Bioresorbable Polymers

Biomedical Applications

Declan Devine, Declan Devine

• Publisher: De Gruyter
• Year: 2019
• ISBN:
  9783110640663
• Edition: 1
• Topic: Medicine
• Subtopic: Medical Theory, Practice & Reference

Bioresorbable implants can be processed via conventional polymer processing methods such as extrusion, injection and compressing moulding, solvent spinning or casting. This book addresses issues and highlights recent advances in the use of biodegradable polymers. It is intended for researchers utilizing biodegradable polymers in areas from tissue engineering to controlled release of active pharmaceuticals, as well as industrial processors.

Gavin Burke, Elaine Kenny, Maurice Dalton, Declan M. Devine, Eilish Hoctor, Ian Major and Luke Geever

1 Biodegradation and biodegradable polymers

1.1 Introduction

Polymers are organic materials consisting of large macromolecules composed of many repeating units called ‘mers’. These long molecules are covalently bonded chains of atoms. Unless they are crosslinked, the macromolecules interact with one another by weak secondary bonds (hydrogen bonds and van der Waals forces) and by entanglement. The mechanical and thermal behaviour of polymers is influenced by several factors, including the composition of the backbone and side groups, the structure of the chains, and the molecular weight (MW) of the molecules.

‘Biodegradable polymers’ can be defined as polymers that are degradable in vivo, either enzymatically or non-enzymatically, to produce biocompatible or non-toxic by-products. These polymers can be metabolised and excreted via normal physiological pathways [1]. In recent years, emphasis in biomaterials engineering has moved from materials that remain stable in the biological environment to materials that can degrade in the human body. Biodegradable materials are designed to degrade gradually and be replaced eventually by newly formed tissue in the body. Biodegradable implants have the advantage of allowing the new tissue, as it grows naturally, to take over load-bearing or other functions without any of the potential chronic problems associated with bio-stable implants [2].

‘Biodegradation’ is used to describe the process of a material being broken down by nature. However, in the case of medical-purpose biomaterials, biodegradation focuses on the biological processes within the body that cause a gradual breakdown of the material. Biomaterials degradation is a very important aspect to consider if they are used for medical purposes because their ability to function for a certain application depends on the length of time needed to keep them in the body [3, 4]

The simple desire of a physician is to have a device that can be used as an implant and does not necessitate a second surgical procedure for removal. In addition to not requiring a second surgical procedure, the biodegradation may offer other advantages. For example, a fractured bone stabilised with a rigid, non-biodegradable stainless-steel implant has a tendency to weaken the bone and re-fracture (i.e., stress shielding). The bone does not carry a sufficient load during the healing process because the load is carried by the rigid stainless steel. However, an implant prepared from biodegradable polymer(s) can be engineered to degrade at a rate that will slowly transfer load to the healing bone [5, 6]. Also, the degradation and resorption kinetics must be controlled in such a way that the bioresorbable scaffold retains its physical properties and thereafter begins to lose its mechanical properties, subsequently being metabolised by the body without a reaction to a foreign body. Apart from the anatomy and physiology of the host, the type of tissue to be engineered has a profound influence on the degree of remodelling. For example, in cancellous bone, the remodelling takes 3–6 months, whereas cortical bone takes ≈6–12 months to remodel. Whether the biodegradable construct will be part of a load-bearing or non-load-bearing site will also significantly influence the needs for mechanical stability of the biodegradable construct because mechanical loading can affect the degradation behaviour directly. A precise timeline for the biodegradation of these materials has been reported inconsistently in the literature. For example, Hutmacher [7] stated that a scaffold must retain its strength for ≥6 months and thereafter gradually reduce over 12–18 months. This finding is similar to that of a recently published study by Gómez-Barrena and co-workers [8], who stated that inadequate formation of bone 6 months after scaffold implantation can be declared a ‘non-union’. Li and co-workers [9] and Kattimani and co-workers [10] reported bone regrowth to have occurred after 3 months, with the latter study stating that grafts healed fully within 8 weeks of implantation, followed by steady bone density after 3 months. However, Mäkinen and co-workers [11] stated that, based on the literature and experience with orthopaedic implants, a scaffold should be resorbed between 1 and 5 years.

The prevailing mechanism for polymer degradation is chemical hydrolysis of the hydrolytically unstable backbone. In semicrystalline polymers, chemical hydrolysis can be divided into two phases. Initially, water molecules infiltrate the polymer structure and selectively target the amorphous part of the polymer. This process results in the breakdown of long polymer chains into shorter, more hydrophilic polymer chains. Once this fragmentation occurs, the polymer suffers a reduction in MW and mechanical strength. This process leads to the second phase, in which enzymatic breakdown of the polymer fragments results in rapid breakdown of the polymer [12, 13, 14].

Secondary to chemical hydrolysis is bulk erosion. This process occurs if the rate of water penetration occurs faster than the polymer can be converted into water-soluble molecules. In this scenario, there is an initial increase in the rate of surface degradation whereas the interior environment of the polymer becomes increasingly acidic with the increasing concentration of polymer-degradation products, which cannot diffuse out of the polymer. This increase in acidity then creates a self-catalysed degradation of the polymer. This form of polymer degradation has been utilised to control polymer-degradation rates [15, 16]. If the opposite is true (i.e., the rate of water penetration is slower than the rate of polymer conversion to water-soluble molecules), then the third mode of polymer degradation, surface erosion, occurs [17]. This polymer variant results in a slow decrease in polymer size with consistent mechanical strength throughout its structure. Common examples of this type of variant are polyanhydrides and poly-orthoesters, wherein the polymers are hydrophobic but the chemical bonds are ...

Table of contents

• Cover
• Title Page
• Copyright
• Preface
• Contents
• Contributors
• 1 Biodegradation and biodegradable polymers
• 2 Biodegradable medical implants
• 3 Controlled release of poorly soluble active ingredients from bioresorbable polymers
• 4 Potential for biodegradable polymer-based nanotechnology in drug delivery
• 5 Processing of biodegradable polymers
• 6 Cytotoxicity and biocompatibility of bioresorbable polymers
• Abbreviations
• Index