Wearable bioelectronic skin patches are a busy research area: part 1

Biomedical skin patches that integrate multiple disciplines for sensing, power, and reporting are receiving significant R&D effort.

Passive, chemical-only, wearable skin patches for medical applications are not new — they have been used for many years to deliver prescription medications that provide pain and nicotine relief. However, more sophisticated biometric wearable patch development and evaluation are underway at universities and other research institutions.

This makes sense not only for the target subject or patient, of course, but for the research team. Biometric patches avoid the many negatives of the material and biocompatibility issues associated with implanted devices. Further, researchers undoubtedly prefer testing a stick-on patch for which they can easily get volunteers while minimizing regulatory and sanitary issues, rather than doing in vitro implanting of their device under test into the carcass of an animal such as a pig (very popular test subject) or even in vivo implanting it into a live one.

A patch also offers the potential glow of “this is a revolutionary breakthrough” (and it may be) even though very few, if any, will be commercialized for various technical, medical, cost, or viability reasons. Patches are very good for getting attention, leading to follow-up grant funding (although that consideration is rarely cited, except in private).

These approaches leverage basic physics, electronics, and sensing principles to achieve their goals. They span self-powered energy harvesting for their electronics to sophisticated sensing of key body parameters via sweat, assessment of blood pressure, energy harvesting or backscatter for their power, and data reporting via Bluetooth or backscatter. Many of these patches are feasible with advanced materials, ultralow-power electronics, 3D printing, and more.

I’ve read many fascinating biomedical skin-patch stories and watched their videos over the past years. Here are nine projects that stood out for their innovation, cleverness, potential usefulness, and clearly written published research papers. They are from different institutions and cover a range of biomedical objectives and implementations:

#1 Screen-printed electrodes

A multi-institutional research effort led by Washington State University has demonstrated electrodes that can be made using screen printing alone. The resultant stretchable, durable circuit patterns can be transferred to fabric and worn directly on human skin. In contrast, current commercial manufacturing of wearable electronics requires fairly expensive processes involving clean rooms. While some other implementations presently use screen printing for parts of the process, this new method relies on screen printing alone.

They used a multi-step process, layering polymers and metal inks to create snake-like electrode structures. The screen printing of the polyimide (PI) layer enables facile, low-cost, scalable, high-throughput manufacturing. PI mixed with polyethylene glycol exhibited a shear-thinning behavior, significantly improving the printability of PI. The premixed Ag/AgCl ink is then used for conductive layer printing. Multiple electrodes are printed onto a pre-treated glass slide, which allows them to be easily peeled off and transferred onto fabric or other material, as shown in Figure 1.

Figure 1. The multistep process involves layering polyimide (PI) and conductive Ag/AgCl inks to create electrode structures for stretchable, conformable patches. (Image: Washington State University via American Chemical Society)

After printing the electrodes, the researchers transferred them onto an adhesive fabric worn directly on the skin by volunteers. The wireless electrodes with an onboard associated circuit, including a 2.4 GHz Bluetooth link, accurately recorded heart and respiratory rates, sending the data to a mobile phone, as shown in Figure 2.

Figure 2. An exploded view of the device provides more perspective on the completed arrangement. (Image: Washington State University via American Chemical Society)

In addition to Washington State University, the team included the Georgia Institute of Technology and Pukyong National University in South Korea. The work is detailed in their American Chemical Society (ACS) paper, “Fully Screen-Printed PI/PEG Blends Enabled Patternable Electrodes for Scalable Manufacturing of Skin-Conformal, Stretchable, Wearable Electronics,” with more details in the Supporting Information file.

#2 Electronic bandage to speed healing

Northwestern University researchers have developed a small, flexible, stretchable bandage – claimed to be a first-of-its-kind – that accelerates healing by delivering electrotherapy directly to the wound site. In an animal study, the new bandage healed diabetic ulcers 30% faster than in mice without the bandage. The bandage also actively monitors the healing process and then harmlessly dissolves — electrodes and all — into the body after it is no longer needed.

