Shape Memory Polymers for Biomedical Applications

L Yahia

• Publisher: Woodhead Publishing
• Year: 2015
• ISBN:
  9780857097057
• Topic: Tecnología e ingeniería
• Subtopic: Ciencias de los materiales

Shape memory polymers (SMPs) are an emerging class of smart polymers which give scientists the ability to process the material into a permanent state and predefine a second temporary state which can be triggered by different stimuli. The changing chemistries of SMPs allows scientists to tailor important properties such as strength, stiffness, elasticity and expansion rate. Consequently SMPs are being increasingly used and developed for minimally invasive applications where the material can expand and develop post insertion. This book will provide readers with a comprehensive review of shape memory polymer technologies. Part 1 will discuss the fundamentals and mechanical aspects of SMPs. Chapters in part 2 will look at the range of technologies and materials available for scientific manipulation whilst the final set of chapters will review applications.

• Reviews the fundamentals of shape memory polymers with chapters focussing on the basic principles of the materials
• Comprehensive coverage of design and mechanical aspects of SMPs
• Expert analysis of the range of technologies and materials available for scientific manipulation

Part One

Fundamentals of shape-memory polymers for biomedical applications

1

Introduction to shape-memory polymers for biomedical applications

L. Yahia [email protected] École Polytechnique de Montréal, Montréal, QC, Canada

Abstract

Shape-memory polymers (SMPs) are a class of smart materials capable of undergoing a programmed shape change upon application of an external stimulus and remembering the original shape. This behavior is known as the shape-memory effect and is a phenomenon observed first in shape-memory alloys, where the memory effect is governed by the thermoelastic behavior of the martensite phase. In this book, a number of research findings have been introduced to show the current and potential applications of SMPs in medicine. Remarkable research works are focusing on the design and evaluation of SMP-based medical devices, such as vascular stents, clot removal tools, and aneurysm occlusion implants.

In this introduction, two critical aspects regarding the safety of SMPs are raised, i.e., the sterilization method to be used and their biocompatibility. More systematic studies both in vitro and in vivo are required to better address the concern of biocompatibility/sterilization before fabricating biomaterials into implants.

Keywords

Shape memory polymers (SMP)

Shape memory polyurethane

Martensitic transformation

Nitinol

(NiTi) sterilization

Biocompatibility

Food and Drug Administration (FDA)

Biomedical applications

1.1 Introduction

The ability of polymers to respond to external stimuli is of high scientific and technological significance. Such stimuli include heat (thermo-responsive materials), stress/pressure (mechano-responsive materials), electrical current/voltage (electro-responsive materials), magnetic field (magneto-responsive materials), pH-change/solvent/water/moisture (chemo-responsive materials), light (photo-responsive materials), and ultrasound-responsive materials (Sun et al., 2012).

Shape-memory polymers (SMPs) are a class of mechanically functional “smart” materials that have generated substantial interest for biomedical applications. SMPs have most notably been promoted for their potential in minimally invasive surgery, where a compacted device could be passed through a smaller incision and deployed to its full shape once inside the body. In addition, SMPs offer the ability to provide structural support, exert stabilizing forces, elute therapeutic agents, and biodegrade.

As new SMPs are developed for medical applications, it is important to consider the potentially harmful effects the SMPs could have when implanted in the body, and the resulting immune response that is elicited. In vivo testing of materials must be conducted to evaluate the local and systemic interactions of the SMPs with the native tissue. Implanted materials elicit a complex wound-healing process, including hemostasis, inflammation, and repair and remodeling. Typically, materials are implanted subcutaneously in soft tissue to determine the inflammatory response, as has been done for some SMPs (Filion et al., 2011), followed by implantation in an animal model to the target site for functional in vivo testing. A safety consideration unique to SMPs for medical applications is the effect the triggering mechanism may have on the body. Heat-triggered SMPs must be triggered at a cytocompatible temperature at or above 37 °C. SMPs with a triggering temperature lower than body temperature may recover prematurely before implantation, whereas SMPs with triggering temperature above 37 °C may cause cell and tissue damage upon triggering. This provides one motivation for the study of non-thermal triggering mechanisms, such as light, solvent, and magnetic/electric fields.

1.1.1 Shape-memory effect

Since its discovery in alloys in 1932 by Chang and Read (1951), the shape-memory effect (SME) has been extensively investigated in metal alloys for its potential use in medicine (Lipscomb and Nokes, 1996). The fundamental phenomenon of the memory effect governed by thermoelastic behavior of the martensite phase was widely reported by Kurdjumov and Khandros (1949). This phenomenon was later observed in polymers (Kim et al., 1996; Lendlein et al., 2001), thus introducing a variety of materials with stimuli responsiveness, which represented a less expensive and more efficient alternative to established shape-memory alloys (Yahia and Ryhänen, 2000; Yahia et al., 2009). More recently, the synthesis of SMPs has found inspiration in biological substances, as naturally occurring bile acids have been used to fabricate amorphous, thermoplastic polyesters with shape-memory properties (Gautrot and Zhu, 2006, 2009). Interestingly, these bioinspired polymers displayed rubber-like elasticity, glass transitions close to body temperature, high strains at low temperatures, and low systemic toxicity. In addition, these natural biodegradable polymers have several advantages inside the body, such as the capacity for the complete dissolution of the implant. The advantage of biodegradable SMPs is the same as traditional biodegradable materials, except that the shape-memory effect imparts additional functionality to the material by easing minimal invasion and self-actuation inside the body (Lu et al., 2007).

1.1.2 Biocompatibility of SMPs

As mentioned above, the development of metallic or polymeric shape-memory materials for biomedical applications is progressing rapidly because of the unique properties of these materials. However, they must fulfill the basic criterion of biocompatibility before they can be fabricated into medical implants or scaffolds for tissue engineering. The period during which a biomaterial remains inside the human body is important to consider in terms of its use. Though the shape-memory alloy nitinol (NiTi) has typically been successfully used in biomedical devices, its biocompatibility remains controversial due to its high content of potentially carcinogenic nickel. Among the numerous studies that have assessed the biocompatibility of NiTi, several have demonstrated the potential cytotoxicity of NiTi-based devices (Berger-Gorbet et al., 1996). Observed cellular death and severe tissue damage were associated with the release of nickel ions from NiTi alloys. However, other studies agree on the safety of NiTi alloy (Yahia and Ryhänen, 2000). Therefore, caution is required when NiTi-based devices are used long term because of the possibility that dissolved nickel ions will be released into the human body and cause both toxic and allergic reactions (Yahia et al., 2009). In contrast to shape-memory alloys, SMPs have been generally accepted as biocompatible. Many research groups have assessed the biocompatibility of various SMPs. Numerous early studies focused on the biocompatibility of commercially polyurethane SMPs, which in general display low cytotoxicity and in vivo inflammatory response. In 2003, Metcalfe et al. tested cold hibernated elastic memory foams for the treatment of lateral wall aneurysms in carotid arteries by using an in vivo model in dogs (Metcalfe et al., 2003). These materials were fabricated from polyurethane-based SMP in the form of open cellular structures (foams) by Mitsubishi Heavy Industry and Jet Propulsion Laboratory (California Institute of Technology, Pasadena, CA, US...