Shape memory polymer biocompatibility

Shape memory polymers are here to stay, not only because of their unique ability to display double existence under the influence of a triggering mechanism, but also because, unlike shape memory alloys, their elastic deformation and recoverable strains are huge, and their transition dependence can be tailored to fit specific requirements as well as having excellent biocompatibility, nontoxicity, ease of manufacture, and, perhaps most importantly, low cost of manufacture. 

As compared to metallic compounds used as shape memory materials, shape memory polymers have low density, high shape recoverability, easy processability, and low cost. Since the discovery by Mitsubishi in 1988, polyurethane SMPs have attracted a great deal of attention due to their unique properties, such as a wide range of shape recovery temperatures (— 30°C to 70°C) and excellent biocompatibility, besides the usual advantages of plastics. A series of shape memory polyurethanes (SPMUs), prepared from polycaprolactone diols (PCL), 1,4-butanediol (BDO) (chain extender), and 4,4 -diphenylmethane diisocyanate (MDI) or toluene diisocyanate (TDI) have recently been introduced [200—202]. 

Zhu et al. studied a biocompatible shape memory polymer blend based on poly(e-caprolactone) (PCL) and polymethylvinylsiloxane (PVMS). Pure PCL was subject to scission rather than cross-linking under irradiahon. In the presence of a small amoimt of PVMS (<20 wt%), both polymers are miscible in the amorphous phase and the radiation cross-linking of PCL is enhanced. Mechanical properties were improved, and a strong shape memory behavior was achieved. Above the melhng point of PCL, the blend exhibited a rubber-like state and could be deformed. The switch temperature was the melting temperature of PCL. With 5 to 15 wt% of PVMS and under 100 kGy y-irradiation, the deformation fixation ratio and the deformation recovery ratio were 100%. 

Cabanlit, M., Maitland, D., Wilson, T., Simon, S., Wun, T., Gershwin, M.E., Van de Water, J., 2007. Polyurethane shape-memory polymers demonstrate functional biocompatibility in vitro. Macromolecular Bioscience 7,48—55. 

Biomedical applications biocompatible shape memory polymers... 

Recently, Lendlein et al. created a series of responsive shape memory polymers which are mechanically tough, biocompatible, and biodegradable, applicable to a number of biomedical apphcations [300,301,305]. Such shape memory polymers are achieved by copolymerizing precursors with different thermal characteristics such as the melting transition temperature. These shape memory polymers can be deformed into a temporary compressed state and they can recover the permanent shape only with the aid of an external stimulus such as temperature. This type of ape memory polymer mainly consists of two components (i) molecular switches—precmsors that can imdergo stimuh responsive deformation and can fix the formed tempo-... 

A review of micro-electromechanical systems (MEMS)-based delivery systems provides more detailed information of present and future possibilities (52). This covers both micropumps [electrostatic, piezoelectric, thermopneumatic, shape memory alloy bimetallic, and ionic conductive polymer films (ICPF)] and nonmechanical micropumps [magnetohydrodynamic (MHD), electrohydrodynamic (EHD), electroosmotic (EO), chemical, osmotic-type, capillary-type, and bubble-type systems]. The biocompatibility of materials for MEMS fabrication is also covered. The range of technologies available is very large and bodes well for the future. 

In this section, the sterilization and biocompatibility of SMPs are discussed jointly. All proposed SMP medical devices evenmally have to be validated with a designated sterilization method before they can be used clinically. The method of sterilization can influence the biocompatibility and performance of a device [104, 105], Subsequently, sterilization can also alter the thermomechanical properties of the polymer, which directly influence shape-memory properties such as shape storage (fixity) and recovery [106]. Currently, there are three types of sterilization methods including heat, radiation, and chemical techniques. 

Once an SMP device is implanted within the body and fully activated, the device ceases to be shape-memory and should have the properties of a typical polymer-based device and are subject to all the same long-term performance concerns. Obviously, long-term biocompatibility and carcinogenicity are a concern of implantable polymeric materials however, mechanical properties of polymers with respect to water absorption and biodegradation will be discussed for the remainder of this chapter. 

Traditionally, chemical catalysts have been used to perform several reactions. By replacing the chemical catalysts with enzymes, final products can proceed in a controlled manner. Enzymatic reactions can be used to upgrade cheap and saturated fats or to add value to commercial fats and oils. Vegetable oil-based polyurethane, polyester, polyether and polyolefin are the four most important classes of polymers, many of which have excellent biocompatibilities and unique properties including shape memory. Many researchers have investigated lipase-catalyzed reactions as an alternative to green processes and as a way to improve the physical properties of final products (Miao et al., 2013). 

P. Singhal, J.N. Rodriguez, W. Small, S. Eagleston, J. Van de Water, D.J. Maitland, T.S. Wilson, Ultra low density and highly crosslinked biocompatible shape memory polyurethane foams, J. Polym. Sci. B Polym. Phys. 50 (2012) 724-737. 

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