Ultra-thin, seaweed-based electronic skin matches clinical devices for vital signs accuracy

August 13, 2024

(Nanowerk in the spotlight) Accurate measurement of vital signs is the foundation of diagnosis and treatment. Blood pressure and body temperature, in particular, are critical indicators for a range of conditions, including cardiovascular disease, infections, and metabolic disorders. Traditionally, these parameters have been measured periodically in clinical settings using specialized equipment such as sphygmomanometers and thermometers. Although accurate, this approach provides only a snapshot of a patient’s condition, potentially missing important variations that occur between measurements.

Recent advances in wearable technology have attempted to address this limitation by offering continuous monitoring capabilities. Consumer devices such as smartwatches and fitness trackers can now provide real-time heart rate and activity data. However, these devices often lack the accuracy required for clinical decision-making, particularly when it comes to blood pressure measurement.

The concept of electronic skin (e-skin) has emerged as a potential solution to bridge the gap between continuous monitoring and clinical accuracy. E-skin devices aim to mimic the properties of human skin while incorporating advanced sensors. The ideal e-skin would be thin, flexible, and virtually invisible, while also being able to measure multiple physiological parameters with precision comparable to standard medical equipment.

Developing such devices involves significant engineering challenges. The materials used to make them must be biocompatible when in prolonged contact with the skin, highly sensitive to small physical changes, and able to maintain accuracy under a variety of environmental conditions. In addition, the device must be discreet enough for patients to wear comfortably during daily activities.

Previous attempts to create clinically accurate e-skins have encountered numerous obstacles. Many have struggled to achieve the necessary combination of transparency, flexibility, and sensitivity. Others have produced highly sensitive sensors, but at the expense of wearability or durability. Perhaps most importantly, few prototypes have demonstrated the ability to match the measurement accuracy of standard medical equipment in real-world human testing.

In this context, a team of researchers from the University of Sussex have developed a novel approach to e-skin production that offers hope of overcoming these challenges. Their work, reported in Advanced Functional Materials (“Transparent, Bioelectronic, Natural Polymer AgNW Nanocomposites Inspired by Caviar”), draws inspiration from an unexpected source – molecular gastronomy. E-skin based on micro-sized electronic networks of food-grade caviar. A) Composition of the microcaviar core-shell structure. B–D) Photographs of a single AgNW/BS microcaviar, a planar microcaviar network on the University of Sussex logo, and a similar network between two silver electrodes. E) Illustrations showing the locations where the e-skin was attached to the wearer. In particular, the radial artery of the wrist (left) and the carotid artery of the neck (right). F–I) Schematics showing the mechanisms of the electromechanical arterial response of the e-skin device worn on the skin. The e-skin placed on the skin will experience a normal force when blood is pumped through the artery, due to the volumetric expansion (ΔV). The e-skin, which consists of a planar microcaviar network between electrical contacts with a resistance of R0in turn, will experience a force F) through direct skin contact. This will load the network, which will cause a change in resistance (ΔR) across the device. (J) Representative fractional change in resistance (ΔR/R0) compared to the compressive strain (-ɛ) and the corresponding electromechanical metrics of the e-skin. K) Comparison of the gauge coefficient (G) as a function of the transmittance of the sensor component from the literature. The challenge area is highlighted in green. (Graphic: Reproduced from DOI:10.1002/adfm.202405799, CC BY)

The key innovation in this work is the use of “microcaviar”—tiny spheres about 290 micrometers in diameter made from a hydrogel derived from brown seaweed. The researchers embedded a network of silver nanowires into each microcaviar bead. These nanowires, chosen for their high conductivity and flexibility, form a sensitive mesh just 20 nanometers in diameter and 12 micrometers long. This combination of biocompatible hydrogel and conductive nanowires creates a highly sensitive sensor that remains flexible and nearly invisible on the skin.

By carefully controlling the manufacturing process, the team was able to create micro-caviar beads with an optimized structure. The network of nanowires in each bead became highly aligned, increasing its sensitivity to deformation. When many micro-caviar beads are assembled into a thin layer, they form a sensor that can detect tiny changes in pressure—such as those caused by the pulsation of blood flowing through an artery.

What sets this electronic skin apart is its unusual combination of properties. The device is nearly transparent, with light transmission of over 99%. This means it is virtually invisible when applied to the skin, addressing the cosmetic concerns that have limited the adoption of previous wearable sensors. Despite this transparency, the sensor exhibits exceptional sensitivity to mechanical deformation, with a measurement factor (a measure of electromechanical sensitivity) exceeding 200. This is significantly higher than conventional strain sensors and most other experimental electronic skins.

In addition, the microcaviar electronic skin exhibits excellent sensitivity to temperature changes, with a temperature coefficient of resistance of 4.58% per degree Celsius. This is about ten times more sensitive than platinum-based temperature sensors commonly used in medical devices. Importantly, the researchers found that the temperature sensitivity of their device did not interfere with its ability to measure mechanical stress, allowing it to simultaneously track both pulse pressure and skin temperature.

What sets this electronic skin apart is not only its unique combination of properties, but also its proven accuracy in real-world testing. Unlike many previous prototypes that struggled to match the precision of medical equipment, this e-skin demonstrated comparable performance to clinically proven devices in human testing.

The researchers conducted the trials by applying the sensor to the volunteers’ wrists and necks, over the radial and carotid arteries, respectively. They then compared the e-skin measurements with those taken with a clinically tested A&D Medical UA-651 blood pressure monitor over six days. For the wrist measurements, the e-skin reported an average pulse pressure of 35.75 mmHg, which was similar to the 34.33 mmHg measured by a commercial blood pressure cuff. This level of accuracy in a portable, nearly invisible device represents a significant advance in the field.

Similarly, skin temperature measurements closely matched those taken with a medical-grade infrared thermometer. The e-skin reported a wrist temperature of 33.86 °C, which was confirmed with a commercial thermal camera.

However, the researchers also identified important limitations that will need to be addressed in future development. They found that covering the e-skin, such as when held in place by the hand around the neck, can affect measurement accuracy. Slight pressure from the covering seemed to artificially increase pulse pressure readings. This highlights the importance of proper attachment methods that do not interfere with sensor function.

The study also points to the need for further integration with wireless modules to make the e-skin truly practical for long-term, real-world use. While the current prototype demonstrates the feasibility of accurate, continuous monitoring of vital signs, developing it into a fully autonomous, wireless device will be key to widespread adoption.

Despite these challenges, the ability of this new device to provide continuous, noninvasive measurements of key vital signs with clinical-grade precision represents a significant step forward. It addresses one of the most persistent obstacles to the development of wearable health monitors: achieving accuracy comparable to standard medical equipment in a wearable format.

By

Michael
Berger

– Michael is the author of three books published by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology, Nanotechnology: The Future is Tiny and Nanoengineering: The Skills and Tools Making Technology Invisible Copyright ©




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