Wearable piezoelectric mass sensor based on pH sensitive hydrogels for sweat pH monitoring

Abstract

Colorimetric and electrochemical (bio)sensors are commonly employed in wearable platforms for sweat monitoring; nevertheless, they suffer from low stability of the sensitive element. In contrast, mass-(bio)sensors are commonly used for analyte detection at laboratory level only, due to their rigidity. To overcome these limitations, a flexible mass-(bio)sensor for sweat pH sensing is proposed. The device exploits the flexibility of piezoelectric AlN membranes fabricated on a polyimide substrate combined to the sensitive properties of a pH responsive hydrogel based on PEG-DA/CEA molecules. A resonant frequency shift is recorded due to the hydrogel swelling/shrinking at several pH. Our device shows a responsivity of about 12 kHz/pH unit when measured in artificial sweat formulation in the pH range 3–8. To the best of our knowledge, this is the first time that hydrogel mass variations are sensed by a flexible resonator, fostering the development of a new class of compliant and wearable devices.

Comprehensive Review on Wearable Sweat-Glucose Sensors for Continuous Glucose Monitoring

“…The other fundamental sensing methods used in wearable sweat-sensing devices can be categorized into electromechanical techniques. These can be divided into the subcategories of piezoelectric [137], surface acoustic wave (SAW) [138], and photoacoustic spectroscopy [139]. A piezoelectric-material-based crystal or plate resonator is used in piezo-based sensing.…”

Hydrogels and Their Role in Biosensing Applications

“…The applications of wearable devices range from the detection of human movements to force sensing to detection of small molecules. [117][118][119][120] For example, a wearable device on skin, a flexible silk fibroin patch with encapsulated enzyme served as substrate for a conducting polymer, which was photocrosslinked on top. This flexible and biodegradable skin like patch served as a free-standing electronic device.…”

“…[139] Wearable sensors involving hydrogels, e.g., in the form of tattoos or patches to monitor electrolytes or biomarkers in sweat can provide information about clinically relevant health conditions. Key targets are metabolites such as lactate, [140] electrolytes, [141] e.g., chloride, sodium, potassium, pH, [118] glucose, [142,143] drugs, [144] and other small molecules. Often the sensors are used together with gellike materials that are called gel patches.…”

The status and perspectives of nanostructured materials and fabrication processes for wearable piezoresistive sensors

“…Hydrogels show a strong capability to absorb a high amount of water, and possess biological and elastic (i.e., softness) compatibility: these are desirable features for biological applications in wearable chemical (bio)sensors, which require mechanical flexibility to result comfortable to the body (Scarpa et al 2020 ). Several designs of resonating MEMSs, in particular piezoresistive-based MEMSs, have been proposed and realized in combination with hydrogels (Millet et al 2012 ).…”