by looking at studies that involved animals — including dogs, dolphins and rhinos.

This work helps reveal what it takes to realistically capture touch, Andrews says. New devices could use such information to convey touch sensations to users. Some might do this using ultrasonic waves or other techniques. And that might someday lead to virtual hugs and other tactile experiences in virtual reality.

Cynthia Hipwell moved into a new house before the pandemic. She looked at some couches online but she couldn’t bring herself to buy one from a website. “I didn’t want to choose couch fabric without feeling it,” says Hipwell. A mechanical engineer, she works at Texas A&M University in College Station.

She imagines that one day, “if you’re shopping on Amazon, you could feel fabric.” We’re not there yet. But touch screens that mimic different textures are being developed. What may make them possible is harnessing shifts in electrical charge or vibrations. Touching the screen would then tell you whether a sweater is soft or scratchy. Or if a couch’s fabric feels bumpy or smooth.

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To make that happen, researchers need to know what affects how a screen feels. Surface features that are nanometers (billionths of a meter) high affect a screen’s texture. Tiny differences in moisture also change a screen’s feel. That’s because moisture alters the friction between your fingers and the glass. Even shifts in electrical charge play a role. These shifts change the attraction between finger and screen. Such attraction is called electroadhesion (Ee-LEK-troh-ad-HEE-shun).

Hipwell’s group built a computer model that accounts for those effects. It also accounts for how someone’s skin squishes when pressed against glass. The team shared its work in the March 2020 IEEE Transactions on Haptics.

Hipwell hopes this computer program can help product designers make screens that can provide a sense of what a displayed object might feel like. Those screens could be used for more than online shopping. A car’s dashboard might have sections that change texture for each menu, Hipwell says. A driver might change temperature settings or radio stations by touch while keeping her eyes on the road.

Virtual doctor visits rose sharply during early days of the COVID-19 pandemic. But online appointments have limitations. Video doesn’t let doctors feel for swollen glands or press an abdomen to check for lumps. Remote medicine with a sense of touch might help at times when the doctor and patient can’t meet up in person. And it could be useful for people who live in areas far from doctors.

People in those places might someday get touch-sensing equipment at home. So might a pharmacy clinic, workplace — even the International Space Station. A robot, glove or other tool with sensors might then touch a patient’s body. The information it collected could then be relayed to a device somewhere else. A doctor at that distant location could then feel like they’re touching the patient.

The holdup right now is crafting the devices needed to translate data on touch into sensations. One option is a flexible patch that attaches to the skin. It’s upper layers hold a stretchy circuit board and tiny vibrating actuators to the patch. Wireless signals — created when someone touches a screen or device elsewhere — control the device. Energy to run the patch can be delivered wirelessly, notes John Rogers. He’s a physical chemist leading the device’s development at Northwestern University in Evanston, Ill. The group reported its initial progress two years ago in Nature.

Rogers’ team has since made its patch thinner and lighter. It also gives the wearer more detailed touch information. Plus, the latest version comes in custom sizes and shapes. Up to six patches can work at the same time on different parts of the body.

The researchers wanted to make the patch work with common electronics. For that, the team created a program to send sensations from a touch screen to the patch. As one person moves her fingers across a smartphone or touch-screen computer, another person wearing the patch can feel that touch. So a child might feel a mom stroking his back. Or a patient might feel a doctor poking to see where their skin feels tender.

Rogers’ team believes its patch could upgrade artificial limbs, too. The patch can pick up signals from pressure on a prosthetic arm’s fingertips. Those signals can then be sent to a patch worn by the person with the artificial limb. 

Other researchers also are testing ways to add tactile feedback to the artificial body parts. This could make the devices more user-friendly. For example, adding pressure and motion feedback helped people with an artificial leg walk with more confidence. The 2019 device also reduced the pain from phantom limbs.

Brown, the Johns Hopkins engineer, hopes to help people control the force of their artificial limbs. Nondisabled people adjust their hands’ force on instinct. For example, Brown takes his young daughter’s hand in a parking lot. If she starts to pull away, he gently squeezes. But he might hurt her if he couldn’t sense the stiffness of her hand.

Brown’s group tested two ways to give users feedback on the force exerted by their electronic limbs. In one, a device squeezed the user’s elbow. The other used a vibrating device strapped on near the wrist. The stiffer an object touched by the artificial limb was, the more pressure or greater the vibrations there were. Volunteers who had not lost limbs tried each setup. The test involved judging the stiffness of blocks with an artificial lower arm and hand.

Both types of feedback worked better than no feedback. But neither type seemed better than the other. “We think that is because, in the end, what the human user is doing is creating a map,” Brown says. Basically, people’s brains match up how much force corresponds to the intensity of each type of feedback. Brown and his colleagues shared this finding two years ago in the Journal of NeuroEngineering and Rehabilitation.

