Lecture 10 - Solid Freeform Fabrication

Question 1

Conventional Scaffold Fabrication Techniques

Answer 1

• Solvent casting/particulate leaching
• Phase inversion/particulate leaching
• Fiber meshing/bonding
• Melt molding
• Gas foaming
• Membrane lamination
• Hydrocarbon templating
• Freeze drying
• Emulsion freeze drying
• Solution casting

Question 2

Rapid Prototyping Scaffold Fabrication Techniques

Answer 2

• Fused deposition modeling
• 3D printing
• 3D plotting
• Selective laser sintering
• Laminated objet manufacturing
• Stereolitographic
• Multiphase jet solidification

Question 3

Solid Freeforms Fabrication (SFF)

Answer 3

• Additive manufacturing
• Can print precise/exact shapes
• Method of rapid prototyping used to build 3D scaffolds having defined shapes through material deposition onto stage
• Computer control by CAD system, data files from medical imaging modalities or any other method of tissue imaging

Question 4

3D Printing: Early Method

Answer 4

• Utilized inkjet printing system directed by CAD
• Spread thin layer of polymer powder/particulate over a piston surface
• The inkjet dispenses a binding liquid in desired pattern on powder
• After short bonding time, piston is lowered by thickness of single layer and subsequent layer of powder is applied
• Inkjet dispense binding liquid on powder
• Removes unbound particulate from finished scaffold

Question 5

Capabilities of 3D Printing

Answer 5

• Scaffold’s microstructure can be tailored by varying printing speed, the flow rate and drop position of the liquid binder to produce highly consistent structures
• Permits the fabrication of complex scaffold designs
• Simplicity and versatility allows the processing of wide range of biomaterials including polymers, ceramics, metals
• Processing of T sensitive materials can occur (room temp)
• Water-based binders or powder blends can be formulated to allow incorporation of biological and/or pharmaceutical agents

Question 6

Limitations of 3D Printing

Answer 6

• Cells could collapse scaffold when introduced because polymer reduces mechanics
• Polymers not stable @ high T, disappear
• Relies largely on use of organic solvents as binders to dissolve polymer powders in printed regions —> could lead to cytotoxicity issues (not biocompatibility)
• Resolution is 1mm for complex geometries —> need higher resolution because cell dimensions very small (doesn’t allow cell growth)

Question 7

Fused-Deposition Modeling (FDM)

Answer 7

• Employs concept of melt extrusion to deposit series of material filaments that forms material layer
• Material fed and melted inside a heated liquifier head before being extruded through a nozzle with small orifice
• Direction of the as-deposited filaments can be changed
• By changing direction of filament orientation and space between the filaments, scaffolds with highly uniform honeycomb-like structures, controllable pore morphology and complete pore interconnectivity can be obtained

Question 8

Advantages of FDM

Answer 8

• Pore morphology can be varied by changing the filament angle, filament width and the spacing between them
• FDM fabricated scaffolds possess good structural integrity and mechanical properties due to use of mechanically stable designs and proper fusion between individual material layers

Question 9

Limitations of FDM

Answer 9

• Needs supporting structures to be constructed alongside the scaffold for complex scaffold designs, use of secondary support materials may carry risk of material contamination (cytotoxicity, do they dissolve)
• Processing T of FDM limits number of scaffolding materials that can be extruded, utilized to fabricate scaffolds from polymers, composites

Question 10

Selective Laser Sintering

Answer 10

• Laser selectivity scans the powder polymer surface, directed by CAD or CT computer program
• Laser beam heats polymer above its melt T and fuses particles into solid structrue
• Additional layers of polymer powder are added to top surface and sintered accordingly
• Technique has been used with biocompatible materials such as PLLA, PCL, PVA, HA
• Fabricated scaffolds are highly porous and accurately reproduce design specifications

Question 11

Advantages of Laser Sintering

Answer 11

• Scaffold has high porosity and pore interconnectivity
• Scaffolds with highly consistent microstructural properties can be obtained by controlling process parameters such as laser power (or exposure density) and scan speed
• Laser sintering is highly capable of producing scaffolds with irregular shapes including structures containing channels
• It is solvent free and does not require any secondary binder system hence minimizing risks of material contamination

Question 12

Disadvantages of Laser Sintering

Answer 12

• Typical pore sizes in laser sintering fabricated scaffolds are limited to smaller pore size ranges
• Since laser sintering technique involves high processing temperatures, the technique is limited to the processing of thermally stable polymers

Question 13

Stereolithography (SLA)

Answer 13

• Uses light to polymerize or cross-link a photosensitive material
• Fine layer of solution of biocompatible polymer, photoinitiator, porogen, and appropriate solvent is placed beneath the laser
• CAD software guides laser in desired pattern
• Laser’s UV light reacts with photoinitiator to crosslink the polymer in specified locations
• Stage then lowered so that the part is covered with fresh layer of polymer and process is repeated