Biodegradable Piezoelectric Force Sensor

Abstract

Measuring vital physiological pressures is important for monitoring health status, preventing the buildup of dangerous internal forces in impaired organs, and enabling novel approaches of using mechanical stimulation for tissue regeneration. Pressure sensors are often required to be implanted and directly integrated with native soft biological systems. Therefore, the devices should be flexible and at the same time biodegradable to avoid invasive removal surgery that can damage directly interfaced tissues. Despite recent achievements in degradable electronic devices, there is still a tremendous need to develop a force sensor which only relies on safe medical materials and requires no complex fabrication process to provide accurate information on important biophysiological forces. Here, we present a strategy for material processing, electromechanical analysis, device fabrication, and assessment of a piezoelectric Poly-l-lactide (PLLA) polymer to create a biodegradable, biocompatible piezoelectric force sensor, which only employs medical materials used commonly in Food and Drug Administration-approved implants, for the monitoring of biological forces. We show the sensor can precisely measure pressures in a wide range of 0-18 kPa and sustain a reliable performance for a period of 4 d in an aqueous environment. We also demonstrate this PLLA piezoelectric sensor can be implanted inside the abdominal cavity of a mouse to monitor the pressure of diaphragmatic contraction. This piezoelectric sensor offers an appealing alternative to present biodegradable electronic devices for the monitoring of intraorgan pressures. The sensor can be integrated with tissues and organs, forming self-sensing bionic systems to enable many exciting applications in regenerative medicine, drug delivery, and medical devices.

Keywords: PLLA; biodegradable; piezoelectric; pressure; sensor.

Fig. 1.

Biodegradable piezoelectric PLLA pressure sensor. (A) Simplified schematic representing the biodegradable piezoelectric PLLA sensor. (B) Optical image of a fabricated biodegradable piezoelectric PLLA sensor (5 mm × 5 mm and 200 µm thick).

Fig. 2.

Characterization of crystallinity and polymer chain orientation for processed PLLA. (A) Results from one-dimensional (1D) XRD of stretched PLLA films with different DRs. (Inset) Crystallinity percentage of the processed PLLA for different DRs, quantified from the 1D XRD spectrum. (B) Two-dimensional XRD images show polymer chain’s orientation of the stretched PLLA films with different DRs.

Fig. 3.

Characterization of piezoelectric PLLA output from vibration and impact modes. (A) Simplified schematics representing the vibration (Left) and impact (Right) methods used to characterize the PLLA. F, force. (B) Voltage output from the treated PLLA with different DRs under a vibration at 200 Hz. (C) Voltage output from an untreated PLLA (red) and treated PLLA (black, DR = 6) (Bottom) under the same impact force (Top).

Fig. 4.

Characterization of biodegradable piezoelectric PLLA sensor. (A) Typical calibration curve generated by a PLLA sensor/charge amplifier circuit assembly. (Inset) Typical output voltage signals from different input forces. (B) Output signals from the biodegradable PLLA sensor (red) and a commercially available piezoelectric quartz sensor (black) under the same applied force/pressure. (C) Output voltages of the PLLA sensor under the same applied pressure on the initial day and after 4 d in phosphate-buffered solution at 37 °C. (D) Optical images showing the sensor at different days in the buffered solution at an accelerated-degradation temperature of 74 °C.

Fig. 5.

In vivo force measurement and biocompatibility test. (A) Optical image illustrates the sensor and a mouse abdominal cavity with diaphragmatic membrane. (B) Surgical wound closed up by medical suture on abdomen of the mouse, which received an implanted PLLA sensor. (C) Data show the distinct force signals generated by the implanted sensor when the mouse was alive and under anesthesia (black), and when the mouse was euthanized by overdose of anesthetics (red). (Inset) Diagram describes the sensor attached to the bottom of mouse diaphragm inside the abdomen. (D–G) Histology images of s.c.-implanted PLLA sensors after 2 and 4 wk, respectively. D and F are histology stained by H&E while E and G are histology stained by Masson’s Trichrome. Asterisks (*) show locations of the implanted sensors. (Scale bars, 100 µm.)