But the brain may not be able to correctly map all types of touch feedback from artificial body parts. One team in Sweden built bionic hands with touch sensors on the thumb. They sent signals to an electrode implanted around the user’s ulnar nerve. That’s in the arm. So, the feedback went directly into the nervous system.

Three people who had lost a hand tested these bionic hands. Users did feel a touch when the thumb was prodded. But that touch felt as if it came from somewhere else on the hand. The mismatch did not improve even after more than a year of use. Bensmaia was part of a team that shared the finding last December in Cell Reports.

That mismatch may have arisen because the team couldn’t match the touch signal to the right part of the nerve. Many bundles of fibers make up each nerve. Different bundles in the ulnar nerve receive and send signals to different parts of the hand. But the implanted electrode did not target the specific bundle of fibers that maps to the thumb.

Studies published in the past two years, though, have shown that people using these bionic hands could better control their grip than when they had used them with only feedback at the upper arm’s surface. People getting the direct nerve stimulation also reported feeling as if the hand was more a part of them.

As with the bionic hands, future haptic technology will likely need substantial refining to get things right. And virtual hugs and other simulated touch may never be as good as the real thing. But haptics may provide new ways to explore our world and stay in touch — both literally and virtually.

activate: (in biology) To turn on, as with a gene or chemical reaction.

actuator: A motor that provides power to make something happen. It can use any source of power, from water to electricity to a hand crank.

Antarctica: A continent mostly covered in ice, which sits in the southernmost part of the world.

application: A particular use or function of something.

cell: The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells.

colleague: Someone who works with another; a co-worker or team member.

computer model: A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.

COVID-19: A name given to the disease that caused a massive global outbreak. It first emerged in December 2019 and is caused by a new coronavirus known as SARS-CoV-2. Symptoms can include pneumonia, trouble breathing, feeling too tired to walk more than a few steps, fever, headaches, low blood-oxygen levels, blood clots and brain “fog.”

dolphins: A highly intelligent group of marine mammals that belong to the toothed-whale family. Members of this group include orcas (killer whales), pilot whales and bottlenose dolphins.

earthquake: A sudden and sometimes violent shaking of the ground, sometimes causing great destruction, as a result of movements within Earth’s crust or of volcanic action.

electric charge: The physical property responsible for electric force; it can be negative or positive.

electrode: A device that conducts electricity and is used to make contact with non-metal part of an electrical circuit, or that contacts something through which an electrical signal moves. (in electronics) Part of a semiconductor device (such as a transistor) that either releases or collects electrons or holes, or that can control their movement.

electronics: Devices that are powered by electricity but whose properties are controlled by the semiconductors or other circuitry that channel or gate the movement of electric charges.

engineer: A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need. (v.) To perform these tasks, or the name for a person who performs such tasks.

factor: Something that plays a role in a particular condition or event; a contributor.

fiber: Something whose shape resembles a thread or filament. (in nutrition) Components of many fibrous plant-based foods. These so-called non-digestible fibers tend to come from cellulose, lignin, and pectin — all plant constituents that resist breakdown by the body’s digestive enzymes.

force: Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.

friction: The resistance that one surface or object encounters when moving over or through another material (such as a fluid or a gas). Friction generally causes a heating, which can damage a surface of some material as it rubs against another.

gland: A cell, a group of cells or an organ that produces and discharges a substance (or “secretion”) for use elsewhere in the body or in a body cavity, or for elimination from the body.

International Space Station: An artificial satellite that orbits Earth. Run by the United States and Russia, this station provides a research laboratory from which scientists can conduct experiments in biology, physics and astronomy — and make observations of Earth.

limb: (in physiology) An arm or leg. (in botany) A large structural part of a tree that branches out from the trunk.

mammal: A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding their young, and (typically) the bearing of live young.

materials science: The study of how the atomic and molecular structure of a material is related to its overall properties. Materials scientists can design new materials or analyze existing ones. Their analyses of a material’s overall properties (such as density, strength and melting point) can help engineers and other researchers select materials that are best suited to a new application. 

mechanical engineer: Someone trained in a research field that uses physics to study motion and the properties of materials to design, build and/or test devices.

nerve: A long, delicate fiber that transmits signals across the body of an animal. An animal’s backbone contains many nerves, some of which control the movement of its legs or fins, and some of which convey sensations such as hot, cold or pain.

nervous system: The network of nerve cells and fibers that transmits signals between parts of the body.

neuron: The main cell type of the nervous system — the brain, spinal column and nerves. These specialized cells transmit information by producing, receiving and conducting electrical signals. Neurons also can transmit signals to other cells with chemical messengers.

neuroscience: The field of science that deals with the structure or function of the brain and other parts of the nervous system. Researchers in this field are known as neuroscientists.

pandemic: An epidemic that affects a large proportion of the population across a country or the world.

physical: (adj.) A term for things that exist in the real world, as opposed to in memories or the imagination. It can also refer to properties of materials that are due to their size and non-chemical interactions (such as when one block slams with force into another).

pressure: Force applied uniformly over a surface, measured as force per unit of area.

prosthetic: Adjective that refers to a prosthesis.

radio: Referring to radio waves, or the device that receives these transmissions. Radio waves are a part of the electromagnetic spectrum that people often use for long-distance communication. Longer than the waves of visible light, radio waves are used to transmit radio and television signals. They also are used in radar. Many astronomical objects also radiate some of their energy as radio waves.

