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Manufacturers in medical space cottoning to the idea of flexible sensors

Ilene Wolff
By Ilene Wolff Contributing Editor, SME Media

Philips, MC10 out in front with wearable sensor system called Guardian, BioStamp nPoint biosensor

There’s a bit of Rob Podoloff in every Dr. Scholl’s Custom Fit Kiosk for foot orthotics: The chief technology officer at Tekscan Inc. is a pioneer in flexible force-sensing resistors (FSRs). He devised the guts in the kiosks that can help tell you which Dr. Scholl’s shoe inserts will put the spring back in your step.

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Ahmed Busnaina of Northeastern University and the NSF’s Nanoscale Science and Engineering Center for High-rate Nanomanufacturing took 10 years to develop a nanoscale printing system for making sensors.

He invented flexible sensors used to track a person’s gait, which are widely used in medical rehabilitation. Dentists worldwide also use his bite sensors to gauge whether upper and lower teeth are functioning together correctly.
Podoloff’s FSRs are one of three types of flexible sensors, a category that also includes photonics (for example, fingertip sensors on smart phones) and electrochemical sensing.

In addition to FSRs, electrochemical sensors are widely used in biomedical applications.

“The area that’s gotten the most commercialization to date is the one that started first, and that is what Tekscan and Interlink Electronics are doing with force-sensing resistors,” said Roger Grace, president of a marketing consulting firm that bears his name.

“What’s being rapidly commercialized are the sensors that are electrochemical in nature. These conformal/stretchable biosensors, when applied to the skin, use readily accessible bodily fluids, for example perspiration, to assess a wide spectrum of physiological phenomena, for example, dehydration,” he added. “Not only are they considered ‘wearable’ but in many applications they can be disposable as well because of their low cost.”

Researchers at universities are doing the bulk of the work on flexible electrochemical biomedical sensors, with just a few manufacturers currently producing at scale.

For example, MC10 has the BioStamp nPoint biosensor, which is FDA-cleared for use in monitoring people’s vital signs in clinical trials. The Dutch electronics giant Philips is marketing Guardian, a wearable sensor system that monitors inpatients’ vitals and notifies caregivers if something is amiss.
Sensor-equipped disposable diapers are already on the market, and other throw-aways in progress are smart band aids for chronic wounds that dispense antibiotics if infection is detected.

Research laboratories have demonstrated sensors that can detect substances, such as glucose, lactate, alcohol and fentanyl.

Simple sensor design helping

For startups focused on electrochemical sensors, scaling up production is facilitated by the relative simplicity of sensor design. In addition, because the flexible sensors are made in much the same way as printed electronics, there is an existing manufacturing infrastructure.

If sensors are going to be used widely for biomedical applications, a market that indicators show holds big promise, they need to be flexible and even stretchable because of the need to adhere to the irregular surfaces of the moving human body where a stiff, inflexible material like silicon wouldn’t make effective contact.

Pulin Wang, StretchMed co-founder and managing director, demonstrated his electronic tattoo sensor, with potential for a more accurate EMG reading, at NextFlex U.S.’ Innovation Day 2019. NextFlex is a public-private technology hub formed under the administration of President Barack Obama to advance flexible electronics.

“Our sensors are skin-soft, hair-thin and ultra-stretchable and can be placed on any part of the body,” Wang said. “In contrast to soft sensors, which can still leave a lot of gaps between the sensor and the skin, our sensor is stretchable and therefore can be conformable to your skin offering the best interface.”

Tekscan is “in the process of working with substances that are stretchable and flexible, which hopefully open up many more medical applications,” Podoloff said. “But, of course, that brings another set of challenges in that you have to have an ink system that can also bend and stretch to stay with the substrate.”

Play between ‘ink’ and plastics important

In 2016, researchers in the NanoEngineering Department at the University of California at San Diego published a paper on a possible solution to the problem of an ink that can be used on stretchable substrates and be used in medical devices.

Amay Bandodkar and colleagues made a self-healing ink that includes ground neodymium magnets.

If the ink “breaks” when a substrate is stretched, the particles in the ink attract each other and reform their bond.

Plastics, such as PET, are the latest materials widely used for the sensors’ base. Paper substrates hold promise in the future, as does fabric—with sensing fibers woven into or embroidered onto a cloth backing.

