Project Objective

In this study, a range of new bio-compatible/degradable materials that are compatible with a commercial 3D direct manufacture system has been explored. There were three stages involved in this project (as presented in Chapter 1) and the conclusion for each stage is summarised:

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8.2.1 The development of a stereolithography system for the optimal formulation of biodegradable photopolymer resin to fabricate 3D scaffolds with controlled microstructures specifically for soft tissue engineering applications such as hydrogels.

In this study, two types of equipment were developed in order to allow some understanding on the techniques and process involved in microstereolithography. A light projector was developed from a decommissioned Envisiontec Perfactory® Mini and UV light engine was built from high power ultra-bright LED array.

Commercial resin (envisionTec’s R11) was successfully cured on the Light Projector but the layer-by-layer formation was limited due to the presence of a photoinhibitor in R11. On the other hand, the Enfis light engine system was specifically designed to fabricate a 3D structure from formulated photo-curable ceramic suspensions. A 3D structure was successfully fabricated from 70wt% hydroxyapatite (HAP)/1, 6 hexanediol diacrylate (HDDA) suspension. The development of both systems was limited due to the absence of a Z-stage. For future work, the addition of a Z-stage into these systems (especially the Enfis system) could produce a promising system. Nevertheless, the decommissioned Light projector and Light engine have given abundance of knowledge and clear understanding on the principle of microstereolithography techniques.

An important finding in this study was the optimisation of dumb-bell sample size and dimension for materials characterisation on the Deben Microtensile machine, which was used for all measurement in this project, in order to obtain reliable and precise measurements.

8.2.2 The development of a range of new bio-compatible/degradable materials for soft tissue application that is compatible with a commercial 3D direct manufacture system (Envisiontec Desktop).

The objective of this section was to investigate the feasibility of the commercial machine to fabricate novel biomaterials. In this study, a new photopolymer resin with different types of multifunctional acrylate polymer was successfully fabricated on a commercial 3D direct manufacture system, envisionTEC Desktop. Table 8.1 shows the formulated photopolymer resin.

Table 8.1 Formulation of multifunctional acrylate for photopolymer resin.

Monomer/polymer Ratio (%)

Diethylene glycol diacrylate (DEDGA)

Dipentaerythritol penta-/hexa-acrylate (HDPA) 9010 Trietyleneglycol diacrylate (TEGDA)

Dipentaerythritol penta-/hexa-acrylate (HDPA) 9010 Polyethylene glycol diacrylate (PEGDA)

Dipentaerythritol penta-/hexa-acrylate (HDPA) 8020

Photoinnitiator and dye for each formulation: 3.75 wt% and 0.15 wt% of the polymer weight respectively.

Mechanical characterisation was performed on the fabricated hydrogels to investigate the properties and their possible application. The polymerised formulation hydrogels exhibited degradation properties, which is a vital requirement for a biomaterial that promise potential for the use of this resin for soft tissue scaffold applications. However, the hydrogels suffered with stress post-curing, which promote delamination and warping of the sample.

Even though the hydrogels faced the problems mentioned above, the formulated photopolymer resin allowed the exploration/functionalization of these resins by investigating the addition of a carbohydrate groups to the polymer system

179 for biomedical application of hydrogels. This is to explore the feasibility of the microstereolithography technique to fabricate new photopolymer resin for further enhance its diversity. The formulation of new photopolymer resin is shown as in Table 8.2.

Table 8.2Formulation of glycopolymers photopolymer resin

Sample Ratio (%)

PEGDA HDPA *GlcAc

Control 2.5Glc 5.0Glc 70.0 72.5 70.0 30.0 25.0 25.0 - 2.5 5.0

*: 1,2,3,4,6-Penta-O-acetyl-beta-D-glucopyranose; Acetyl 2,3,4,6-tetra-o- acetyl-beta-D-glucopyranoside

The glycopolymer resin was successfully fabricated on the machine and was characterised to study the effect of carbohydrate group to the formulation. The modification and increases of GlcAc has increased the molecular weight of the polymer mixture and as a result increased the swelling ratio of the cured polymer. The addition of GlcAc also assists in the erosion process during degradation. However, sugar-based glycopolymers has no significant effect on the tensile strength of the cured polymers due to reoccurring stress post-curing on the samples.

8.2.3 The optimal formulation of polymer ceramic resin compatible with a commercial 3D direct manufacture system (envisionTEC Desktop) specifically for hard tissue engineering applications such as bone.

In this study, three types of calcium phosphate have been used; synthesised calcium pyrophosphate (CPP-A), as-received calcium phosphate (CPP-B) and hydroxyapatite (HAP), as part of a new synthesized photopolymer resin. It was shown that a 3 dimensional ceramic composite structure can be fabricated on an EnvisionTEC Desktop machine. It has been shown that particle size, composition, and viscosity of calcium phosphate within the photopolymer resin have a significant effect on the decomposition temperature and porosity of the polymer composite. Table 8.3 represent the polymer ceramic resin prepared in this study.

Table 8.3 Formulation of photopolymer ceramic resin for fabrication on envisionTEC Desktop.

Calcium

Phosphate Weight %(wt%)i Abbreviation Photopolymer_Resin Ratio (%) ii CPP-A 50 70 100 50A 70A 100A *HDeDA *DPA *BPA PI Dye 70 20 10 3a 0.15b CPP-B 50 70 100 50A 70A 100A HAP 50 70 100 HAP50 HAP70 HAP100 i_{: weight percent of total weight of prepolymer (*)} ii_{: ratio based on total volume of prepolymer (*)}

a_{: weight percent of PI based on the total weight of prepolymer (*)}

Different sintering times were designed in order to study its effect on the mechanical properties and microstructure of the composites. The mechanical strength of both type of CPP ceramic were found to increase with increasing sintering times from 5 to 10 hours at 1250°C. Generally as sintering temperature is increased the

181 compressive strength was increased but after 12 hours, micro-cracks were induced on the particles surface. On the other hand, the elastic modulus of the HAP-based composite increased as the sintering time increased. This is due to the small particles size and high diffusion rate between particle contacts during the sintering process. Nevertheless, the compressive properties of the heat treated composite were quite close to those of cancellous bone (2–17 MPa) [116].

In bioactivity study, increasing the sintering time has enhanced the apatite nucleation onto the composite surface. A study on sintering times has provided significant insight into the re-precipitation and microstructure of apatite formation that could be used in bone substitutes and tissue engineering applications.

As a preliminary study, a 3D extruder machine (BFB 3000 3D extruder) was used to study the feasibility and the potential of the machine as a tool for manufacturing a 3D polymer composite for tissue engineering application. A composite filament of polycaprolactone (PCL)/ hydroxyapatite (HAP) was successfully fabricated on the machine. However, further study on the sintering temperature has to explore in order to obtain a 3D ceramic composite for tissue engineering application.