This document discusses various biomaterial fabrication techniques. It begins with an introduction to biomaterials and their uses. It then describes common materials used as biomaterials and the evolution of biomaterials. Several scaffold fabrication techniques are outlined, including solvent casting, melt molding, gas foaming, freeze drying, and fiber bonding. Limitations of these techniques are noted. Rapid prototyping and nanofabrication techniques that allow for more control and precision are also summarized.

1. ARJUN G NAMBOODIRI Polymer processing Laboratory 4/6/10 BIOMATERIAL FABRICATION TECHNIQUES

2. OVERVIEW INTRODUCTION USE OF BIOMATERIALS MATERIALS USED AS BIOMATERIALS EVOLUTION OF BIOMATERIALS SCAFFOLD FABRICATION TECHNIQUES LIMITATIONS RAPID PROTOTYPING TOWARDS NANOTECHNOLOGY CONCLUSION

3. INTRODUCTION “ Non viable material used in medical devices intended to interact with biological systems” (Williams 1987) A biomaterial is "any substance (other than drugs) or combination of substances synthetic or natural in origin, which can be used for any period of time, as a whole or as a part of a system which treats, augments, or replaces any tissue, organ, or function of the body". BIOMATERIAL ONE MUST HAVE EITHER VAST KNOWLEDGE OR DIFFERENT COLLABORATORS WITH DIFFERENT SPECIALITIES INORDER TO DEVELOP BIOMATERIALS IN MEDICINE AND DENTISTRY

4. USE OF BIOMATERIALS REPLACEMENT OF DISEASED OR DAMAGED PARTS ASSIST IN HEALING IMPROVE FUNCTION CORRECT FUNCTIONAL ABNORMALITIES AID TO DIAGONISE AID TO TREATMENT CORRECT COSMETIC PROBLEMS

5. MATERIALS FOR USE AS BIOMATERIALS Polymer: Nylon, Polytetrafluoroethylene, Polyurethane, Silicone rubber, polycaprolactone Metals: Ti, Co-Cr alloy, Stainless Steel, Pt, Au etc Ceramics: Aluminum oxide, Calcium phosphate, Hydroxyapitite, Carbon etc Composites: Fiber reinforced bone cements etc

6. Evolution of Biomaterials Structural Functional Tissue Engineering Constructs (Scaffolds) Soft Tissue Replacements First generation Second Generation Third Generation

7. SCAFFOLD FABRICATION TECHNIQUES Solvent Casting and Particulate Leaching Melt molding Gas Foaming Fiber bonding Freeze drying

8. SOLVENT CASTING/ PARTICULATE LEACHING Incorporation of Salt particles Polymer/solvent solution e.g. PLLA/chloroform Casting Vacuum dry Immerse in water salt particles of a specific diameter to produce a uniform suspension (Mikos et al. , 1994,1996).

9. Advantage - Highly porous scaffold with porosity up to 93% and an average pore diameters up to 500 u m can be prepared using this technique. Disadvantage - A disadvantage of this method is that it can only be used to produce thin wafers or membranes up to 3mm thick.

10. MELT MOLDING This process involves filling a mould with polymer powder/melt and obtaining the shape of the mould. MELT MOULDING COMPRESSION MOULDING INJECTION MOULDING

11. In the work done by Thompson et al in 1995 they used the COMPRESSION MOULDING PRINCIPLE where a TEFLON MOULD was used with PLGA and gelatin micro spheres of specific diameter, and then heating the mould above the glass-transition temperature of PLGA while applying pressure to the mixture (This treatment causes the PLGA particles to bond together. Once the mould is removed, the gelatin component is leached out by immersing in water and the scaffold is then dried.

12. GAS FOAMING Another approach to using gas as porogen was developed by Nam et al. (Park, 1999; Nam et al. 2000). This technique includes both melt moulding and particulate leaching aspects. Porosities as high as 90% with pore sizes from 200-500 um are attained using this technique.

13. Fabrication process Ammonium bicarbonate is added to a solution of polymer in methylene chloride or chloroform. The resultant mixture is highly viscous and can be shaped with a mold . The solvent is then evaporated and the composite is either vacuum dried or immersed in hot water.

