Features Physics World  May 2018
(2018 Takao Someya Research Group)

Making health digital

With wearable tech now a staple of modern life, it’s never been easier to keep track of your health. But what will be the next big innovation? Jess Wade gives her top eight technologies-in-the-making that will lead to a new generation of health aids

There was a time when keeping on top of your health, activity and diet was archaically analogue: you’d waste time waiting for follow-up appointments to monitor recovery, obsessively check food labels to keep track of your diet, and fastidiously log exercise when preparing for sporting events. But with the help of wearable gadgets, these time-consuming problems are becoming a thing of the past. Nowadays, there are “smart” watches, rings, necklaces, bracelets and even headphones that can keep track of your steps, heart rate, sleep, stress and so much more. But despite consumer enthusiasm – 38 million wearable devices were shipped worldwide at the end of 2017 – market growth is slowing, and sophisticated shoppers are looking for more.

So far, the functionality of commercial wearable devices has been limited: they rely on rigid electronic components mounted in plastic, and few are biocompatible, washable or breathable. Future innovations will exploit softer electronics, and indeed we are already starting to see energy-harvesting threads woven into beach towels, sweat-resilient piezoelectric components embedded into running shoes and responsive textiles that can release controlled doses of medicine. Thanks to recent advances in material design, manufacture and fabrication, researchers are exploring new platforms and creating more malleable electronic devices.

To obtain reliable and detailed health metrics, it is vital that wearable and implantable sensors make constant contact with the skin or cells, and not irritate the wearer. With these details in mind, here are my top eight technologies-in-the-making that could continue revolutionizing how we track and maintain our health.

 

Weaving electronics Conductive threads can be woven into fabric to create clothing that, for example, stores data collected by wearable devices. (Mi Jung Lee)

1 Woven data storage

The more data we collect with our wearable devices, the more space we’ll need to store that information. To solve this problem, Mi Jung Lee at Kookmin University in South Korea and colleagues are using conductive threads to weave data storage into our clothing. Lee’s technology is based on resistive random access memory (RRAM), in which a dielectric material changes its resistance under a strong electric field or current. They use carbon fibres and aluminium-coated cotton, where the metal forms aluminium oxide in air, which is stable and resistant to washing (Adv. Funct. Mater. 27 10.1002/adfm.201605593).

For Lee’s fabric, bipolar resistive switches form at the interface between the aluminium oxide and the carbon threads. The aluminium oxide acts as the resistive switching layer, in which an applied voltage triggers an electrochemical redox reaction that creates conducting pathways between the aluminium and carbon electrodes. The threads switch from a low- to a high-resistance state, which can be used to write (or erase) information. To convince themselves of the reproducibility and scalability of their memory system, which is non-volatile, Lee’s team demonstrated that these data-storage fibres could be woven using a recommissioned commercial loom, after the team refused to learn how to knit.

 

Handy tech An integrated “skin electronics” system, such as that developed by Takao Someya’s team at the University of Tokyo, allows health monitoring at home. (2018 Takao Someya Research Group)

2 Fabric photovoltaics

If we are going to functionalize our fabrics, we’ll need to harness energy from the textiles themselves. According to Takao Someya’s group at the University of Tokyo in Japan, the answer is washable, stretchable and ultrathin photovoltaics. These solar cells are created from a blend of polymers and small molecules sandwiched between two carefully chosen elastomers that protect the active layer from water, while also being able to stretch and have good optical transmission. Light absorbed by the polymer creates bound electron-hole pairs (known as excitons), which are separated at the interface with an acceptor molecule and then navigated through the device to be collected at the electrodes. Someya’s devices maintain an efficiency of 8%, and can survive even if mechanically compressed and dunked for up to 100 minutes in water.

3 Cell monitors

Sticking sensors and electronic components into biological tissue can trigger inflammation and wound healing (a “foreign-body” response), preventing the tissue from properly contacting the device. Róisín Owens from the University of Cambridge in the UK and her team of tissue engineers are therefore seeking to understand the mechanisms by which cells can stick to and move across polymer surfaces. In particular, she is building organic electrochemical transistors (OECTs), which can measure the health of individual cells.

As organic materials can be transparent, Owens’ devices are able to measure conductivity and take images of cells at the same time. OECTs include an organic polymer film in contact with an electrolyte, where a gate electrode controls the doping level within the polymer (Nat. Rev. Mat. 3 17086). The transistors can be used to sense low levels of metabolites (such as glucose) continuously and in vivo. The biorecognizing enzymes used for sensing aren’t usually very good at transporting electrons to neighbouring electrodes, but organic electronic materials can help. They can operate in severe biological environments and are able to store biorecognizing enzymes, allowing for fast conduction pathways to nearby electrodes. Owens’ team is trying to identify the precise mechanisms of cell adhesion and migration on polymer surfaces, which are crucial for next-generation medical diagnostic devices and toxicology.

4 Bionic eyes

Rylie Green from Imperial College London is one of eight researchers to win a share of last year’s £8m Healthcare Technologies Challenge fund from the UK’s Engineering and Physical Sciences Research Council (EPSRC). She’s combining conducting polymers with proteins to mediate the interactions between implants and tissues. When implants are rejected, the formation of non-conductive scar tissue destroys the conducting interfaces with cells or neurons. Instead of having inert implants, Green is developing organic polymers that can regenerate the tissues around them.

The surface of the electrodes can be coated with a functional tissue layer, which, for neuroprosthesis, could build synaptic connections and change the way devices are driven (Adv. Funct. Mat. 28 1702969). Her electrode coating has a bilayer of conducting and biosynthetic hydrogels, which supports the development of neural tissues at the interface with an electrode while protecting it from high voltages across the electrode. The work could help with artificial eyes.

