Most of the conventional techniques used to create scaffold fabrication such as fiber bonding, solvent casting and melt moulding [2] yield out with random porous architectures which could not necessarily produce an appropriate homogeneous environment for bone information. Moreover non-uniform microenvironment can provide the region with inadequate nutrient concentration that will make cultured tissue grow in poor cellular activity prevent the formation of homogeneous quality new tissues.
In tissue engineering field, rapid prototyping is one of the most efficient techniques to design and create a highly porous artificial extracellular matrix (ECM) or scaffold that allows accommodating and guiding the proliferation of new cells. A scaffold is a polymeric porous structure made of biodegradable material such as poly-lactic-acid (PLA) and poly-glycolic-acid (PGA) [3]. To regenerate new tissues successfully, the whole process mainly relies on the structural formability of the tissue scaffold and bioreactors to provide appropriate environment for new cell feasibility and function. Rapid prototyping technique is capable to produce complex product quickly from the computer model based on the data of the patient CT. However RP techniques still have limitations and shortcomings such as its mechanical strength, interconnected channel and pores distribution to be resolved [1]. It still need to be improved in order to produce well-defined tissue engineered scaffolds with appropriate chemical and mechanical microenvironments. In this review, we will discuss further developments of RP techniques in tissue engineering based on its major aspects: methods and materials.
Rapid Prototyping Technologies [5]
Rapid prototyping is an advanced technology based typically based on development of computer technology and manufacturing. It is currently being used by investigators to produce scaffolds for use in tissue engineering. Rapid prototyping methods can be categorized into –liquid-based, solid-based and powder-based. In RP process, the 3D model is created layer by layer at a time based on the data defined by a computer-generated till the whole product is complete.
Main systems of RP technique mostly used in tissue engineering fields are
(1) Stereo lithography Apparatus (SLA)
(2) Selective Laser Sintering (SLS)
(3) Fused Deposition Modeling (FDM)
(4) Three-dimensional printing (3-DP)
The advantages and limitations of each of the rapid prototyping technology applied in TE can be summarized as described in table below.
Table. Advantages and limitations of SFF fabrication techniques [5]
Technique
Advantages
Limitations
SLA
-easy to remove support and trapped materials
-can get small features accurately.
-the development of photopolymerizeable and biocompatible, biodegradable liquid polymer material are limited
SLS
-can get good compressive strengths
-have greater material choice
-don’t need to use solvent
-processing temperatures is high
-difficult to remove difficult to remove trapped material in small inner features.
FDM
-no trapped material within small features
-don’t need to use solvent
-can get good compressive strengths
-support material is required for irregular structures
-have anisotropy between XY and Z directions
3D-P
-wider field for material choice
-heat effect is not high on raw material
- difficult to remove difficult to remove trapped material in small inner features.
-need to use toxic organic solvents
-mechanical strength is not good enough
According to the facts described above, it can be clearly seen that main limitations are the use of materials, toxic binders and poor feature symmetry [5].
Selective Laser Sintering Process (SLS) [6]
At first, CAD data files of the object in the .STL file format are transferred to the RP system where they are sliced into layer of equal thickness by mathematically. From this point SLS process start to operate as follow,
-A thin layer of heat-fusible is deposited onto the part-building chamber.
-The bottom-most cross-sectional slice of the CAD part to be fabricated is selectively scanned on the layer of powder by a carbon dioxide laser. The intersection of the laser beam with the powder elevates the temperature to the point of melting, fusing the powder particles to form a solid mass.
-New sintered layer of powder and previously formed layers are fused together to form the object.
Fig shows the process chain of SLS technique.
Fig.1. Schematic layout of the SLS process. [6]
Improvements of SLS process in order to create smaller features by using smaller laser spot size, powder size and thinner layer thickness are expected to produce the desired scaffolds for TE [1]. The degree of easy to remove trapped loose powder is also one of the criteria in current techniques. Existing solutions such as ultrasonic vibration, compressive air, bend blaster, and/or suitable solvent [1].
The bio-materials used by SLS system are non-biocompatible and bio-inert in nature. Because of that fact, SLS application in scaffold production is still limited. Moreover SLS in fabricating TE scaffolds often needs to use organic solvent in order to remove trapped materials [1] which can harm the inner organs when the structure is implanted in human body [6].
K.H. Tan et. al described the bio-compatible polymers such as Polyetheretherketone (PEEK), Poly(vinyl alcohol) (PVA), Polycaprolactone (PCL) and Poly(L-lactic acid) (PLLA) and a bioceramic namely, Hydroxyapatite (HA) to fabricate TE scaffolds [6]. By using these polymers the post-process doesn’t need to use any organic solvent in order to remove trapped material.
The properties and sources of these polymers are described below:
Molecular
Weight
(Mw)
Melting
Point
(Tm)
Glass-
Transitional
Temperature
(Tg)
Density
Avg.
Inherent
Viscosity
Particle
Size
Source
PCL
10,000
60.C
-60.C
Polyscience
Inc. (USA)
PLLA
172.C ~
186.8.C
60.5.C
2.53 dl/g
PURACAsia Pacific Pte.
