麻省理工大学：《生物材料的分子结构》教学讲义（英文版）Lecture 6：Biodegradable Polymers for Tissue Engineering

BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 Lecture 6: Biodegradable Polymers for Tissue Engineering Last time: enzymatic degradation of solid polymers Engineering biological recognition of polymers Toda Designing polymers for tissue engineering Reading Tissue engineering-current challenges and expanding opportunities, L.G. Griffith and G Naughton, Science 295, 1009(2002) Overview of Biomaterials in Tissue Engineering Let's review the main approaches and applications in Tissue Engienering before getting into the details of materials for TE We will review the fundamental approaches that have been taken here and return to this topic later when we discuss integration of biological molecules in synthetic biomaterials TE scaffolds seek to provide a surrogate for natural ECM o Provide functions of native ecm o Create a space for new tissue development Deliver cells to site o Direct macroscopic size/shape of new tissue Tissue Engineering Approaches ·3 major approaches o In vitro tissue genesis in vivo application o In vivo tissue genesis→ in vivo application Schematic comparison of in vitro and in vivo tissue engineering approaches N VITRO SYNTHESIS N VITRO SYNTHESIS IN VIVO SYNTHESIS IN VIVO SYNTHESIS Lecture 6- Biodegradable Polymers in Tissue Engineering 1 of 5

BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Lecture 6: Biodegradable Polymers for Tissue Engineering Last time: enzymatic degradation of solid polymers Engineering biological recognition of polymers Today: Designing polymers for tissue engineering Reading: ‘Tissue engineering- current challenges and expanding opportunities,’ L.G. Griffith and G. Naughton, Science 295, 1009 (2002) Overview of Biomaterials in Tissue Engineering Let’s review the main approaches and applications in Tissue Engienering before getting into the details of materials for TE We will review the fundamental approaches that have been taken here and return to this topic later when we discuss integration of biological molecules in synthetic biomaterials • TE scaffolds seek to provide a surrogate for natural ECM o Provide functions of native ECM o Create a space for new tissue development o Deliver cells to site o Direct macroscopic size/shape of new tissue Tissue Engineering Approaches • 3 major approaches o In vitro tissue genesis → in vivo application o In vivo tissue genesis → in vivo application Schematic comparison of in vitro and in vivo tissue engineering approaches1 : Skin: bone: Lecture 6 – Biodegradable Polymers in Tissue Engineering 1 of 5

BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 o In vitro tissue genesis> ex vivo application Fig. 1. There are three commo from (70). In vasc chemical boads (diameter in vitro application e.g. tissue on a chip approaches Cells organized into tissue-like structures Culture ilter controls Perfusion through"tissue Fig. 2. A microfabricated bioreactor for perfusing 3D liver tissue engi- nels. (C)Hepatocytes seeded onto the scaffold of the bioreactor attach to owing attached to the inside walls of the narrow channels of the 3d structures that are reminiscent of liver cords. bile c organize to for peered in vitro(54, 55).(A)A cross section showing tissue aggregates the walls of the channels(four channels are shown) and reorganize to for silicon-chip scaffold Culture medium flows across the top of the scaffold junctions can be seen with high-power microscopy(54, 55). Live cells are as well as through the narrow channels, enabling tissue aggregates to green and dead cells are red as visualized with the calcein AM/ethidium extract oxygen and nutrients. The des of the scaffold promotes homodimer stain.( D) Scanning electron micrograph showing vessel-like self-assembly of the cells into tissues.( B)a bioreactor containing a structures assembled from endothelial cells at the fluid-tissue interface in 0. 2-mm-thick silicon-chip scaffold etched with 0.3-mm-diameter chan- the bioreactor channels [llustration: Preston Morrighan Macroscopic TE Scaffold Structure Early attempts at designing scaffold for tissue engineering simply used forms of processed polymers o PGA mesh fibers From here, the need for a higher surface area and more 'enclosed structure were recognized and polymer foams were develope Freeze-dried scaffolds o particulate-leached scaffolds( Mikos 1994, Lu 2000) o Supercritical CO2-based scaffolds (Hile et al 2000, J Contrl Rel 66, 177) o Effervescent salt leaching (Yoon et al 2001, JBMR 42, 396) Lecture 6- Biodegradable Polymers in Tissue Engineering 2 of 5

BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 o In vitro tissue genesis → ex vivo application2 o In vitro tissue genesis → in vitro application e.g. tissue on a chip approaches3 : Macroscopic TE Scaffold Structure • Early attempts at designing scaffold for tissue engineering simply used forms of processed polymers: o PGA mesh fibers • From here, the need for a higher surface area and more ‘enclosed’ structure were recognized and polymer foams were developed: o Freeze-dried scaffolds o particulate-leached scaffolds (Mikos 1994, Lu 2000) o Supercritical CO2-based scaffolds (Hile et al 2000, J Contrl Rel 66, 177) o Effervescent salt leaching (Yoon et al 2001, JBMR 42, 396) Lecture 6 – Biodegradable Polymers in Tissue Engineering 2 of 5

BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 More elegant approaches are now being considered o Colloidal crystal templating Poly(methyl methacrylate) Optical micrograph/20 um pores Fluorescence micrograph/60 um pores 83三 Hydrogel o Nanofiber-based structures(P. Ma) Researchers have also investigated natural materials as scaffolds for tissue engineering- using processed ECM for tissue engineering(we wont pursue here, covered in Biomaterials-Tissue Interactions) o Example materials Decellularized tissues( Badylak 1998, Hilbert 1989) Collagen-based gels(Ellis et al 1996) o Advantages Native cues present Can preserve natural tissue microstructure o Disadvantages: Poor mechanical properties in some cases Difficult to process Poor reproducibility High cost ellular Interactions with Synthetic Degradable Solids Used as Scaffolds? Review older literature looking directly at cells on PLGA, PLA, etc Molecularly-Designed Surfaces for TE Reconstruction at scaffold surfaces Lecture 6- Biodegradable Polymers in Tissue Engineering 3 of 5

BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 • More elegant approaches are now being considered: o Poly(methyl methacrylate) microspheres Colloidal crystal templating Hydrogel precursor polymerize Dissolve microspheres Ordered porous structure Optical micrograph/20 µm pores Fluorescence micrograph/60 µm pores 60 µm o Nanofiber-based structures (P. Ma) o • Researchers have also investigated natural materials as scaffolds for tissue engineering- using processed ECM for tissue engineering (we won’t pursue here, covered in Biomaterials-Tissue Interactions) o Example materials ƒ Decellularized tissues (Badylak 1998, Hilbert 1989) ƒ Collagen-based gels (Ellis et al 1996) o Advantages: ƒ Native cues present ƒ Can preserve natural tissue microstructure o Disadvantages: ƒ Poor mechanical properties in some cases ƒ Difficult to process ƒ Poor reproducibility ƒ High cost Cellular Interactions with Synthetic Degradable Solids Used as Scaffolds? Review older literature looking directly at cells on PLGA, PLA, etc. Molecularly-Designed Surfaces for TE Reconstruction at Scaffold Surfaces Lecture 6 – Biodegradable Polymers in Tissue Engineering 3 of 5

BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 Controlled Release in Tissue Engineering Cytokine delivery from scaffolds ase study: Induction of vascularization in TE scaffolds Structure of vasculature Dual growth factor delivery from degradable scaffolds for de novo blood vessel synthesis D 15 10215 (days) Figure 1 Schematic of scaffold fabrication process and growth factor release kinetics(A) Growth factors were incorporated into polymor coaffoids by orthor moeng with polymor particle bofors proooeeing into coaHfoide VEGP) or pri-ancapsulating the factor(PDGP)into polymer microspheres used to form scaffolds. The VEGF incorporation contrast, the PDGF incorporation approach is pre cistn buton of factor th polymer with release regulated by the dagradation of the polymer usad to fm microspheres. The two growth factors tha same scafo ds by mixing polymar microspheres containing pre-ancapsulated PDGF with lyophil zed VEGF before utilized for dual growh factor release C)n vitro release kinetics ofVEGe Lnm scaffolds fabricated from PLG(85: 15 smallar than the symbol controlled release scaffolds induce formation of more blood vessels with larger diameters Lecture 6- Biodegradable Polymers in Tissue Engineering 4 of 5

BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Controlled Release in Tissue Engineering Cytokine delivery from scaffolds Case Study: Induction of vascularization in TE scaffolds • Structure of vasculature • Dual growth factor delivery from degradable scaffolds for de novo blood vessel synthesis4 : controlled release scaffolds induce formation of more blood vessels with larger diameters: Lecture 6 – Biodegradable Polymers in Tissue Engineering 4 of 5

BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 G n ual delivery of VEGF and PDGF rapidly forms w图m听b( ond ton(G.ind catas statistcs significance relatve to bla laksA E dNa delivery from scaffolds Objective- in situ gene therapy Microenvironments for Stem Cells Application Focus: Engineering Vasculatu References Yannas, I V Tissue and Organ Regeneration in Adults(Springer, New York, 2001) 2. Langer, R.& Vacanti, J. P. Tissue engineering Science 260, 920-6(1993) Griffith, L G& Naughton, G. Tissue engineering--current challenges and expanding opportunities. Science 295 1009-14(2002) Richardson, T. P, Peters, M. C, Ennett, A.B.& Mooney, D. J. Polymeric system for dual growth factor deliver Nat Biotechnol 19, 1029-34(2001) Lecture 6- Biodegradable Polymers in Tissue Engineering 5 of 5

BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 DNA delivery from scaffolds Objective – in situ gene therapy Microenvironments for Stem Cells Application Focus: Engineering Vasculature References 1. Yannas, I. V. Tissue and Organ Regeneration in Adults (Springer, New York, 2001). 2. Langer, R. & Vacanti, J. P. Tissue engineering. Science 260, 920-6 (1993). 3. Griffith, L. G. & Naughton, G. Tissue engineering--current challenges and expanding opportunities. Science 295, 1009-14 (2002). 4. Richardson, T. P., Peters, M. C., Ennett, A. B. & Mooney, D. J. Polymeric system for dual growth factor delivery. Nat Biotechnol 19, 1029-34 (2001). Lecture 6 – Biodegradable Polymers in Tissue Engineering 5 of 5