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.
Highlights
Colorimetric and electrochemicalsensors are commonly employed in wearable platforms for sweat monitoring; they suffer from low stability of the sensitive element
The flexible aluminum nitride (AlN) microbalances were fabricated on Kapton HN, laminated onto a rigid silicon (Si) support, starting from the protocol described in a previous work[35]
The flexible AlN membranes were realized exploiting sputtering deposition for materials followed by UV-photolithography (Mask Aligner SUSS MA8/BA8) and etching/lift-off processes for defining electrodes and active piezoelectric elements
Summary
Colorimetric and electrochemical (bio)sensors are commonly employed in wearable platforms for sweat monitoring; they suffer from low stability of the sensitive element. Advances in the fabrications methods of micro and nano electromechanical systems (MEMSs and NEMSs) and the availability of new (bio)sensing platforms have allowed the commercialization of wearable and portable (bio)sensors for checking health s tatus[1,2,3,4] Such microsystems can continuously monitor the physiological conditions by tracking physical (e.g. heart rate, blood pressure and temperature) and/or chemical parameters (biologically relevant molecules) in a non-invasive w ay[5,6]. A wearable gravimetric sensor for sweat pH monitoring is shown It consists of flexible piezoelectric resonators surmounted by pH responsive cylindrical hydrogel microstructures. Characterization by laser Doppler vibrometry (LDV) in artificial sweat demonstrates the sensitivity of the fabricated flexible microbalances to acidic and basic pH conditions
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