Rayleigh waves: A type of seismic wave generated by earthquakes and underground explosions. Rayleigh waves, which travel only along Earth’s surface, have a rolling motion very similar to surface waves on the ocean. Rayleigh waves typically are larger and cause more damage than the faster-moving seismic P-waves and S-waves.

receptor: (in biology) A molecule in cells that serves as a docking station for another molecule. That second molecule can turn on some special activity by the cell.

rehabilitation: The act of restoring something to its original state. Often called “rehab” for short, the term is used commonly for both physical injuries (such as regaining muscle strength after an accident, for example) and mental problems (such as addiction to drugs, alcohol or other substances).

robot: A machine that can sense its environment, process information and respond with specific actions. Some robots can act without any human input, while others are guided by a human.

sensor: A device that picks up information on physical or chemical conditions — such as temperature, barometric pressure, salinity, humidity, pH, light intensity or radiation — and stores or broadcasts that information. Scientists and engineers often rely on sensors to inform them of conditions that may change over time or that exist far from where a researcher can measure them directly. (in biology) The structure that an organism uses to sense attributes of its environment, such as heat, winds, chemicals, moisture, trauma or an attack by predators.

silk: A fine, strong, soft fiber spun by a range of animals, such as silkworms and many other caterpillars, weaver ants, caddis flies and spiders.

smartphone: A cell (or mobile) phone that can perform a host of functions, including search for information on the internet.

tactile: An adjective that describes something that is or can be sensed by touching.

technology: The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts.

virtual: Being almost like something. An object or concept that is virtually real would be almost true or real — but not quite. The term often is used to refer to something that has been modeled — by or accomplished by — a computer using numbers, not by using real-world parts. So a virtual motor would be one that could be seen on a computer screen and tested by computer programming (but it wouldn’t be a three-dimensional device made from metal). (in computing) Things that are performed in or through digital processing and/or the internet. For instance, a virtual conference may be where people attended by watching it over the internet.

virtual reality: A three-dimensional simulation of the real world that seems very realistic and allows people to interact with it. To do so, people usually wear a special helmet or glasses with sensors.

wave: A disturbance or variation that travels through space and matter in a regular, oscillating fashion.

wireless: (in telecommunications) An adjective that describes the ability of certain devices to send and receive radio signals over the air. It often refers to Wi-Fi networks and the networks operated by cell-phone companies to transmit data called up by phone users.

Journal: M. Ortiz-Catalan. Chronic use of a sensitized bionic hand does not remap the sense of touch. Cell Reports. Vol. 33, December 22, 2020. doi: 10.1016/j.celrep.2020.108539.

Journal: X. Li et al. Electrowetting: A consideration in electroadhesion. IEEE Transactions on Haptics. Vol. 13, July – August  2020, p. 522. doi: 10.1109/TOH.2020.2979439.

Journal: A. Middleton and M. Ortiz-Catalan. Neuromusculoskeletal arm prostheses: Personal and social implications of living with an intimately integrated bionic arm. Frontiers in Neurorobotics. Vol. 14, July 24, 2020. doi: 10.3389/fnbot.2020.00039.

Journal: X. Yu et al. Skin-integrated wireless haptic interfaces for virtual and augmented reality. Nature. Vol. 575, November 20, 2019, p. 473. doi: 10.1038/s41586-019-1687-0.

Journal: F.M. Petrini et al. Sensory feedback restoration in leg amputees improves walking speed, metabolic cost and phantom pain. Nature Medicine. Published online September 9, 2019. doi: 10.1038/s41591-019-0567-3.

Journal: N. Thomas et al. Comparison of vibrotactile and joint-torque feedback in a myoelectric upper-limb prosthesis. Journal of NeuroEngineering and Rehabilitation. Vol. 16, June 11, 2019. doi: 10.1186/s12984-019-0545-5.

Journal: E. Mastinu et al. Grip control and motor coordination with implanted and surface electrodes while grasping with an osseointegrated prosthetic hand. Journal of NeuroEngineering and Rehabilitation. Vol. 16, April 2019. doi: 10.1186/s12984-019-0511-2.

Kathiann Kowalski reports on all sorts of cutting-edge science. Previously, she practiced law with a large firm. Kathi enjoys hiking, sewing and reading. She also enjoys travel, especially family adventures and beach trips.

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