One-Step Injection moulding is a novel technique, that was firstly introduced by Szydzik et al. in 2016 42. It allows the integration of components (such as membranes) into multi-layer microfluidic structures in one simple step. It also allows the integration of microfluidic valves into microfluidic channels as explained in chapter 2.2.. It therefore simplifies production, while increasing functionality. 42
A commonly found alternative technique is 2D microfabrication, which has not seen much improvement over the last years. It needs multiple manual alignments, which make it unsuitable for mass fabrication 9, 21, 24, 25, 27, 42. Another form of fabrication is 3D fabrication. There, PDMS is casted on a sacrifice structure which is then removed or dissolved 3, 15, 19. This gets rid of the alignment problem, but still bares new challenges in handling like the removal of the sacrificial structure 42. The last possible fabrication method is the “Sandwich method” as presented by Whiteside et al. 32. The process consists of multi-layering channels by sandwiching PDMS pre-polymer between a glass mould and a passivated PDMS mould 42. This leads to a simpler process and easy mould removal 42. The laid-out technology of one-step injection moulding is a consequent development of this technology. In comparison to conventional sandwiching it offers the opportunity to implement more complex structures such as valves and implement features such as membranes on a multi-layered chip. One-step injection moulding is of high importance for this project because it allows us to implement features like the filtering membrane and microfluidic valves in a fashion that could be upscaled to mass fabrication easily and therefore has a big advantage over all other proposed technologies.
The procedure to fabricate multi-layer channels can be seen in Figure 2. Two complimentary mould have to be aligned in the first step and then brought into contact. After that, PDMS is injected, which will fill up all void spaces between the two moulds. This will then need to be cured at around 70 °C for an hour before obtaining the final microfluidic structure that can be bonded to glass slide to seal the channels. Vertical interfaces can be biopsy punched. 42
Figure 3 “Conceptual illustration of PDMS injection moulding fabrication technique. (A) Complementary mould halves are initially loaded with a functional substrate for integration, (B) the moulds are then brought into contact and clamped, (C) degassed PDMS is injected through an opening and fills voids between the mould structures. The mould assembly is then placed in an oven at 70 °C for one hour until cured (D and E) upon removal of the mould structure the resultant double sided PDMS slab is cut to size, and channels on either side of the slab are sealed with glass slides.” 42
Figure 3 shows the encapsulation of a membrane into a microfluidic device. Therefore, two complementary moulds are positioned in such a way, that a membrane is sandwiched between two channels. The channels on the membrane can be seen as slightly elevated and coloured parts in the figure. Any injected PDMS will then only fill the void space and leave two channels that are separated by a membrane. 42
Figure 4 “Concept for integration and encapsulation of prefabricated functional components. (A) … permeable support membrane is placed on a point of intersection of the microfluidic mould structures, (B) the complimentary mould structures are then brought into intimate contact and clamped, (C) PDMS is then injected through an opening and fills the voids between moulds, (D) during this process PDMS infiltrates all areas of the membrane not mechanically compressed, resulting in clear areas at the channel intersection and mechanically encapsulated areas without.” 42
Figure 4 shows the fabrication process for microfluidic valves. To realise this structure, again two complementary moulds are aligned over one another. The Valve consists of a 50 ?m gap between two channels. The green channel over it has a chamber exactly over the valve gate gap, which will later be filled with low pressure air to lift up the gate. To make this possible, the actuation chamber has to be positioned in a defined distance from the gate. Therefore additional pillar structures are implemented around the gate to ensure a specific height. 42
Figure 5 “Concept for fabrication of active elastomer structures. (A) The mould halves are designed so as to not quite contact at the location of the valve. (B) The mould halves are again brought into contact leaving a narrow void in the region of the valve, (C) PDMS is injected forming a membrane between the two channels, (D) the valve gate can be actuated through application of negative pressure in the actuation chamber.” 42
Before any of the priory described PDMS fabrication can take place, the mould itself has to be fabricated. To accomplish this, a number of techniques are utilised. The first one is photolithography. It is used to pattern two silicon wafers. The silicon wafers are then put into petri dishes, which are subsequently filled with PDMS. It is then cured under room temperature for 24 hours. The exact fabrication process for this project can be found in the Appendix.