There are wearables embedded with sensors, largely for the fitness market. They can monitor heart rate, breathing, distance walked or run, and temperature. The Athos brand of fitness gear even has micro-electromyography sensors to monitor muscle activity.

“It’s gone from an electronics game to a materials game,” Grace said. “The play between ‘ink’ and plastics is the name of the game here.”

Materials both common and rare

More insight into the materials Grace referred to comes from Ahmed Busnaina, chairman of mechanical and industrial engineering at Northeastern University and a director for the Nanoscale Science and Engineering Center for High-rate Nanomanufacturing at the NSF (National Science Foundation).
He spun off a company to make a nanoscale offset printing system, NanoOPS, where he is chief technology officer.

Currently in its second generation, the machine uses existing semiconductor industry robotics and is completely software driven. It prints down to 20nm.
In addition to flexible plastics, such as Kapton, PET, PETG and PU, “NanoOPS can print on silicon wafers, glass, ceramics, metal … any surface that’s flat,” Busnaina said. “We have not been given a substrate that we have not been able to print on.”

The ink solutions that Busnaina and his team have printed are all conductors, semiconductors and insulators, including copper, gold, titanium, platinum, silver, aluminum, and a variety of inorganic and organic semiconductors, as well as graphene and nanotubes.

Manufacturing on the NanoOPS differs greatly from traditional ways of making sensors.

“Because it’s a printing process you do not need a chemical reaction (as in electroplating or chemical vapor deposition) and that’s the limitation that the conventional industry has,” he said. “Or you need a vacuum. We don’t need any of that.”

Sensor from Busnaina could make history

The professor also patented nanoelectronics and a printed electrochemical sensor, a single-walled carbon nanotube-based biosensor to detect glucose, lactate and urea in sweat. The sensor is based on modification of the nanotubes with a linker that non-covalently associates with the nanotubes and covalently couples to an enzyme. The enzyme interacts with sweat to increase the resistance of the nanotubes within the sensor.

Busnaina envisions use of the sensor for patient monitoring in a clinical setting.

To make this possible, the sensor is outfitted with electronics and a radiofrequency signal generator that enables communication to a relay station, smart phone or a remote receiver. Still in progress are microfluidics to capture and direct the sweat, and a power source.

If that happens, Busnaina will be making his mark with electrochemical sensors just as Podoloff’s legacy is with FSRs.

Inspiration for key technology inside Dr. Scholl kiosks came from a dentist

Rob Podoloff was working toward a master’s degree in mechanical engineering at MIT in the mid-1980s when he got the inspiration for a technology destined for use in the ubiquitous Dr. Scholl Custom Fit Kiosk for foot orthotics—from an inquisitive dentist who had taken an adult education class in personal computers.

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Rob Podoloff, CTO at Tekscan Inc., invented the world’s first force-sensing resistor.

The prosthodontist was “very interested in the timing of the tooth contacts as somebody closed their mouth,” he said.

Because Podoloff was working in MIT’s artificial intelligence lab, he happened to know the state of the art for sensors. And he knew the question the dentist was posing had not been answered. There was no thin, flexible sensor you could bite on to give you that kind of information.

“Both my roommate and I were hungry grad students and if this dentist wanted to pay us a little money to look into it, we were more than happy,” he said.

While the students were brainstorming and trying different things, Podoloff remembered his dad, who represented a company that made heaters to defrost cars’ side mirrors. The heaters used flexible, copper-etched circuits.

The students took them and realized that if they could make a grid of some kind on the circuit they’d have a way of creating individual sensing cells within each intersection of the grid. They hacked something together out of an old heater cell and some conductive rubber.

“I made this clunky sensor that I was at least able to tell where I was pressing on it,” Podoloff said. “And that was enough of a breakthrough to say, ‘Can we start a company and pursue this more?’”

Two years after the dentist first approached Podoloff, the students were the first to print a matrix-based sensor, a model of the sensor the prosthodontist was looking for—a prototype that eventually became known as the “T-Scan” digital occlusal analysis system. During this time, they also established Tekscan, Inc.

Over the last several decades, Tekscan has expanded its application portfolio beyond dental occlusion and into other industries, including automotive design, medical devices and robotics. Tekscan currently owns 18 patents in printed force- and pressure-sensor technologies—the most recent of which was issued in the last year.

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