Design of ultrasensitive flexible bend sensor using silver-doped oriented PVDF nanofiber web for respiratory monitoring.

We propose a design strategy to fabricate a flexible bend sensor (BS) with ultrasensitivity towards airflow using all PVDF nanofiber web based sensing element and electrode to monitor human respiration. The unique electrospinning (rotational speed of collector of 2000 rpm and tip-to-collector distance of 4 cm) with silver nanoparticles interfacing was introduced to prepare an Ag-doped oriented PVDF nanofiber web with high β-phase content as sensing element (AgOriPVDF, β-phase crystallinity ~ 44.5 %). After that, a portion of the prepared AgOriPVDF was processed into flexible and electrically conductive electrode through electroless silver plating technique (SP-AgOriPVDF). Interestingly, the encapsulated AgOriPVDF BS with SP-AgOriPVDF electrode exhibited superior piezoelectric bending response (open-circuit peak-to-peak output voltage, Vp-p ≈ 4.6 V) to injected airflow, which is more than 200 times higher than that of the unpackaged randomly aligned PVDF nanofiber web BS with conductive tape electrode (Vp-p ≈ 0.02 V). In addition, the factors influencing the bend sensitivity of BS such as β-phase content, nanofiber orientation, flexibility of electrode and so forth were thoroughly analyzed and then discussed. We also demonstrated that the AgOriPVDF BS has sufficient capability to detect and identify various respiratory signal, presenting a great potential for wearable applications, e.g. smart respiratory protective equipment.