Physical forms and processing of degradable polyurethanes

Many fabrication methods exist for processing degradable PUs, whose final forms affect their degradability [145]. 

Another scaffold fabrication technique is thermally induced phase separation (TIPS) [34,46,84,147-149]. TIPS involves decreasing the temperature of a polymer solution to obtain a polymer-rich and polymer-poor phase. Following phase separation, the solvent is removed using one of a number of methods (freeze drying, evaporation) resulting in the formation of pores in the polymer stracture. TIPS can also be combined with the use of porogens to increase void fraction, have better control over pore size, or improve pore interconnectivity. 

Another modification involves removing the potentially cytotoxic solvent from the electrospinning equation in a process called melt electrospinning [159]. This process involves tight control of temperature, and for the case of a PU based on BDl, PCL, 1,4-butanediamine (BDA), and BDO involved temperatures of 220-240 °C [160]. 

The use of oil/water (OAV) emulsions has also been employed to fabricate PU NPs [162-165]. In this method, the diisocyanate (IPDI) is first dissolved in an oil/ surfactant mixture (90/10, saturated medium chain triglyceride/polysorbate 80 [polyoxyethylene 20-sorbitan monooleate]). Addition of the aqueous phase with PEG 400 (diamine or diol) to the 0/S mixture in dropwise fashion (to obtain 90% aqueous component) occurs under mechanical stirring to obtain nanoemulsions, followed by heating to 70 °C to allow polymerization and achieve PU or PU urea NPs, which can be isolated by ultracentrifugation [165]. This method works by having IPDI present in the core of oil nanodroplets in the 0/W nanoemulsion, which react with the diols or diamines at the surface of the oil droplet, resulting in the formation of the NPs with a size distribution from 40 to lOOnm. 

Core-shell PU NPs can also be prepared by appropriate choice of isocyanate [166]. Blocked amphiphilic prepolymers were prepared by Cheong et al., where the hydrophobic block is composed of IPDI-polytetramethylene adipate polyol (PTMA) and the hydrophilic block is MDI-DMPA. The resulting polymer is added drop-wise to water under stirring, resulting in a core-shell structure with the hydrophobic IPDI-PTMA in the core and hydrophilic MDI-DMPA in the shell (80-100 nm diameter) [166]. 

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