Advances in Polyurethane Biomaterials

Stuart L. Cooper, Jianjun Guan

• Publisher: Woodhead Publishing
• Year: 2016
• Print ISBN: 9780081006146
• eBook ISBN: 9780081006221
• Topic: Medicine
• Subtopic: Medical Technology & Supplies

Advances in Polyurethane Biomaterials brings together a thorough review of advances in the properties and applications of polyurethanes for biomedical applications. The first set of chapters in the book provides an important overview of the fundamentals of this material with chapters on properties and processing methods for polyurethane. Further sections cover significant uses such as their tissue engineering and vascular and drug delivery applications Written by an international team of leading authors, the book is a comprehensive and essential reference on this important biomaterial. - Brings together in-depth coverage of an important material, essential for many advanced biomedical applications - Connects the fundamentals of polyurethanes with state-of-the-art analysis of significant new applications, including tissue engineering and drug delivery - Written by a team of highly knowledgeable authors with a range of professional and academic experience, overseen by an editor who is a leading expert in the field

Part One

Chemistry, processing and applicationsof polyurethane biomaterials

Outline

1. Hierarchal structure–property relationships of segmented polyurethanes

2. Surface characterization techniques for polyurethane biomaterials

3. Design of biodegradable polyurethanes and the interactions of the polymers and their degradation by-products within in vitro and in vivo environments

4. Novel applications of urethane/urea chemistry in the field of biomaterials

5. 3D printing of polyurethane biomaterials

6. Nanoparticle-induced phenomena in polyurethanes

7. Polyurethane nanoparticles, a new tool for biomedical applications?

8. Polyurethanes for controlled drug delivery

9. Antibacterial polyurethanes

1

Hierarchal structure–property relationships of segmented polyurethanes

T.J. Touchet, and E.M. Cosgriff-Hernandez∗ Texas A&M University, TX, USA
^∗ Corresponding author: [email protected]

Abstract

The highly tunable mechanical and physicochemical properties of biodegradable polyurethanes make them promising candidates in the rapidly growing market of resorbable devices. Incorporation of hydrolytically or enzymatically cleavable moieties into the polyurethane structure confers biodegradability; however, these modifications can also affect other physical properties. The impact of these modifications on the resulting properties is determined by a number of structural and morphological factors. Successful design of biodegradable polyurethane devices depends on balancing the mechanical property requirements and the desired degradation rate. To this end, key structure–property relationships of biodegradable polyurethanes are discussed. An overview of the effects of structural components, such as soft segment chemistry, hard segment chemistry, and hard segment content, on mechanical properties and degradation rate is provided.

Keywords

Biodegradable; Polyurethane; Structure–property relationships; Tissue engineering

1.1. Introduction

The use of polyurethanes in medical devices has been well documented since 1965 [1–7]. More recently, efforts have focused on the design of biodegradable formulations for use in tissue-engineered scaffolds and other resorbable implants. Biodegradable polyurethanes have a unique set of design requirements that include the use of biocompatible components, tissue-like mechanical properties, bioactivity, and an appropriate degradation rate. To accommodate these design criteria, the traditional polyurethane structure is often modified to incorporate degradable linkages or cell-responsive moieties. The impact of these modifications on the resulting properties is affected by a number of factors including phase mixing, soft segment chemistry, hard segment chemistry, hard segment content, and molecular weight [8–12]. To enable rational design for these applications, an in-depth understanding of the effects that structure has on the properties is necessary. Characterization of polyurethane structure–property relationships has historically been focused on biostable formulations [4,9,13–17]. This chapter will provide an overview of the effect of polyurethane structure on physical properties with an emphasis on biodegradable polyurethane elastomers.

1.2. Structure of segmented polyurethanes

1.2.1. Polyurethane reactions

Polyurethane reactions fall into the category of step growth or condensation polymerization. In this process, bifunctional monomers react in a stepwise manner to produce long chains of the reacting monomers [18]. Step growth and polycondensation polymerizations typically expel a small molecule such as water or CO₂; however, there is no by-product in the segmented polyurethane synthesis. Segmented polyurethanes comprise a low Tg soft segment that is commonly a low molecular weight macrodiol ranging from 400 to 6000 kg/mol and a glassy or semicrystalline hard segment of diisocyanate and chain extender. In a typical segmented polyurethane synthesis, the macrodiol is reacted with an excess of isocyanate to form a prepolymer. The prepolymer is then reacted with a chain extender to build molecular weight and form a linear block copolymer with alternating blocks of hard segment and soft segment. In contrast to the one-shot method in which the isocyanate, polyol, and chain extender are all reacted at once, the prepolymer method yields more ordered structure and control of properties [18]. Figure 1.1 provides a schematic comparing the polyurethane reaction using the one shot-method and the prepolymer method.

Figure 1.1 Polyurethane polymerization based on the one-shot method and prepolymer method.

The central reactions of polyurethane synthesis are the formation of the carbamate or urethane linkage that occurs when an isocyanate reacts with an alcohol and the urea linkage that occurs when an isocyanate reacts with an amine. Isocyanates are a unique functional group that has several resonant structures and allows for the reaction with both nucleophiles and electrophiles [6,7,18]. Isocyanates react readily with primary alcohol functional groups but will also react with both secondary and tertiary alcohols at slower rates. The reaction kinetics can be influenced by factors such as steric hindrance that can slow down the reaction or proximity of electron-withdrawing groups that increase the rate of reaction [6,7]. The nitrogen of the urethane can also undergo a secondary reaction with excess isocyanate to form allophanates. Urea linkages can undergo similar reactions with excess isocyanate to form biurets [6,7,18]. These reactions provide thermally labile crosslinks and provide additional structural diversity in polyurethane design (Figure 1.2).

1.2.2. Segmented polyurethane elastomers

Elastomers are a class of polymers that can be repeatedly strained and then return to the approximate original length on release of the load. Traditional elastomers such as rubber are able to achieve this elastic behavior by having a low glass transition temperature and a small number of chemical crosslinks that form a permanent network for high recovery. Similar to rubber, thermoplastic polyurethane elastomers have soft segments with low glass transition temperatures but are reinforced...

Table of contents

• Cover image
• Title page
• Table of Contents
• Related titles
• Copyright
• List of contributors
• Woodhead Publishing Series in Biomaterials
• Preface
• Part One. Chemistry, processing and applicationsof polyurethane biomaterials
• Part Two. Polyurethanes for vascular applications
• Part Three. Polyurethane scaffolds for tissue engineering
• Index
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