5 Artificial muscles

Carbon-based materials are stretchy, soft, cheap and simple to process, making them perfect candidates for exploring innovations in wearable technology. And as they can transport electrons, holes and ions, such materials are compatible with conventional solid-state electronics as well as being able to interface with biological systems. The conductivity of organic materials arises from their sp2 carbon bonding, in which overlapping pz electron orbitals along a conjugated polymer chain permit the delocalization of π-electrons. This type of bonding means that organic polymers are intrinsically anisotropic and – in some cases – that thermal expansion along one axis is therefore not the same as along another.

In Sweden, Ali Maziz of Linköping University and co-workers are using conjugated polymers to weave artificial muscles. The “textile actuators” are formed from polymer-coated fabrics that are immersed in an electrolyte, which provides a sea of mobile ions. When the polymer is chemically reduced by applying a current, positively charged cations from the surrounding electrolyte move in to the threads, making them expand. When the current is reversed, the polymer is oxidized and the ions move out, causing the thread to shrink and the fabric to contract. Textile muscles can be used in limb prosthetics and exoskeletons, providing silent and soft movement at low current.

 

Digital stickers John Rogers and colleagues have developed a throat sensor (above) that uses stretchable electronics to monitor the recovery of stroke patients, giving doctors remote access to real-time data (below). (Elliott Abel / Shirley Ryan AbilityLab)
(Elliott Abel / Shirley Ryan AbilityLab)

6 Throat sensors

John Rogers and colleagues at the Bio-Integrated Electronics Lab at Northwestern University in the US have been creating biosensors since they first developed techniques to stretch ultrathin layers of silicon in 2006. Their most recent inventions are stretchable, stick-on sensors that help with the rehabilitation of stroke patients. The sensors are lightweight – with mass densities as low as the outer layer of your skin – and can be attached to the throat to monitor a patient’s muscle movement and the vibrations of their vocal chords.

Specifically, the sensors detect mechano-acoustic waves – mechanical waves from natural physiological activity that move through tissues and fluids in the body. The sensors include a commercially available low-power accelerometer, low-pass and high-pass filters, a preamplifier and capacitive electrodes, all sandwiched between an ultralow-modulus elastomer (stretchy polymer) shell (Sci. Adv. 2 10.1126/sciadv.1601185). Working with a rehabilitation lab in Chicago, the stretchable sensors can send real-time data about a patient’s swallowing and patterns of speech back to their doctors, who can monitor progress and set up alerts for when their patients are in trouble.

 

Stuff of sci-fi DuoSkin’s fabrication process involves sketching skin circuitry with graphic design software and creating stencils of the circuitry (top), then applying gold leaf and mounting electronics (middle). The DuoSkin device is applied to the user’s skin via water-transfer and can take the form of a tattoo or jewellery (bottom). (Microsoft Media / MIT Media Lab; Jimmy Day / DuoSkin)

7 Skin interfaces

We are not far from having instant digital access to our own detailed health information – but what if you can’t work out how to use the interface? An ageing population means that those who are most in need of regular health updates will struggle the most to interpret them. For them, Someya and his group in Tokyo have designed a flexible and deformable skin display. A micro-array of light-emitting diodes (LEDs) mounted onto a breathable nanomesh electrode and attached to a lightweight sensor can visualize vital signs and relay them to friends, family members or nearby medical staff.

In an attempt to make skin-based technology more accessible in and outside the medical field, Cindy Hsin-Liu Kao at the MIT Media Lab has created do-it-yourself temporary tattoos called DuoSkin. Made from gold leaf available at any craft store, the tattoos turn bodies into an interface. Once applied to the skin, they can sense touch input, display an output, and share data with other devices using near-field communication technology. The team behind DuoSkin is collaborating with Microsoft Research, and has shown how users can design their own tattoos, which are durable and skin-safe. The group even launched DuoSkin at New York Fashion Week, where they sent information about clothing to the smart phones of audience members (PNAS 115 3504).

8 Epilepsy treatments

Anyone who has been diagnosed with epilepsy will have probably had an electroencephalogram (EEG), in which electrodes are attached to your scalp to detect electric fields from the action potentials speeding along your neurons. While EEG is good at indicating when events occur, it cannot easily pinpoint exactly where in the brain the signals are coming from and the skull isn’t a very good conductor of electricity. Epileptic seizures are typically controlled with anti-convulsant drugs, but despite advances in pharmaceutical design, about a third of sufferers have epilepsy that is resistant to medicines.

When drugs cannot penetrate the blood–brain barrier, treatment options are highly invasive and can include having one or more parts of the brain removed. Unfortunately, these “epileptogenic zones” cannot be extracted if there are too many of them, or if they’re inaccessible or hard to locate. That’s where organic electronic devices could do wonders. Working with Magnus Berggren of Linköping University, George Malliaras from the University of Cambridge has developed a microfluidic ion-pump system that can deliver targeted anti-epileptic drugs on demand (Adv. Mater. 27 3138). Malliaras’ team uses brain-penetrating sensors that continuously monitor neural signals, detect hyperactivity and immediately release a charged neurotransmitter through organic ion pumps. Both the sensor and pump use organic materials, which, like the EEG, feel the electric field of hyperactive neurons and begin to pump the drug with high spatiotemporal precision. So instead of sticking a bunch of electrodes on top of your head to tell you that some part of your brain has had a seizure, Malliaras’ system will detect and treat the seizure as soon as it happens.

The future is now

Technology will undoubtedly transform medical diagnostics, combining sensitive sensing with precise therapeutics. It will revolutionize the lives of patients, help doctors and encourage the advancement of science. To succeed, interdisciplinarity is essential – and that means engineers, life scientists and physicists talking to each other more than ever before. The result could be clothes, tattoos and implants that help us live long and healthy lives.