Perivascular patch using biodegradable polymers: Investigation of mechanical and drug elution characteristics

Intimal hyperplasia (IH) is the primary cause for the vascular graft stenosis. Perivascular devices offer a potential treatment option to reduce the impact of intimal hyperplasia by providing mechanical support and local administration of therapeutic agents to control cellular overgrowth. In the present study, a perivascular patch primarily made up of biodegradable polymer, Poly L-Lactide, has been designed with adequate mechanical strength and ability for sustained drug elution of anti-proliferative drug (Paclitaxel). The elastic modulus of the polymeric film has been optimized by blending the base polymer with different grades of biocompatible polyethylene glycols. Using design of experiments, the optimized parameters were obtained as PLLA with 2.5% PEG-6000 and have shown 3.14 MPa elastic modulus. The film prepared based on optimum conditions has been employed for prolonged drug delivery (about four months) under simulated physiological conditions. The addition of drug release rate enhancer (polyvinyl pyrrolidone K90F) has improved the drug elution rate and ∼83% drug was released over entire study period. The molecular weight of the base biodegradable polymer was estimated by gel permeation chromatography (GPC) which remained unchanged during the drug release study duration. Evidences of Paclitaxel drug crystallization were found to contribute to the sustained drug elution. The SEM examination of the surface morphology post-incubation revealed micropores on the surface, contributing to the overall drug release rate. The study concluded that perivascular biodegradable films could be tailored for their mechanical properties, and sustained drug elution could also be formulated with reasonable choices of biodegradable polymer and biocompatible additives.

Key Factors of Mechanical Strength and Toughness in Oriented Poly(l-lactic acid) Monofilaments for a Bioresorbable Self-Expanding Stent

The investigation of the strength and toughness of poly(l-lactic acid) (PLLA) monofilaments is essential as the fundamental element of a biodegradable braided stent. However, the determining factor remains poorly addressed with respect to influencing the mechanical behavior of PLLA monofilaments. In this work, the electron beam (EB) with different radiation doses was utilized to sterilize PLLA monofilaments. Properties of the monofilaments, including the breaking strength, elongation at break, molecular weight, orientation, and microstructure of the fracture, were characterized. Results showed that a random chain scission of PLLA resulting from EB during this process could cause the decrease in molecular weight, which led to the decline in breaking strength. Meanwhile, the irradiated monofilaments were found to have almost the same elongation at break below a dose of 30 kGy and declined by 71.41% up to a dose of 48 kGy. It was also found that the ductile fracture connection of the monofilament translated to the brittle fracture by comparing the microstructure without and with sterilization. These phenomena could originate from the destruction of the long molecular chains connecting the crystal plates into shorter ones by radiation. PLLA monofilaments with 0, 30, and 48 kGy were used to braid carotid stents. Compared with a carotid Wallstent, the PLLA stent can better provide radial supporting to the carotid lesion. This study provides preliminary experimental references to evaluate and predict the mechanical performance of PLLA braided stents.

Different properties of poly(L-lactic acid) monofilaments and its corresponding braided springs after constrained and unconstrained annealing

Thermal annealing is widely applied to enhance the mechanical performance of PLLA monofilaments, which brings in a variety of expected strengths through different constrained methods. In this work, samples with constrained and unconstrained annealing process were both prepared and characterized, including mechanical performance, surface morphology, radial supporting performance and axial flexibility. Experimental results revealed that the monofilaments under constrained annealing showed higher elastic modulus with 6.4 GPa, which were higher than those without any constraint. While the maximal elongation at break with 51.11% were observed in unconstrained annealed monofilaments. Few changes were presented in the molecular weight between the two types of samples. Moreover, the springs under constrained annealing inhibited the most reliable radial supporting performance with higher radial compression force and chronic outward force, 0.665 N/mm and 0.14 N respectively. However, unconstrained annealing springs showed better flexibility with 0.178 N bending stiffness and 1.58 N maximum bending force. These results suggested that filaments and springs with various properties can be obtained under different annealing conditions.