14. FREEZE DRYING The pore size can be controlled by the freezing rate and pH; a fast freezing rate produces smaller pores. Freeze-drying works by freezing the material and then reducing the surrounding pressure and adding enough heat to allow the frozen water in the material to sublime directly from the solid phase to the gas phase. Yannas et al., 1980 Collagen scaffolds have been made by freezing a dispersion or solution of collagen and then freeze drying . Dagalakis et al., 1980; Doillon et al., 1986

16. FIBER BONDING PGA fibers are immersed in PLLA solution.

17. LIMITATIONS Poor mechanical integrity Residual organic solvents Lack of structural stability Some techniques can only be used to make very small membranes. All the materials cannot be used for all the processes. Difficult to control membrane porosity and morphology.

18. RAPID PROTOTYPING TECHNIQUE 3D Solid modeling Data preparation Part Building Redesign Pass Reject A family of fabrication processes developed to make engineering prototypes in minimum lead time based on a CAD model of the item

19. BENEFITS: 1) Reduced lead times to produce prototype components. 2) Improved ability to visualize the part geometry due to its physical existence. 3) Earlier detection and reduction of design errors. 4) Increased capability to compute manufacturing properties of components and assemblies.

20. RAPID PROTOTYPING PROCESSES Three Dimensional Printing (3DP) Stereolithography (SLA) Selective Laser Sintering (SLS) Fused Deposition Modeling (FDM) Organ printing Membrane lamination

21. Technology invented at MIT by Bredt et al (1998) 1. Layer of powder spread on platform 2. Ink-jet printer head deposits drops of binder* on part cross-section 3. Binder dissolves and joins adjacent powder particles 4. Table lowered by layer thickness 5. New layer of powder deposited above previous layer 6. Repeat steps 2-4 till part is built 7. Shake powder to get part *Materials used: starch, plaster-ceramic powder Three Dimensional Printing (3DP)

22. Advantages Easy process Achievable pore size=45–500 um High porosity High surface area to volume ratio Independent control of porosity and pore size Wide range of materials Disadvantages Use of toxic organic solvents Lack of mechanical strength

23. 3D printed testpart with interconnecting channels. (a) Whole structure. (b) Detail view of the interconnecting channel structure with diameter of about 500 μ m. HA scaffolds seeded with MC3T3-E1 cells Binder (Schelofix)

24. STEREOLITHOGRAPHY 1. Raw material: photocurable monomer by a laser beam 2. Part constructed in layers of thickness 3. Supporting platform  in container at depth . UV laser solidifies part cross- section 4. Platform lowered by 5. Part cross-section computed at current height 6. Repeat Steps 4, 5 7. Removed completed part, 8. Break off supporting structures 9. Cure the part in oven. Polymerization occurs by the exposure of liquid resin to laser. He-Cd Laser UV beam Rotating mirror High-speed stepper motors Focusing system Liquid resin Part Platform Elevation control Support structures He-Ne Laser Sensor system for resin depth

25. Advantages Relative easy to remove support materials. Relative easy to achieve small feature. Disadvantage Limited by the development of photo polymerisable liquid monomer material

26. Porous polylactide constructs Light microscopy images showing the spreading of mouse pre-osteoblasts after 1 d of culturing on PDLLA network

27. SELECTIVE LASER SINTERING Moving laser beam sinters heat‑fusible powders in areas corresponding to the CAD geometry model one layer at a time to build the solid part After each layer is completed, a new layer of loose powders is spread across the surface Layer by layer, the powders are gradually bonded by the laser beam into a solid mass that forms the 3-D part geometry In areas not sintered, the powders are loose and can be poured out of completed part

28. Advantages High porosity Achievable pore size=45–200 um High surface area to volume ratio Complete pore interconnectivity Good compressive strengths Wide range of materials Solvent free Disadvantages High processing temperatures

29. (a) STL design file of porous scaffold. (b) PCL scaffold fabricated by SLS. cortical shell and areas of trabeculated structures within the marrow space

30. FUSED DEPOSITION MODELING FDM uses a moving nozzle to extrude a fibre of polymeric material (x- and y-axis control) from which the physical model is built layer-by-layer. The model is lowered (z-axis control) and the procedure repeated. Although the fibre must also produce external structures to support overhanging or unconnected features that need to be manually removed

