RP techniques for tissue engineering purposes Author: Evgeny Barabanov

Tissue engineering Image source

Scaffold in tissue engineering  Scaffold in tissue engineering is an artificial structure capable of supporting three-dimensional tissue formation.  Cells are often implanted or 'seeded' into a scaffold  Scaffold purposes

Example - carbon nanotube Image source

Requirements To achieve the goal of tissue reconstruction, scaffolds must meet some specific requirements

Requirements  A high porosity and an adequate pore size

Requirements  A high porosity and an adequate pore size To facilitate cell seeding and diffusion throughout the whole structure of both cells and nutrients.

Requirements  A high porosity and an adequate pore size To facilitate cell seeding and diffusion throughout the whole structure of both cells and nutrients  Biodegradability

Requirements  A high porosity and an adequate pore size To facilitate cell seeding and diffusion throughout the whole structure of both cells and nutrients  Biodegradability To allow absorption by the surrounding tissues without the necessity of a surgical removal

Requirements  A high porosity and an adequate pore size Necessary to facilitate cell seeding and diffusion throughout the whole structure of both cells and nutrients  Biodegradability To allow absorption by the surrounding tissues without the necessity of a surgical removal  Customizability

Requirements  A high porosity and an adequate pore size To facilitate cell seeding and diffusion throughout the whole structure of both cells and nutrients  Biodegradability To allow absorption by the surrounding tissues without the necessity of a surgical removal  Customizability To allow fabrication into various shapes and sizes for matching the each patient’s individual needs

Limitations of conventional methods  Lack of precise control of scaffold properties  Exploitation of organic solvents as a part of the synthetic polymers dissolution process (toxic and cancerogenic).

Limitations of conventional methods – example Inhomogeneities of pore distribution Irregular pore size distribution Image source

Rapid prototyping of bone and cartilage

Ideally, bone grafts should be porous, be able to promote new bone formation, and they should possess proper mechanical and physical properties. Image source

Rapid prototyping of bone and cartilage  First used in cranio-maxillofacial surgery  Pioneered by Griffith and coworkers at MIT  In 1996 Griffith and Halloran reported the fabrication of ceramic parts by stereolithographyreported

Stereolithography (SLA) Image source

Stereolithography (SLA) Hydroxyapatite (HA) scaffolds fabrication for orbital floor implants (by Levy et al.) Image source

Stereolithography (SLA) Minimum pore size of 100 μm is required for mineralized tissue ingrowth. CAD model of the scaffold SLA fabricated scaffold Micro-CT image of the scaffold The Micro-CT scan reveals that the scaffold has a very regular pore size distribution in the range of μm Image source

Stereolithography (SLA) Disadvantages:

Stereolithography (SLA) Disadvantages:  Requires the use of supporting structures

Stereolithography (SLA) Disadvantages:  Requires the use of supporting structures To attach the part to the elevator platform To prevent deflection due to gravity To hold the cross sections in place so that they resist lateral pressure from the re-coater blade.

Stereolithography (SLA) Disadvantages:  Requires the use of supporting structures To attach the part to the elevator platform To prevent deflection due to gravity To hold the cross sections in place so that they resist lateral pressure from the re-coater blade. Although supports are generated automatically during the preparation of CAD models, they must be removed from the finished product manually.

Stereolithography (SLA) Disadvantages:  Requires the use of supporting structures  Limited materials (photo polymers)

Stereolithography (SLA) Disadvantages:  Requires the use of supporting structures  Limited materials (photo polymers)  Extremely expensive

Stereolithography (SLA) Disadvantages:  Requires the use of supporting structures  Limited materials (photo polymers)  Extremely expensive Advantages:

Stereolithography (SLA) Disadvantages:  Requires the use of supporting structures  Limited materials (photo polymers)  Extremely expensive Advantages:  Relatively fast (functional parts can be manufactured within a day)

Three-dimensional printing (3DP)  Was developed at the Massachusetts Institute of Technology (MIT)  Uses a liquid adhesive that binds the material  Uses a powder as a material

Three-dimensional printing (3DP) Image source

Three-dimensional printing (3DP) Advantages:  Does not require supporting structures

Three-dimensional printing (3DP) Advantages:  Does not require supporting structures The remaining free standing powder supports the part during the build

Three-dimensional printing (3DP) Advantages:  Does not require supporting structures The remaining free standing powder supports the part during the build  Inexpensive

Three-dimensional printing (3DP) Advantages:  Does not require supporting structures The remaining free standing powder supports the part during the build  Inexpensive Disadvantages:

Three-dimensional printing (3DP) Advantages:  Does not require supporting structures The remaining free standing powder supports the part during the build  Inexpensive Disadvantages:  Accuracy, surface finish, and part strength are not quite as good as some other additive processes

Selective laser sintering (SLS)  Was developed and patented by Dr. Carl Deckard and academic adviser, Dr. Joe Beaman at the University of Texas in Austin in the mid-1980s  A combination of SLA and 3DP

