Fabrication, Characterization And Preliminary Applications Of Biodegradable Tissue Engineering Porous Scaffolds Composed Of Biodegradable Polyester

As an interdisciplinary field, tissue engineering applies the principles ofengineering and the life sciences toward the development of biological substitutes thatrestore, maintain, or improve tissue function. Tissue engineering will take people intothe new eve of tissue manufacture away from traditional transplantation.Three-dimensional porous scaffolds fabricated from biodegradable polymers playcritical roles in tissue engineering and in vivo tissue reconstruction. High porositiesand interconnected pores of porous scaffolds are required to facilitate cell seeding andadhesion, extracellular matrix secretion and eventual tissue regeneration. Acomplicated external shape is highly desirable from the clinical point of view becausethe final anatomical shape of a regenerated tissue is basically dependent on the shapeof the associated scaffold. The degradation behaviors of porous scaffolds affect cellvitality, cell growth and even host response. Other properties of scaffolds, such asmechanical properties, surface behavior and so on, also have great influence on itsfunctions. So the study of tissue engineering porous scaffolds become a veryimportant research topic.So far, many investigations have been done of tissue engineering scaffolds.Nevertheless, it is still worthy of improvement of highly-repeatable techniques tofabricate porous scaffolds with a precise external shape and well-controlled internalpores. The fundamental research of theology of the complex fluids composed ofbiodegradable polymers and inorganic porogens is also rather challenging. Thepresent Ph. D thesis is focused upon these two aspects. In this thesis, biodegradable polyesters such as poly(D,L-lactide) (PLA) andpoly(D,L-lactide-co-glycolide) (PLGA) were synthesized by ring openingpolymerization, and porous scaffolds with anatomical shapes were then fabricated byroom-temperature compression molding/particulate leaching approach androom-temperature injection molding/ particulate leaching approach based onpolyester/solvent/particles mixture. The rheological behaviors of (biodegradablepolyester)/(organic solvent)/(inorganic particles) suspensions were studied for the firsttime. The properties of porous scaffolds, such as porosity, pore structure, shrinkageduring processing and degradation, were investigated. Some animal experimentsrelated to vein grafts and oesophagus repair were, in a cooperative way, performed.The main achievements are listed as follows:1. Rheology of suspension of polyesters and inorganic salts as a processingsystem for fabrication of tissue engineering scaffolds. A kind of suspensioncomposed of biodegradable polyester, volatile organic solvent, and solid inorganicparticles of size of hundreds of microns has been a very important system infabrication of porous scaffolds in the field of tissue engineering. This thesis performsthe basic rheological investigations of this complex fluid. After trial and analysis, aCouette geometry covered by silicon oil was found to be an appropriate geometry tokeep the stability of the rheological measurements. The suspension was a Newtonianfluid at low shear frequencies and exhibited a power-law relation at high shearfrequencies which was well fitted by Bird-Carreau model. Suspension viscosity wasincreased with the particle's volume fraction, and the increase extent was much largerthan that predicted by the Einstein suspension equation, which implied the existenceof interparticle interaction or particle reorganization. Both start-up dynamics at theinception of a steady shearing and relaxation after an abrupt change of oscillatoryshear frequency in the suspensions showed significantly different behaviors fromthose in the associated polymer solutions. These transient observations furtherconfirmed the mechanism of particle reorganization in a concentrated hybrid suspension upon change of a rheological state.2. Room-temperature compression molding technique of polyester porousscaffold with a complicated shape. Polyester three-dimensional porous scaffolds fortissue engineering with anatomical shapes (ear, joint, tube, cylinder) were fabricatedby room-temperature compression molding/particulate leaching approach (RTCM/PL).The pores were interconnected, and porosity can be over 90%. The availability of this"room-temperature" compression approach comes from the solvent assistance, but thepossible post-molding scaffold shrinkage turns out to be a new problem and was thusexamined with emphasis. The shrinkage was found to be tolerable under usualfabrication conditions with high salt contents, which is just what the preparation ofhighly porous scaffolds requires. Furthermore, the resultant porosities after saltleaching were measured as well as the initial scaffold shrinkages after solventevaporation, and the relation between them was revealed by theoretical analysis andconfirmed by comparison with experimental measurements. This convenientfabrication approach is prospective to tailor porous scaffolds for a variety of potentialapplications in tissue engineering and tissue reconstruction.3. Room-temperature injection molding technique of polyester porousscaffold with a complicated shape. A "room-temperature" injection moldingapproach combined with particulate leaching (RTIM/PL) was used to fabricatethree-dimensional porous scaffolds composed of biodegradable polyesters for tissueengineering. In this approach, a "wet" composite of particulate/polymer/solvent wasused in processing, and thus the injection was not performed at melting state.Appropriate viscosity and flowability were facilely obtained at a certain solventcontent so that the composite was able to be injected into a mould under low pressureat room temperature, which was very beneficial for avoiding thermal degradation ofpolyesters. As a demonstration, tubular and ear-shaped porous scaffolds werefabricated from biodegradable poly(D,L-lactide-co-glycolide) (PLGA) by thistechnology. Porosities of the resulting scaffolds were as high as 94%. The pores were well interconnected. The primary biological experiment to reconstruct tissueengineering canine esophagus proved that the porous scaffolds were suitable tissueengineering carrier of artificial esophagus.4. Characterization studies of polyester and associated scaffolds. Biodegradablepoly(D,L-lactide) (PLA) and poly(D,L-lactide-co-glycolide) (PLGA) weresynthesized by ring opening polymerization with Sn(Oct)₂ as catalyst. TheMark-Houwink constants of PLA and PLGA in tetrahydrofuran at 25℃werecalibrated via GPC-intrinsic viscosity method. The degradation behaviors of porouspolyester scaffolds at different temperatures were studied and a method to extrapolatedegradation behaviors rapidly was brought forward. We measured viscosity, weightloss and modulus change of scaffolds during degradation in PBS at differenttemperatures. By examining the half-life of molecular weight under differenttemperatures, we confirmed that the biodegradation of PLA porous scaffolds obeysArrhenius equation, and the activation energy has been calculated. Fast degradation athigh temperature might be used to estimate degradation rate of associated scaffolds atthe body temperature.5. Preliminary applications in stents of vein grafts and oesophagus repair.PLGA external stents fabricated by melt injection were used to protect vein grafts.The influence of processing temperatures on PLGA stents was studied. Thedegradation behaviors of PLGA stents in PBS at 37℃were studied. The primarybiological results showed that PLGA external stent could reduce hypertrophy ofcanine vein grafts. The tube-like PLGA porous scaffolds were also tried to repairoesophagus, and the associated animal experiments of tissue engineering oesophaguswere done in canine.