Helical Conformation Endows Poly-<span style="font-variant: small-caps;">l</span>-Lactic Acid Fibers with a Piezoelectric Charge under Tensile Stress

Abstract

ABSTRACT. Poly-l-lactic acid (PLLA) has been clinically used as a bioabsorbable material and attains a piezoelectric charge upon molecular orientation by the application of a shear force to the C-axis of the crystal line region. Previous studies showed that implanted drawn PLLA films or rods accelerate the ossification due to piezoelectric effect. In this study, we originally designed helically-twisted PLLA fiber to produce piezoelectricity in bioabsorbable suture upon tensile stress. The piezoelectricity of the helical PLLA fibers was evaluated using a lock-in amplifier system in vitro. The ossification induced by helical PLLA fibers was examined by implanting them in the rat patellar ligament supporting a physiological tensile load. We observed that 57° and 45° twisted PLLA fibers generated a higher piezoelectric potential than did 27° twisted fibers. The animal experiment showed that the formation of osseous tissue around helical PLLA fibers was more significant than around non-helical control fibers at 4 weeks after their implantation. These results suggest that helical PLLA fiber may be useful for the surgical suture or artificial ligament, which connects to the bone. KEW WORDS: animal model, helical conformation, ossification, piezoelectricity, poly-l-lactic acid.doi: 10.1292/jvms.12-0323; J. Vet. Med. Sci. 75(9): 1187-1192, 2013 Ligament injury, such as anterior cruciate ligament rupture, is a common cause of lameness in small animal practice [6,15]. Various types of artificial ligaments including bioabsorbable material have been studied to reconstruct the injured ligament [3,27,31]. It has been suggested that early osteointegration of the grafted artificial ligament to the bone is important to improve the ligament function and durability [39].Poly-l-lactic acid (PLLA) is well known as a bioabsorbable material and has been clinically used as bioabsorbable mini-plate, screw and pin [2,13,14,30,33]. On the other hand, PLLA is known as a piezoelectric material. Fukada reported that an external mechanical stress to a drawn PLLA film gives rise to electrical polarization [10], and this piezoelectric effect is ascribed to the change in the microscopic orientation of crystals with a fundamentally asymmetrical structure [5,10,22,23]. Animal experiments showed that the piezoelectricity of drawn PLLA films and rods accelerated callus formation on bone [10,22,35]. Shimono et al. prepared three types of PLLA piezoelectric films with different oriented directions (0°, 45° or 90° to the drawing direction) and implanted them on the periosteum of the rabbit tibia [35]. These drawn PLLA films promoted ossification to a greater extent than undrawn PLLA films, and the effect of drawn PLLA films in promoting ossification was greatest when a shearing stress was applied at 45° to the axis of orientation. Ikada et al. prepared drawn PLLA rods with different draw ratios and implanted these rods intramedullary in the osteotomized tibia of cats [22]. Fracture healing was clearly promoted with increased callus formation as the draw...

Piezoelectric materials as stimulatory biomedical materials and scaffolds for bone repair

“…This has been attributed to the displacement of the C=O bond in PLLA in response to mechanical stress leading to generation of a net dipole moment and charge [145][146][147]. PLLA has been used to fabricate films, fibres and rods with piezoelectric behaviour [148][149][150][151] and it has been observed that crystallinity and polymer orientation play key role in piezoelectric characteristic [152]. It presents an additional advantage of fabricating degradable scaffolds [153][154][155].…”