Selective laser sintering (SLS) Image source

Selective laser sintering (SLS) SLS provides a cost-effective, efficient method to construct scaffolds to match the complex anatomical geometry of craniofacial or periodontal structures Image source

Selective laser sintering (SLS) Advantages:  A wide range of materials can be used (including metals) In fact any powdered biomaterial that will fuse but not decompose under a laser beam can be used to fabricate scaffold by SLS.  Accurate (very complex geometries can be created directly from digital CAD data)  Fabricated prototypes are porous  Does not require the use of any organic solvent

Fused deposition modeling (FDM)  Was developed by S. Scott Crump in the late 1980s and was commercialized in 1990 by Stratasys in Eden Prairie, Minnesota  Uses semiliquid-state thermoplastic polymer as a material  Two heads with a fixed distance in between

Fused deposition modeling (FDM) Image source

Fused deposition modeling (FDM) Can be used as a bone patch to repair holes in the skull PCL (Polycaprolactone) scaffold Image source

Fused deposition modeling (FDM) Advantages:  Easy material changeover Disadvantages:  Support design / integration / removal is difficult

Soft tissue scaffolds by the means of RP

 The requirements of soft tissue implants differ from hard tissue replacements  Soft tissue has a very high content of water, so from the chemical point of view it is a hydrogel.

Hydrogels  Polymers  Can absorb water even 10 times specimen’s original weight without disintegrating (only swelling)  Can be used as simple scaffold structures, like sheets, fibers, wovens or non-wovens  Proven to be excellent candidates for substituting soft tissues

Hydrogel scaffolds Advantages:

Hydrogel scaffolds Advantages:  Flexible

Hydrogel scaffolds Advantages:  Flexible  Similar to the extracellular matrix

Hydrogel scaffolds Advantages:  Flexible  Similar to the extracellular matrix  Permeability to oxygen and metabolites

Hydrogel scaffolds Advantages:  Flexible  Similar to the extracellular matrix  Permeability to oxygen and metabolites Disadvantages:

Hydrogel scaffolds Advantages:  Flexible  Similar to the extracellular matrix  Permeability to oxygen and metabolites Disadvantages:  Mechanical stability of hydrogels does not allow the use in stress-loaded implants

Hydrogel scaffolds Advantages:  Flexible  Similar to the extracellular matrix  Permeability to oxygen and metabolites Disadvantages:  Mechanical stability of hydrogels does not allow the use in stress-loaded implants  Cannot be produced with SLA, SLS, 3DP and FDM due to their processing conditions

3D Bioplotter  Developed at the Freiburg Materials Research Center  Can produce hydrogel scaffolds

3D Bioplotter Image source

3D Bioplotter Advantages:  Allows to integrate living cells into scaffold fabrication process  No support structure is needed (the liquid medium compensates for gravity)

Two-photon polymerization  Uses two-photon absorption and subsequent polymerization  Allows fabrication of any computer generated 3D structure by direct laser “recording” into the volume of a photosensitive material  Allows real-time monitoring of the polymerization process

Two-photon polymerization Overlap of photons from the ultra short laser pulse leads to chemical reactions between monomers and starter molecules within transparent matrix. Image source

Two-photon polymerization Advantages:  Provides much better resolution than other RP methods  Can handle very complex structures

Potential advantages and challenges of rapid prototyping processes in tissue engineering

Advantages

 Production of three-dimensional scaffolds with complex geometries and very fine structures

Advantages  Production of three-dimensional scaffolds with complex geometries and very fine structures  High customizability

Advantages  Production of three-dimensional scaffolds with complex geometries and very fine structures  High customizability  Control of the scaffold porosity

Advantages  Production of three-dimensional scaffolds with complex geometries and very fine structures  High customizability  Control of the scaffold porosity  Speed - three-dimensional parts can be manufactured in hours and days instead of weeks and months

Advantages  Production of three-dimensional scaffolds with complex geometries and very fine structures  High customizability  Control of the scaffold porosity  Speed - three-dimensional parts can be manufactured in hours and days instead of weeks and months  Several RP techniques operate without the use of toxic organic solvents

Challenges

 Surface roughness

Challenges  Surface roughness  Resolution

Challenges  Surface roughness  Resolution  Internally trapped materials

Challenges  Surface roughness  Resolution  Internally trapped materials  Environment requirements

Challenges  Surface roughness  Resolution  Internally trapped materials  Environment requirements  Temperature

Challenges  Surface roughness  Resolution  Internally trapped materials  Environment requirements  Temperature  Sterility

Summary Although RP methods already can serve as a link between tissue and engineering, every RP process has its own unique disadvantages in building tissue engineering scaffolds. Hence, the future research should be focused into the development of RP machines designed specifically for fabrication of tissue engineering scaffolds.

References A review of rapid prototyping techniques for tissue engineering purposes Two-photon polymerization: A new approach to micromachining Additive fabrication Rapid prototyping Tissue engineering