Functionalised material for 3D printing : microfluidic and microanalytical devices

Additive manufacturing, or 3D printing, continues to impact manufacturing and prototyping industries, through the provision of a quick and low-cost alternative to the more conventional subtractive manufacturing techniques. However, due to the intrinsically limited functionality of most proprietary resins and commercially available pristine feedstock polymers, there is now a critical need to develop printable composites with high-performance and improved physico-mechanical features and varied functional properties. This research project focuses on exploring the potential of new functional 3D printable materials, such that 3D printing technology can develop further and deliver in new application areas. This thesis documents research on the development of novel, print-compatible functional composites, followed by the investigation of their physico-chemical properties, with potential applications in the area of microfluidic, microanalytical and micro-electronic-mechanical platforms. Chapter 1 begins with a brief introduction of the origin of micro-systems, followed by a comparative analysis of the conventional manufacturing techniques with 3D printing technology. This Chapter gives an overview of functionalised 3D printable materials developed over the last 5 years. The integration of functional fillers, such as carbonaceous, inorganic, stimuli-responsive polymers and biopolymers, into 3D printable resins has been discussed briefly. The Chapter concludes by describing the aims and objectives of this research project. Chapter 2 gives an insight into the state-of-the-art of 3D printing for microfluidics, focusing on the four most frequently used printing approaches: inkjet (i3DP), stereolithography (SLA), two-photon polymerisation (2PP) and extrusion printing (focusing on fused deposition modelling). It discusses their working principles, achievements and limitations, and the opportunities and advances necessary to reach 3D printing's full potential in the development of microfluidic and microanalytical devices. Chapter 3 presents a print, cast and peel approach for the rapid fabrication of robust thermally conductive polydimethylsiloxane (PDMS) based microfluidic chips. The intrinsically low thermal conductivity of PDMS microfluidic chips was enhanced by doping with 60 wt % synthetic high pressures high temperature (HPHT) micro-diamond. The composite microfluidic chips were fully characterised to reveal their homogeneity, hydrophilicity, flexibility and thermal properties. The elastic modulus increased from 1.28 for pure PDMS to 4.42 MPa, for the 60 wt % composite, along with the three-fold increase in thermal conductivity. An IR camera was used to study the thermal performance of fluidic chips at different concentrations of micro-diamond and flow rates. At a flow rate of 1000 ˜í¬¿L/min, the gradient achieved for the 60 wt % composite chip was equal to a 9.8 ¬¨‚àûC drop across a 3 cm channel, more than twice as compared with the control PDMS chip. This study offered a practical solution for the thermal management of electrofluidic embedded electronics and mechanical-electronic microsystems. Chapter 4 focuses upon the direct printing of a synthetic-diamond-acrylonitrile butadiene styrene (ABS) composite, using an FDM printer, by developing feedstock filaments containing 37.5 and 60 wt % micro-diamond. The composite filaments were extruded multiple times to attain homogenous distribution of the micro-diamond. The impact of micro-diamond on the thermo-mechanical properties of the ABS composite were investigated. The thermal conductivity of the ABS composite material increased from 0.17 to 0.94 W/(m¬¨‚àëK), more than five-fold following the incorporation of micro-diamonds. The elastic modulus for the 60 wt % micro-diamond containing composite material increased by 41.9 % with respect to pure ABS, from 1050 to 1490 MPa. The low-cost FDM printer was customised for robust printing using the abrasive diamond containing filament. The print performance of the new composite filaments and their practical utility were evaluated via printing test heat sink designs. Heat dissipation measurements demonstrated that a 3D printed heat sink containing 60 wt % diamond increased the heat dissipation by 42 %. The thermally conductive and electrically insulating diamond-ABS printable composite printable material offered a potential practical solution for cooling of miniaturised micro-electronic and analytical devices. Chapter 5 describes the fabrication of a chitosan-ABS composite filament for FDM printing. The printing compatibility of the composite filament was investigated by varying (5, 10 and 30 wt %) the content of chitosan. Up-to 10 wt % chitosan-ABS filament was printed successfully with macroporous features. As a proof of concept, the antibacterial activity of 3D printed ABS and composite discs containing 5 and 10 wt% chitosan was tested against the model bacteria Escherichia coli. The 10 wt% chitosan composites inhibited the growth of bacterial cells. The composite filaments were also used to print monoliths to investigate their ability to extract Cu2\\(^+\\) ions from solution. Overall, the 5 wt% chitosan-ABS monolith achieved ~28% Cu2\\(^+\\) extraction efficiency, while the 10 wt% composite monolith attained 78% extraction efficiency after 144 hours exposure. These proof of concept results indicate that 3D printed monoliths can be used as a low-cost device for the removal of heavy metals such as Cu2\\(^+\\) ions and for water disinfection with potential application in remote areas where clean water supply infrastructure is not available. Finally, Chapter 6 includes some concluding remarks with some recommendations and future directions of this research.