Ltd [ ]
PEEK
343.C
143.C
25µm
Victrex PIC
Lancashire
UK
PVA
89,000~
98,000
220~
240.C
58~
85.C
100 µm
Aldri
Chemical
Company
HA
3.05 g/cm3
Below
60 µm
Coulter
Counter
Analysis
Among all these bio-materials,HA is highly compatible and can provide well bonding between tissue and the ceramic material[6]. In the process, the released ions of calcium and phosphate ions cause bone-induced osteogenesis and provide the linking of ceramic implant to the bone [6].
The experimental results of their optimized parameters in laser sintering process are described in the table below [6]:
Materials
Part bed Temperature
(.C)
Laser Power
(W)
Scan Speed
(mm/s)
PCL
40.C
2-3
3810
PLLA
60.C
12-15
1270
PVA
65.C
13-15
1270-1778
PEEK
140.C
16-21
5080
PEEK/HA
140.C
16
5080
K.H. Tan et. al reported that in sintering of PEEK/HA bio-composite blend, it is found by reducing the composition percentage of PEEK in the powder made the scaffold to be fragile and that fact made it is not practical to use in laser sintering. Their experiment result shows that the composition percentage of HA 40 wt% can provide the structure in good integrity. And furthermore this composition ratio should be kept at this value in order to get good result.
And from this research it can be clearly seen that (i) part bed temperature, (ii) laser power and (iii) scan speed are three main parameters to control the micro-porosity of the structure [6].
Another powder-based RP technique is 3D-printing (3DP). The bioresorbable polymers and copolymers (based on either polycaprolactone or polylactic and polyglicolic acids) [1, 7, 8] are used in this technique.3D system deposits a liquid binder by means of multiple injects onto a powder bed. The powder particles are glued each other in layer and down to nearly 0.08mm thickness. Depends on the different proposed solutions, biomaterials are incorporated in either the powder bed the liquid binder or post-process infiltrating agent. Well-defined porosity can be achieved by a careful selection of binder printing parameters or by mixing powder with salt, eventually melt in water. 3DP process has the advantage of being conducted at room temperature related to binder toxicity and mechanical strength of built parts.
The process chain of 3D-printing is as shown in Figure below.
Fig. 3-Dimensional Printing Schematic [10]
Organ printing[11]
Based on the concept of 3D-printing technique, one of the development of rapid prototyping technology in TE is organ printing.
Vladimir et al.[11] demonstrated in their report that organ printing is the bio medically relevant technique of RP technology which uses the behaviour of tissue fluidity. Its computer-assisted deposition material is cells, cells aggregates or matrix. The components used in organ printing is jet-based cell printers, cell dispensers or bio plotters, the different types of 3D hydro gels and varying cell types.
Fig- (a) CAD-based cell printer (b) Bovine aortic endothelial cells-printed in 50-micron size drops in a line (c) Cross-section of the p (NIPA-co-DMAEA) gel showing the thickness of sequentially placed layer (d) Real Cell Printer (e) The cell printer connected to a PtdCho via bidirectional parallel together with 9 jets extent of mixing. (f) Endothelial cell aggregate ‘printed’ on collagen before their fusion (g) After their fusion. This information is taken from [11]
The sequential process of organ printing includes (i) preprocessing (ii) processing and (iii) postprocessing[10]. Preprocessing is the development of computer aided design(CAD) or blue print of specific organ structure. The required 3D design can be achieved from digitised image construction of a natural organ or tissue. This imaging data is derived from various modalities such as noninvasive scanning of the human body (e.g. MRI or CT) or a detailed 3D reconstruction of serial section of specific organ. In processing, CAD design of specific organ structure is printed as layer-by-layer by jet-based cell printer. Postprocessing is the process of perfusion of printed organ and its biomechanical conditioning to both direct and accelerated organ maturation.
Fig- Schematic representation of cell printing, assembly and their perfusion of 3D printed vascular unit. Red-Endothelial cells aggregates, Blue-Smooth muscle cell aggregate [11]
Basically organ printing is an advantage of the fusion phenomena of embryonic cell or tissue which are viscoelastic fluids can flow and fuse [11].
Vladimir et al.[11] described that based on achievements: development of a printer which can print cells and/or aggregates; demonstration of procedure for layer-by-layer sequential deposition and solidification of a thermo-reversible gel or matrix and fusion property of embryonic cell, cell aggregate or tissue that if they are put closely fused into ring-like od tube-like structure within the gel; all these achievements and advantage pointed out that organ printing is a feasible advanced technique for TE.
In application of rapid prototyping technique to TE, vascular density of desired organ is one of the most crucial factor for adequate organ perfusion and supply of oxygen and functioning [10]. The tissue-engineered organ could not survive and develop if they don’t have adequate vascularisation.
The authors had recommended that the advantage of organ printing technique is a unique opportunity to eventually print a sophisticated branching vascular tree during the whole process of printing the specific organ.
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