Shape Memory Scaffold Lab Report

CHAPTER 2 LITTERATURE REVIEWSCHEME
2.1 Introduction

Before starting the experiments, it is essential to gain a basic concept and knowledge of shape memory polymer and scaffold first, and it covers a wide range of textile, chemistry and biology technology. After studying these reviews, some critical points of shape memory polyurethanes and scaffold would be found out.
2.2 Introduction of Shape memory polyurethanes (SMPUs)

Shape memory polymers (SMPs) are stimuli-responsive smart materials which having the ability to change their shape upon by external force, and then recovery the primary shape controllable. Changing in shape is caused by a variation of temperature called the thermally induced shape memory effect. Shape memory effect has
The polymeric network chains act as “switching segments” whose thermal transition temperature essentially serves as Ttrans for triggering shape memory effect. The molecular mobility of switching segments changes significantly above and below Ttrans and the modulus of materials can be changed in a narrow temperature range around Ttrans. The network chain segments are flexible at a temperature above Ttrans but stiff in a temperature below Ttrans where the mobility of the chains is frozen or limited. The polymer materials can develop great deformation at a temperature above Ttrans and afterwards, also can be fixed into a temporary shape at temperature below Ttrans. Because of these distinctive shape memory effects, the SMPs have drawn huge attentions in the
There are two phases of SMPUs which are fixed phase and reversible phase, or defines as hard segment and soft segments. The soft and hard segments have a tendency to phase separate and aggregate into soft and hard phase due to the thermo-dynamic incompatibility caused by huge difference of polarity2. Phase transition try to a great change in physical properties of SMPUs. The principle is a vast of modulus and micro-Brownian motion on heating via glass transition or melting temperature can be used in the design of shape memory behavior, large glassy state modulus result in larger shape fixity upon cooling and unloading, while a large rubbery state modulus guide to large shape recovery around heating and standing at high temperature, like the figure 1. Shape memory polyurethane would be deformed if the reversible phase was heated to a soften state. When the reversible phase was cooled to a hardened state, it could maintain the deformation

Typically synthetic polyols of low molecular weight (Mw-2000 g/mol) as the high density of cross-links in these low molecular weight polyols are critical for high mechanical strength and good shape memory effect5. What’s more, higher cross-linked SMPUs have better tensile properties and higher content of cross-link points between the hard segments, thus higher shape recoverability can be resulted, rather