(Nanowerk Spotlight) The accurate measurement of vital signs is fundamental to medical diagnosis and treatment. Blood pressure and body temperature, in particular, serve as critical indicators for a range of conditions including cardiovascular diseases, infections, and metabolic disorders. Traditionally, these parameters have been measured intermittently in clinical settings using specialized equipment such as sphygmomanometers and thermometers. While precise, this approach provides only snapshots of a patient’s condition, potentially missing important fluctuations that occur between measurements.
Recent advances in wearable technology have attempted to address this limitation by offering continuous monitoring capabilities. Consumer devices like smartwatches and fitness trackers can now provide ongoing heart rate and activity data. However, these devices often lack the accuracy required for clinical decision-making, especially 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 sophisticated sensors. The ideal e-skin would be thin, flexible, and virtually invisible, yet capable of measuring multiple physiological parameters with precision comparable to standard medical equipment.
Developing such devices poses significant engineering challenges. The materials used must be biocompatible for long-term skin contact, highly sensitive to minute physical changes, and able to maintain accuracy across varying environmental conditions. Additionally, the device must be unobtrusive enough for patients to wear comfortably during daily activities.
Previous attempts at creating clinically accurate e-skins have faced numerous obstacles. Many struggled to achieve the necessary combination of transparency, flexibility, and sensitivity. Others produced highly sensitive sensors but at the cost of wearability or durability. Perhaps most critically, few prototypes have demonstrated the ability to match the measurement accuracy of standard medical equipment when tested on human subjects in real-world conditions.
In this context, a team of researchers from the University of Sussex has developed a novel approach to e-skin fabrication that shows promise in 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 food caviar networks. A) Composition of micro-caviar core-shell structure. B–D) Photographs of individual AgNW/BS micro-caviar, a micro-caviar planar network on the University of Sussex logo, and a similar network between two silver electrodes. E) Illustrations showing the locations in which the e-skin was attached to a wearer. Specifically, the radial artery of the wrist (left) and carotid artery of the neck (right). F–I) Schematics presenting the mechanisms for skin-on electromechanical arterial response of e-skin device. E-skin placed on the skin will experience a normal force as blood pumps through the artery, due to a volumetric expansion (ΔV). The e-skin, which consists of a planar network of micro-caviar between electrical contacts with a resistance R0, will in turn experience a force F) via its direct contact with the skin. This will strain the network, resulting in a change in resistance (ΔR) across the device. (J) Representative fractional resistance change (ΔR/R0) versus compressive strain (-ɛ) and the corresponding electromechanical metrics of the e-skin. K) Comparing gauge factor (G) as a function of sensing component transmittance from literature. The challenge region is highlighted in green. (Image: Reproduced from DOI:10.1002/adfm.202405799, CC BY)
The key innovation in this work is the use of “micro-caviar” – tiny spheres approximately 290 micrometers in diameter, made from a hydrogel derived from brown seaweed. Within each micro-caviar bead, the researchers embedded a network of silver nanowires. 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 production process, the team was able to create micro-caviar beads with an optimized structure. The nanowire network within each bead became highly aligned, enhancing its sensitivity to strain. When multiple micro-caviar beads are assembled into a thin layer, they form a sensor that can detect minute changes in pressure – such as those caused by the pulse of blood flowing through an artery.
What sets this electronic skin apart is its remarkable combination of properties. The device is nearly transparent, with a light transmittance of over 99%. This means it is virtually invisible when applied to the skin, addressing the cosmetic concerns that have limited adoption of previous wearable sensors. Despite this transparency, the sensor demonstrates exceptional sensitivity to mechanical strain, with a gauge factor (a measure of electromechanical sensitivity) exceeding 200. This is far higher than conventional strain sensors and most other experimental electronic skins.
Additionally, the micro-caviar electronic skin shows 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 strain, allowing it to simultaneously track both pulse pressure and skin temperature.
What sets this electronic skin apart is not just its remarkable combination of properties, but its demonstrated accuracy in real-world testing. Unlike many previous prototypes that struggled to match the precision of medical equipment, this e-skin has shown comparable performance to clinically validated devices when tested on human subjects.
The researchers conducted trials applying the sensor to volunteers’ wrists and necks, over the radial and carotid arteries respectively. They then compared the e-skin’s measurements to those taken with a clinically tested A&D Medical UA-651 blood pressure monitor over the course of six days. For measurements taken at the wrist, the electronic skin reported an average pulse pressure of 35.75 mmHg, which aligned closely with the 34.33 mmHg measured by the commercial blood pressure cuff. This level of accuracy in a wearable, nearly invisible device represents a significant advancement in the field.
Similarly, the 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 using 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 holding it in place with a hand at the neck, could affect measurement accuracy. The slight pressure from the covering appeared to artificially increase the pulse pressure readings. This highlights the importance of proper attachment methods that don’t interfere with the sensor’s function.
The study also points out 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 vital sign monitoring, transforming it into a fully self-contained, wireless device will be crucial for widespread adoption.
Despite these challenges, the ability of this new device to provide continuous, non-invasive measurements of key vital signs with clinical-grade precision represents a significant step forward. It addresses one of the most persistent obstacles in the development of wearable health monitors: achieving accuracy comparable to standard medical equipment in a form factor suitable for constant wear.
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