Electron Based Techniques

Developments in metals processing are being made at Arcam AB, Sweden, in which fully dense parts are built from metal powder. The technology is based on electron beam melting (EBM), and the parts are built by melting the metal in a layer-by-layer manner. Advantages of using an electron beam as the energy source, as opposed to laser processing, include very small spot sizes (^100 mm); very high beam-material coupling efficiencies, high scanning speeds (up to ^1000 m/s); and allowing beam deflection without the use of moving mirrors. The material is processed in a vacuum, which increases material quality. The use of extremely low processing pressures minimizes the adverse effects of impurities such as oxides and nitrides and allows for processing of reactive materials such as titanium. A schematic of the Arcam system is shown in Fig. 6.7.

Fig. 6.7 Schematic of the Arcam electron beam melting (EBM) system. The electron beam is generated in an EB gun, and the beam is deflected to the top of the powder layer by two magnetic fields. The first acts as a magnetic lens and focuses the beam to the desired diameter. The second deflects the focused beam to the desired location [www.arcam.com]

Fig. 6.7 Schematic of the Arcam electron beam melting (EBM) system. The electron beam is generated in an EB gun, and the beam is deflected to the top of the powder layer by two magnetic fields. The first acts as a magnetic lens and focuses the beam to the desired diameter. The second deflects the focused beam to the desired location [www.arcam.com]

The maximum build size of the Arcam EBM system measures 200 x 200 x 160 mm, with a reported accuracy of ±0.3 mm]. The EBM process is able to use standard biocompatible materials, such as Ti6Al4V ELI, Ti Grade 2 and Cobalt-Chrome and lends itself particularly well to metal implant manufacturing. The process has been verified for tool steel, low alloy steel, alloyed titanium, and nickel alloys. Mechanical properties of samples fabricated of the Ti6Al4V are 30-35 HRc hardness, 930 MPa/135 ksi tensile strength (Rm), 880 MPa/125 ksi yield strength (Rp0.2). Examples of custom implant are shown in Fig. 6.8. Disadvantages of electron beam technology are: the need for a vacuum, the production of gamma rays and the requirement for conductive materials.

6.6.3 Extrusion Based Techniques Fused Deposition Modelling (FDM)

FDM employs the concept of melt extrusion to deposit a parallel series of continuous material beads that form a layer. In FDM, filament material stock (generally thermoplastics) is softened and extruded through a heated nozzle and

Fig. 6.8 (a) customised skull plate implant (material: Ti6Al4V, 4 hours) (b) hip stemimplant (material: Ti6Al4V, build time: 20 pieces in 24 hours) (c) customised bone plate deposited in such a pattern to form a layer. By changing the direction of material deposition for consecutively deposited layers and the spacing between the material roads, scaffolds with highly uniform internal honeycomb-like structures, controllable pore morphology and complete pore interconnectivity may be obtained. In order to fabricate scaffold designs with overhanging features, removable supporting structures are deposited alongside the scaffold to support such features.

An interdisciplinary group in Singapore has studied and patented the parameters for processing PCL and several composites (PCL/HA, PCL/TCP etc.) by FDM [34, 10]. These first generation scaffolds (PCL) have been studied for several years with, and without, cells in a clinical setting (www.osteopore-inntl.com). The second generation scaffolds for bone engineering using FDM were made of polymer and CaP composites because they confer favourable mechanical and biochemical properties, including strength via the ceramic phase, and toughness and plasticity via the polymer phase, with favourable degradation and resorption kinetics and graded mechanical stiffness. Other advantages include improvements to cell seeding and the enhanced incorporation and immobilization of growth factors. Endres et al. [35] and Rai et al. [36]

have tested these PCL/CaP composite scaffolds for bone engineering and reported encouraging results. Shao et al. [37] have evaluated the feasibility and potential of a hybrid scaffold system for large and high-load-bearing osteochondral defect repair. The implants were made of medical-grade PCL (mPCL) for the bone component whereas fibrin glue was used for the cartilage element using FDM. Both matrices were seeded with allogenic bone marrow-derived mesenchymal cells (BMSC) and implanted in the defect (4 mm diameter x 5 mm depth) on medial femoral condyle of adult New Zealand White rabbits. Mature trabecular bone regularly formed in the mPCL scaffold at both 3 and 6 months post-operation. Micro-Computed Tomography showed progression of mineralization from the host-tissue interface towards the inner region of the grafts. After 3 months, the specimens showed good cartilage repair. In vivo viability of the transplanted cells was demonstrated for the duration of 5 weeks. The results demonstrated that mPCL scaffolds provided a potential matrix for osteochondral bone regeneration and that fibrin glue does not possess the physical properties to allow for cartilage regeneration in a large and high-load-bearing defect site.