The joint team developed a small, flexible bandage that softly wraps around the injury site. One side of the smart regenerative system contains two electrodes: a tiny flower-shaped electrode on top of the wound bed and a ring-shaped electrode on healthy tissue to surround the entire wound. The other side of the device contains an energy-harvesting coil to power the system and a near-field communication (NFC) system for real-time updates, as shown in Figure 3.

Figure 3. A bioresorbable, battery-free wireless electrotherapy system designed to accelerate wound healing. (a) The device structure and its components, including a wireless platform and molybdenum electrodes. (b) System operation diagram, showing power harvesting and real-time monitoring. (c) Electric field distribution between electrodes. (d) The device lifecycle from application to full bioresorption. (Image: Northwestern University via Science Advances)

If desired, a more in-depth analysis of Figure 3 can be found here:

(a) Schematic illustrations of a transient, wireless, battery-free system for electrotherapy mounted on a wound on the foot (left) and in an enlarged view (right) that highlights the different components.
(b) Operational diagram of the entire system. (RF, radio frequency; ISO, interconnection system operation; LDO, low-dropout regulator)
(c) FEA results of the electric field between the positive (+) and negative (−) electrodes. Scale bar, 3 mm.
(d) Schematic illustrations of the mode of use, device on a wound (i) before and (ii) after healing, (iii) removed by cutting the traces to the anode, (iv) partially bioresorbed after a period of therapy, and (v) fully bioresorbed; the semitransparent orange color represents the healed skin.

The team also included sensors to assess how well the wound is healing. The device can be operated remotely without wires, allowing the physician to decide when to apply the electrical stimulation and monitor the wound’s healing progress.

Physicians can monitor progress by measuring the resistance of the electrical current across the wound. A gradual decrease in current measurement relates directly to the healing process. So, if the current remains high, physicians know something is wrong.

The work is explored in their paper in Science Advances, “Bioresorbable, wireless, and battery-free system for electrotherapy and impedance sensing at wound sites” which also includes the appended Supplemental Information; there’s also a two-minute video here.

#3 Sweat sensor with immediate readout

Sweat is a “gold mine” of useful information about the body. Recognizing its virtues, Engineers at the University of California San Diego have developed a thin, flexible, and stretchy sweat sensor that can show the level of glucose, lactate, sodium, or pH of sweat at the press of a finger.

They claim it is the first standalone wearable device that allows the sensor to operate independently — without any wired or wireless connection to external devices — to visualize the measurement result directly. The design of this small disk-shaped patch includes all the essential components required for wearable sensors: two integrated batteries, a microcontroller, sensors, the circuit, and a stretchable display.

The fabrication of the device involves the formulation of nine different stretchable inks, which were used to print the batteries, circuits, display panel, and sensors. The device is printed layer-by-layer onto stretchable polymer sheets and assembled with hydrogels and microcontroller chips into the complete device. Each ink was optimized to ensure its compatibility with other layers while balancing its electrical, chemical, and mechanical performance, as shown in Figure 4.

Figure 4. A fully integrated, stretchable epidermal sweat sensing patch with an electrochromic display. (a) Layer-by-layer composition of the device. (b) System flow and module breakdown. (c) Patch operation, showing the patch used for epidermal sweat sensing by revealing the target concentration (i). Illustration of the display that changes with the electrolyte concentration and potentiometric sensor readout (ii) and the intermittent discharge mode of the Ag₂O–Zn battery power supply (iii). (d) Demonstrates mechanical durability under bending and stretching, especially when stretching the connection between the interconnect and MCU. (Image: University of California San Diego via Nature Electronics)

When a negative voltage is applied, the polymer changes from light sky blue to dark navy blue and turns back when a positive voltage is applied. By tuning the ink formulation with PEDOT:PSS, they can make it printable and stretchable; the patch can be stretched 20% repeatedly without affecting its performance.

The researchers designed a display panel composed of 10 individual pixels, which is programmed to display the concentration of the chemicals by turning on different numbers of the pixel. The pixels take only 500 milliseconds to change color, so they consume just 80 microwatts of power on average.

Full details are in the Nature Electronics paper “A stretchable epidermal sweat sensing platform with an integrated printed battery and electrochromic display,” which also has appended Supplemental Information.

Part 2 of this article looks at three more biometric skin patches.