Flexible and stretchable piezoelectric bio-integrated sensors and energy harvesters based on polyvinylidene fluoride (PVDF)

Piezoelectricity is a phenomenon that electric energy is generated in a certain material when the material is subjected to mechanical load, and vice versa. Piezoelectric devices with high stretchability and flexibility for bio-integrated applications, including skin-conformal strain sensors and energy harvesters, have been developed for many years. However, the manufacturing process for conventional ceramic-based piezoelectric materials is sophisticated, expensive, and time-consuming. In contrast, piezoelectric polymer sheets, such as polyvinylidene fluoride (PVDF), are commercially available, mechanically robust, biocompatible, and less demanding to manufacture. This dissertation introduces low-cost, stretchable bio-integrated sensors and energy harvesters based on PVDF, and a novel subtractive digital manufacturing method for them called the “cut-and-paste” method. To systematically investigate our devices, we developed analytical, numerical, and experimental methods and obtained consistent results. As one demonstration, we created a filamentary serpentine (FS) network of 28-μm-thick PVDF as the soft and stretchable vibration sensor and attached the sensor on human chest to measure seismocardiography (SCG), which reflects chest vibration associated with heartbeats. The SCG measured by our PVDF sensor was comparable with conventional accelerometers, but PVDF sensor was much softer and imperceptible to wear. To optimize the SCG sensing location and to clarify the effect of the skin-sensor elastic mismatch, full-field displacement and strain analysis of the chest motion was conducted via three-dimensional digital image correlation (3D DIC) method. Integrating the FS PVDF-based SCG sensor with Au-based electrocardiogram (ECG) sensors, a skin-soft electro- and mechano-acoustic cardiovascular (EMAC) sensing tattoo has been created. It can synchronously collect electrical and mechanical cardiovascular signals which when combined, can fully reveal the cardiac mechanics such as the pre-ejection period (PEP), the isovolumetric contraction time (ICT), etc. Based on the strong negative correlations between systolic time intervals and systolic/diastolic blood pressures, continuous blood pressure (BP) could be non-invasively tracked via the EMAC sensing tattoo. As another demonstration, we created serpentine PVDF based stretchable energy harvester. We had to optimize the electrode design to minimize the counter effect due to the opposite signs of strain developed in the serpentine ribbon when subjected to stretch. After optimization, the serpentine ribbon exhibits much higher stretchability but comparable attainable electrical outputs with the straight PVDF ribbons