Drawbacks of the FDM techniques include the need for input material of a specific diametric size and material properties to feed through the extrusion nozzle. Any changes in the properties of the material require efforts to recalibrate and optimise the feeding parameters. As a result, FDM has a narrow processing window. The resolution of FDM is relatively low, at 250 mm. In FDM, a limited range of materials can be used, with almost complete exclusion of natural polymers, as the material used must be made into filaments and melted into a semi-liquid phase before extrusion. The operating temperature of the system is too high to incorporate biomolecules into the scaffold. Moreover, the material deposited solidifies into dense filaments, blocking the formation of microporosity [38]. Precision Extruding Deposition (PED) and Precise Extrusion Manufacturing

The precision extruding deposition (PED) system was developed at Drexel University to fabricate PCL scaffolds with a pore size of 250 mm [39]. The major difference between PED and conventional FDM is that the scaffold material can be directly deposited without preparation of a filament. Pellet-formed PCL is liquefied by two temperature controlled heating bands and is then extruded by pressure created by a precision turning screw.

Xiong et al. [40] have developed precise extrusion manufacturing (PEM) to fabricate bone tissue scaffolds. In the PEM process, the thermoplastic material feeds into a heated extrusion sprayer capable of moving in X-Y directions. The sprayer extrudes a fine filament of the material onto a working platform capable of moving in the Z direction. So the material is extruded and deposited layer by layer in areas defined by the CAD model to build a 3D object. PEM

was used to fabricate poly(l-lactic acid) (PLLA) scaffolds with controllable porous architectures from 200 to 500 mm. With the aim of preserving bioac-tivities of scaffold materials, a new process of low-temperature deposition manufacturing (LDM) [41] was developed to fabricate PLLA/tricalcium phosphate (TCP). Instead of melting solid materials by heating, PLLA grains, TCP powders and dioxane were mixed for use as a slurry. The material slurry was fed into a screw pump nozzle and deposited in low temperature environment (<0°C). The frozen scaffolds were then freezed-dried to remove the solvent. The scaffolds had macro-pores of approximately 400 mm and micro-pores of approximately 5 mm, with a porosity of 89.6%. The mechanical properties of the manufactured scaffolds are close to those of human spongy bone and much lower than those of human compact bone. The scaffolds loaded with bovine bone morphogenic protein (bBMP) were used to repair 20 mm segmental defects in canine radiuses in implantation experiments using mature beagle dogs. The results showed that bone and bone marrow like tissue were regenerated and the scaffold degraded after 24 weeks with no trace of aseptic inflammation found in the histological section image. The LMD was further developed for use in a multi-nozzle deposition system (MDM) [42]. It achieved the deposition of PLLA/TCP materials with different composition by bi-nozzle deposition and spraying BMP particles into the scaffold during its forming process by tri-nozzle deposition. Support structures could be built using water, which is nontoxic and easy to remove. Bio-Plotter

The Bio-Plotter, developed by Landers et al. [43] at the Freiburg Materials Research Centre is a 3D dispensing technique that uses a pressure-controlled dispenser movable in each direction. 3D objects can be fabricated layer by layer with each layer composed of single parallel strands. By setting a certain distance between the stands, porous structures can be built. The key feature of this method is the 3D dispensing of liquids and pastes into a liquid medium with matched density. Initially, this technique was used to fabricate hydrogel soft tissue scaffolds [44]. Carvalho et al. [45] reported the use of this technique to fabricate hydroxyapatite with polyvinylo alcohol. The scaffolds were freeze-dried and sintered in an oven. Li et al. [46] used this technique to fabricate porous titanium scaffolds for orthopaedic implants and bone tissue applications. A Ti6Al4V slurry prepared from the mixture of Ti6Al4V powder and an aqueous solution of methylcellulose and stearic acid was deposited by a 3D dispensing unit consisting of a syringe and nozzle at room temperature. The scaffold samples were first dried at room temperature and then sintered in a high vacuum furnace. The result showed that Ti6Al4V slurry was successful for building 3D scaffolds and the scaffolds were not only non-toxic but also favourable for cell attachment, proliferation and differentiation. The drawbacks of this process are the lower limit of the nozzle size, anisotropy of the deposited structure and some pore occlusion at boundaries.

0 0

Post a comment