31. Advantages High porosity Achievable pore size=250–1000 um Complete pore interconnectivity Macro shape control Independent control of porosity and pore size Good compressive strengths Solvent free Disadvantage High processing temperatures Limited material range Inconsistent pore opening in x-,y and z-directions Requires support structures for irregular shapes Materials: ABS, Polycarbonate (PC) Z-motion Melting head with XY-motion Build material wire spools: (a) Part (b) Support Extrusion nozzles Part Support Foam base

32. PCL scaffold with a lay-down pattern fabricated by FDM HA–PCL scaffolds have a fine apatite coating 3-dimensional distribution of cells within the scaffolds. PCL HA-PCL

33. ORGAN PRINTING (Mironov) Similar To Ink Jet Printer Print Gels That Are Thermo responsive Cells Are Sprayed Onto The Solidifying Thin Layer Of polymer solution Polymer Solution CELL TYPE1 CELL TYPE 2

34. DISADVANTAGES Cell Aggregates Are Formed Within Droplet Cells Maybe Damaged Choice Of Different Types Of Materials Are Limited

35. Computer aided design-based presentation of model of cell printer. (b) Bovine aortic endothelial cells were printed in 50-micron size drops in a line. (c) Cross-section of the p(NIPAAm-co-DMAEA) (d) Picture of the real cell printer and part of the print head with nine nozzles. Endothelial cell aggregates ‘printed’ on collagen before There fusion

36. MEMBRANE LAMINATION Membrane Of 500-2000µm IS USED It Is Cut By Laser To Form The Shape Required It Is Then Wet And The Next Layer Is Cut And Placed On Top Of It And Pressure Is Applied To Adhere The Two Layers Then Finally The Solvent Is Evaporated NOT VERY PRECISE SO MORE PRECISE METHODS ARE NOW REPLACING THIS TECHNIQUE

37. TOWARDS NANOTECHNLOGY !!!

38. Cellular interaction with the extracellular matrix is dynamic and demanding. Membrane bound receptors are constantly recycled and renew to bind to the matrix.

40. NANO FABRICATION TECHNIQUES ELECTROSPINNING SELF ASSEMBLY

41. SELF ASSEMBLY Self-assembly involves the spontaneous organization of individual components into an ordered and stable structure with preprogrammed non-covalent bonds complex laboratory procedure that is limited to only a select few polymer configurations (diblock copolymers, triblocks from peptide-amphiphile, and dendrimers). The most common of these for the production of nanoscale fibers are the peptide-amphiphiles (PA). complexity of the procedure and the low productivity of the method limit it as a large-scale tissue engineering option

42. SAPNS repair for the animal brain. ( a ) Molecular model of the RADA16-I molecular building block. ( b ) Molecular model of numerous RADA16-I molecules undergo self assembly to form well ordered nanofibers with the hydrophobic alanine sandwich inside and hydrophilic residues on the outside. ( c ) The SAPNS is examined by using scanning electron microscopy. (Scale bar, 500 nm.)

43. When the electrical force at the surface of a polymer solution or polymer melt overcomes the surface tension, a charged jets is ejected. ELECTROSPINNING FIRST DESCRIPTION Electrospinning was in 1902 when J. F. Cooley filed a United States patent entitled ‘Apparatus for electrically dispersing fibres’ Electro-spinning uses an electrical charge to form a mat of fine fibers.

44. Poly styrene fibers Polyvinyl pyrolidone fibers

46. In summary, biomaterials fabricated by traditional techniques are inadequate for the growth of thick cross-sections of tissue due to the diffusion constraints posed by foam structures . Rapid prototyping fabrication systems provide a solution to this problem by creating scaffolds with controlled internal microarchitecture , which should increase the mass transport of oxygen and nutrients deep into the structure. Yet with all these technique available we do not have any guidelines to which type of technique is best for which kind of polymers The development of new nanotechnology Techniques to develop better and more promising biomaterials is on the go CONCLUSION

47. 1. The Design of Scaffolds for Use in Tissue Engineering. Part II. Rapid Prototyping Techniques, TISSUE ENGINEERING Volume 8, Number 1, 2002. 2. Processing and Fabrication of Advanced Materials VIII by K. A. Khor, T. S. Srivatsan M. Wang, W. Zhou, F. Boey on 1999 3. Biomaterials and bioengineering handbook, Donald L Wiss,2003. 4. Three-dimensional tissue fabrication, Valerie Liu Tsang, Sangeeta N. Bhatia, Advanced Drug Delivery Reviews 56 (2004) 1635– 1647 REFERENCES

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