Scaly sensor
Some functions of the human body are accompanied by mechanical deformations of the skin – from muscle contraction and movement in the joints to the pulse beating on the wrist. These changes can be detected and tracked by measuring the configuration of the skin surface in various parts of the body.
Currently, various portable sensors are being actively developed that assess skin tension for health monitoring. Some of these sensors can detect high-level mechanical tension (40-100%), for example, associated with finger movements and limb joints, others detect medium-level tension (10-40%) accompanying swallowing and facial movements, and still others are sensitive to low-level movements (less than 10%), for example, pulse beat or vibration from vocal cords.
Due to the high level of conductivity and stability, the most preferred material for tension sensors is poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). Previously, very sensitive PEDOT:PSS strain gauges were developed, which can detect deformation from very weak movements (<1%), but the poor extensibility of PEDOT:PSS leads to insufficient performance and functionality when used to measure medium and large deformations (>20%). Attempts to solve this problem by adding elastic polymers or elastomers led to an increase in extensibility, but reduced sensitivity when detecting small deformations.
A research team from the Terasaki Institute of Biomedical Innovation has developed a portable device capable of effectively detecting skin deformities in a wide range. To maximize the extensibility of this sensor, the researchers drew inspiration from nature. Snakes are known for their ability to stretch many times their normal body size when absorbing prey. Snake skin is covered with overlapping scales; when mechanical force is applied, these scales slide and shift relative to each other, giving the skin exceptional elasticity.
The researchers used this design concept in the manufacture of the new sensor. A thin layer of PEDOT:PSS was applied to an elastomeric tape. This layer was then stretched to an experimentally optimized strain level of 50%. This process led to the appearance of cracks and the disintegration of the PEDOT:PSS layer into small fragments with open areas between them. A second layer of PEDOT:PSS was applied to the resulting open areas, which was then further stretched to 100% of the deformation level, which led to fragmentation and the appearance of new islands of PEDOT:PSS, which naturally aligned with those of the first layer. Upon termination of the tensile force, the sensor acquired the structure of overlapping fragments that imitated snake scales. The new design allowed the sensor to measure a wide range of deformations with a high degree of sensitivity.
The PEDOT:PSS bilayer is coated with hydrogel – this layer will be in contact with the skin, providing biocompatibility and comfort. Copper wires and an elastomeric seal were added to the sensor, and then various experiments were carried out to test its ability to detect deformations of a wide range.
In low-range stretching tests, the wrist pulse was measured at rest and after training. Measurements of the movement of the skin and tissues on the neck during the operation of the speech organs were also performed. Measurements of eyebrow movement and laryngeal movement up and down during swallowing were carried out to identify mid-level stretching. And in high-level tensile tests, various degrees of flexion in the elbow joint were measured.
The results of the experiments showed that the new sensor gives clearly perceptible signals with a wide range of sensitivity. The signals accurately transmitted the degrees and angles of the detected movements. In addition, the sensor has demonstrated excellent conductivity, endurance and reproducibility.
The versatility of the scaly sensor can be useful for a variety of biomedical needs, for example, monitoring the work of the cardiovascular system, helping patients with difficulties in vocalization or swallowing, as well as physical rehabilitation and evaluation of sports results. It can also be used to improve communication when working in a noisy environment or monitoring psychological states related to facial expression.
Article H.Liu et al. Harnessing the wide-range strain sensitivity of bilayered PEDOT:PSS films for wearable health monitoring is published in the journal Cell.
Aminat Adzhieva, portal "Eternal Youth" http://vechnayamolodost.ru based on the materials of the Terasaki Institute: Wearable Sensors with Wide-Ranging Strain Sensitivity.