Prototyping of microfluidic devices presents a number of challenges. Fabrication techniques such as clean room-based microfabrication, injection molding and milling and bonding are generally slow and expensive, and traditional 3D printers are unable to generate fluidically-sealed devices able to cope with the high pressures required. In addition, conventional 3D printing materials are unsuitable for microfluidic applications, as they are not chemically or biologically compatible, or transparent. Until now, this has prevented the adoption of 3D printing technology for microfluidic applications.
Microfluidics is a fast-growing field showing great potential for a wide range of applications, including point-of-care diagnostics, analytics, drug development, organ-on-a-chip, education, chemical synthesis and biomedical assays, as well as research and development. There is a clear need to complement existing techniques for the production of microfluidic devices with more rapid, cost-effective 3D printing technology that can create fluidically-sealed devices able to withstand the high pressures used in many applications.
Options and challenges
Traditionally, prototype microfluidic devices are produced using clean room-based microfabrication techniques such as lithography. Although these are extremely accurate and deliver the highest quality possible, they are also very expensive and slow. An alternative technique, developed in the late 1990s by the Whitesides Research Group at Harvard University, allowed micron-scale features to be replicated by casting polydimethylsiloxane-silicon rubber (PDMS), creating an open channel that then had to be closed by a lid. This method has formed the backbone of academic research in microfluidics.
3D printing techniques are appealing in nature, since a design can theoretically be replicated very quickly and cost effectively. However, traditional 3D printers are not designed for microfluidic applications and this creates certain challenges. These printers create microfluidic devices using stereo lithography (SLA), selective laser sintering (SLS) and fused deposition modeling (FDM) techniques, which tend not to work effectively, either because they use the wrong type of material, or because they result in the internal voids being filled with support material. The difficulty with printing hollow structures —for example, microfluidic chips with minute, twisting channels —is that when the lid is printed it collapses, as it has no support. SLA and SLS techniques provide the necessary support with UV-curable photopolymers and powdered polymers respectively, but the material used is impossible to remove from the microfluidic channels afterwards. FDM techniques may also require the use of a support material, although there are circumstances in which this can be dispensed with. The security of fluidically-sealed pathways cannot be guaranteed and, in addition, the UV-curable photopolymers used in SLA are not representative of the materials used in biological applications.
3D printing focused on microfluidics
Until now, there has not been a 3D printer designed specifically for the fabrication of fluidically-sealed devices. For most applications, the primary function of a 3D printer is the creation of a prototype device, for example, a mobile phone, where the priority is the external appearance. In microfluidics, the opposite is true; the most important consideration is the channel structure inside the microfluidic chip.
The recently launched Fluidic Factory—the first commercially available 3D printer for fluidically-sealed devices — addresses this shortcoming. It uses FDM techniques, melting a fiber of a given polymer and depositing it in layers to build a model. This is a rapid, cost-effective technique that is ideal for prototyping, and has been carefully engineered to avoid the need for a support material during printing. It is also the first 3D printer to accommodate cyclic olefin copolymer (COC), an FDA-approved material offering many advantages over other polymers. This material is transparent, chemically and biologically compatible, does not auto fluoresce and has very low water absorption. Traditional 3D printers compromise on the internal form,which is usually unseen as the materials used are not generally transparent. However, the Fluidic Factory works from the inside out, focusing instead on the internal structure to ensure the precise geometry of the microfluidic channels; the external appearance is a secondary consideration.
Most FDM printers work by heating the polymer until it becomes molten, and then ejecting it in the form of a circular cross-section bead, which solidifies. However, spaces remain between the beads, creating a leak path (Figure 1). The Fluidic Factory overcomes this problem by dispensing beads with a ‘squashed’ cross-sectional area. As additional layers are printed, the polymer seamlessly flows into the areas above and below each of these beads, ensuring reliable, leak-free sealing of fluidic channels (Figure 1). This one-step process is a significant advantage, allowing the creation of precise channel geometries – circular, triangular or rectangular – and various features not possible using etching, embossing, molding or machining techniques.
A wide range of devices can be produced in this manner, including droplet and emulsion chips, micromixers, microreactors, connectors, custom fluid manifolds, sensor cartridges, valves and medical devices. Open format print files are downloaded from the system’s Design Library, or custom designs created using CAD software and uploaded to the printer via a USB connection, allowing structures of any shape and geometry to be generated. This enables straightforward, cost-effective production of individual microfluidic devices, allowing ad hoc changes to the structural design and geometry, and easy reprinting of devices.
This launch of a 3D printer specifically designed for fluidically-sealed devices is an important step forward in microfluidics, offering rapid, reliable and cost-effective printing. It is ideal for the production of small numbers of prototype devices to test a specific concept, complementing traditional fabrication techniques, such as injection molding, and micro-milling and bonding. This fast, affordable fabrication of individual prototypes increases the speed of development of microfluidic devices for subsequent bulk production by large-scale manufacturing processes, benefitting the industry by helping to shorten the time to market.