Abstract:

Objective Postoperative monitoring of vascular diseases is crucial for evaluating repair efficacy and preventing complications. However, existing clinical monitoring methods are associated with inherent limitations, including reliance on large-scale equipment, cumbersome operational procedures, and the lack of continuous monitoring capability. In order to address these pressing issues, this study aims to develop a flexible implantable sensor based on poly(L-lactic acid) (PLLA) that enables long-term, continuous monitoring of the repair status of vascular diseases. PLLA was selected as the core material by virtue of its excellent biocompatibility, biodegradability, and inherent piezoelectric properties, which are essential for constructing implantable devices with minimal biological side effects.

Method Although PLLA is a medically degradable material with intrinsic piezoelectricity, the nanofiber membranes fabricated via electrospinning typically exhibit low piezoelectric output, which severely restricts their practical application in sensor devices. Moreover, the underlying mechanism regulating the piezoelectric properties of PLLA nanofibers remains unclear. In order to overcome these short comings, a systematic experimental approach was adopted. In particular, different electrospinning parameters and post-treatment conditions were selected to fabricate a series of PLLA nanofiber membranes. Comprehensive characterizations were performed to investigate the influences of these parameters on the fiber morphology and molecular crystal structure of the PLLA nanofibers, as well as their subsequent impacts on piezoelectric performance.

Results The experimental results demonstrated that both fiber morphology and crystallinity are critical factors governing the piezoelectric output performance of PLLA nanofiber membranes. PLLA nanofibers with a smaller and more uniform diameter exhibited the optimal piezoelectric response, as such morphological features facilitate the efficient generation and transmission of piezoelectric charges. When the fiber morphology was maintained at an optimal state, the piezoelectric output of PLLA nanofibers increased linearly with the enhancement of α-phase crystallinity. In contrast, heat treatment of the nanofibers induced the formation of α'-phase crystals, and notably, an increase in α'-phase crystallinity led to a significant decrease in piezoelectric performance. Under the optimized electrospinning and post-treatment parameters, the PLLA nanofiber membrane achieved a maximum output voltage of 2.933 V (under the condition of 87.7 N load and 1 Hz frequency), an output current of 766.26 nA, a charge density of 1.95 μC/m², and a maximum output power of 4.23 mW/m². Furthermore, the sensor maintained linearity in the pressure range of 8.3-186.4 kPa, which fully covers the physiological pressure range of human blood vessels, indicating its suitability for vascular pressure monitoring applications. Additional tests using an in vitro vascular simulation device confirmed that the flexible PLLA sensor could effectively perceive cyclic pulsating strains similar to those generated by blood vessel contraction and relaxation.

Conclusion This study clarifies the regulatory mechanisms of the piezoelectric performance of PLLA nanofiber membranes and optimizes such performance via parameter modulation. Specifically, fiber morphology and crystallinity are confirmed as key determinants: smaller fiber diameters without bead-like structures enhance piezoelectric output; with favorable morphology, output increases with α-phase crystallinity, while α'-phase formation after heat treatment reduces piezoelectricity despite higher crystallinity. Under optimal parameters, the PLLA nanofiber membrane achieves 2.933 V output voltage (87.7 N, 1 Hz), 766.26 nA current, 1.95 μC/m² charge density, 4.23 mW/m² maximum output power, and excellent linearity under 8.3-186.4 kPa. In vitro vascular simulation tests verify its feasibility for practical monitoring by effectively sensing cyclic pulsating strain. Collectively, the PLLA-based flexible implantable sensor exhibits excellent sensitivity, stability, and biocompatibility, meeting the demands of real-time continuous postoperative vascular repair monitoring. It thus holds great clinical application potential, offering a novel solution to the limitations of existing clinical monitoring methods.

Key words: piezoelectric sensor, flexible sensor, poly (L-lactic acid), electrospinning, piezoelectric property, pulsation monitoring, biomedical textiles, nanofiber membrane