Currently, the most compelling application of LOC technologies is in the early and accurate detection of infectious agents in developing countries where resources are severely limited. However, microfluidic research and development is often limited to countries with significant resources for research since current prototyping methods for microfluidic devices are costly and access to materials is limited in low-resource settings [ 1 ]. There is often a disconnect between the design of LOC devices and the realities of field application that can be overcome by widening participation in microfluidic prototyping to low-resources setting.
Microfluidics prototyping consists of two main steps: creation of the master mold and replication into a polymeric microfluidic prototype. Master molds are generally created using photolithography, which is a complex and expensive procedure. In the photolithographic process, high-power, collimated ultraviolet UV light is projected through a photomask onto a photoresist SU-8 layer to fabricate patterns [ 2 ]. The process is often performed in a clean-room environment, which requires significant capital and maintenance costs.
Cheaper alternatives to photolithography include removing the need for master molds entirely such as vinyl cutters xurography [ 3 , 4 ], laser cutting [ 5 ], shrinking polymer [ 6 , 7 ], polyester-toner [ 8 ], paper-wax [ 9 ], and NOA adhesive [ 10 , 11 ], which have been explored to create master molds and microfluidic chips but lack the superior resolution of the photolithography [ 1 ]. Even though 3D printing is also a cheap, robust, and scalable method for producing master mold [ 12 , 13 , 14 ], there are still challenges that have prevented the adoption of 3D-printing by microfluidic developers, which are resolution, throughput, and resin biocompatibility [ 15 , 16 ].
Attempts to lower the cost of the technique have focused on the UV lighting [ 17 ] and alternatives to chrome-plated masks, but the use of a photoresist component, SU-8, has remained constant. However, SU-8 can be prohibitively expensive and complicated to source especially for researchers in low-resource countries. The lack of a high-reproducibility fabrication method that is inexpensive, robust, scalable, and sturdy to use under harsh and resource-limited environments and simple to use is a complication for low-resource countries and prevents them in joining the microfluidics research community.
In the early s, methyl methacrylate MMA was tested as a photo-resistance for microfluidic chips with pre-polymerized MMA placed in a cavity and exposed to UV [ 18 ]. However, the volume shrinkage during molding resulted in bubble formation in the PMMA channel plates.
Once the master mold is created, it is used as a template for fabrication of the polymeric device. To date, most microfluidic prototypes are fabricated through soft lithography, which was originally popularized by the Whitesides group [ 19 ]. A pre-polymer, most commonly poly dimethylsiloxane PDMS , is cured on top of the master mold and, after curing, a PDMS-negative stamp of the mold is created and bonded irreversibly to glass, which creates the chips. In recent years, it has been recognized that, while PDMS has been widely adopted in academic settings and in a few commercial products Fluidigm, Cellectricon, etc.
The polymer can be soft embossed on the master mold to replicate its features, can be surface treated for permanent hydrophilicity, and bonded without plasma treatment to glass bonding strength 1 bar or polymers bonding strength up to 3 bars [ 26 ].
We demonstrate that the method is able to create master molds with high accuracy and high reproducibility. We also show that the resulting master molds are compatible with two common fabrication methods in microfluidics: soft lithography with PDMS and soft embossing with Flexdym.
Photomasks were created using two different printing methods. The first method involves using a dpi laser printer Canon LBP, Tokyo, Japan onto a transparent film Polyethylene terephthalate PET A4 transparent sheet , which is commonly used for printed circuit board fabrication. This spray dissolves the toner, which enables merging of neighboring ink spots and filling open spaces.
This results in darker patterns. The second method involves a widely used printing called offset printing from a printing provider Saigon 3 Printing Co. The photolithographic process is described in detail below and in Figure 1. Ningbo , China were cleaned using acetone, ethanol, and distilled water. Approximately 1. The photomask was then placed on top of the chamber in contact with the MA solution.
An even spread was ensured by scrolling a rod along the long edge of the chamber. Under UV illumination, the methacrylate crosslinked into a hardened pattern and adhered to the glass slide. After UV illumination, the photomask was peeled off and discarded. The UV exposed methacrylate layer was washed several times with a washing buffer of acetone-ethanol volume ratio of to remove the uncured methacrylate.
Browse By Subject
For sophisticated patterns, the glass slide was dipped into the washing buffer and a paintbrush was used to gently wipe the uncured methacrylate away. To prove the compatibility of our master molds to create microfluidic prototypes, we tested them with two fabrication methods: soft lithography and soft embossing. PDMS slabs were bonded with microscope slides after air plasma treatment evacuating for 30 s, s at high RF power 18 W by manually pressing the two parts together. It has the advantage of using conventional manufacturing processes to be molded hot embossing, extrusion [ 26 ], which permits an easy transition from prototyping to mass production of microfluidic devices.
In addition, it offers better hydrophilic properties and easy bonding to most surfaces [ 26 ]. Once the molding was done, the Flexdym channels were peeled off and bonded to another Flexdym sheet by simple contact pressure.
In developed countries, photomasks made of chrome-plated glass can be purchased from suppliers. In Vietnam, no photomasks suppliers could be identified. The preciseness and resolution of the mask are critical to the quality of the lithography. As shown in Figure 2 A, the mask printed using the office laser printer is not homogeneous and the edges of the pattern are coarse [ 27 ]. This is due to the printing method in which microdots of electrically-charged powder are dispensed and spaced apart to form the image on the film.
The gaps let the UV light pass by leasing to an inhomogeneous curing of the methacrylate and rough channel walls. By using a toner aided spray to enhance the contrast of the photomask Figure 2 B , the small gaps can be filled, but the edges are still irregular, which leads to rough channel walls.
With the offset printing technique Figure 2 C , the edges of the pattern are smooth and the printed surface is uniformly dark. The offset printing technique sequentially uses four plates with four colors cyan, magenta, yellow, and black and a rubber blanket to imprint the image onto the film, which results in a higher quality in the image transfer. UV curing using an offset-printed photomask resulted in channels with sharp walls and uniform cross-linked methacrylate. Photomasks were created using printing methods readily available for low-resource research environments.
A Laser printing from an office printer presents a rough edge. B Laser printed masks sprayed a solvent-based solution are more regular but still present coarse walls and overall roughness. C Offset printing showed sharp edges and high contrasts suitable for use as a photomask. The use of an offset printed transparency represents an excellent alternative to glass photomasks in the low-resources environment. To overcome the lack of access to photolithography facilities, we explored the use of a LED array. The low power requirement allows the system to run on AA batteries instead of a high-voltage power supply, which is a feature that further enhances its accessibility for low-resource settings.
Since the methacrylate manufacturer did not provide specific information on the energy intensity required for cross-linking, we measured the intensity of the UV light at various distances directly under the light source Figure 3 to link it with crosslinking performance.
Materials and methods for the microfabrication of microfluidic biomedical devices - ScienceDirect
Since the array of light is not equipped to generate parallel light beams, we explored its impact on wall perpendicularity. We adjusted the distance between the light source and the chip to reduce the divergence angle and create perpendicular sidewalls. We observed a direct correlation between the perpendicular distance between the lamp and the chip and the variation of the wall angle.
We established the reproducibility of our method by comparing lines of various width from the photomask and that of the resulting MA pattern. When measured from the bottom of the line, the ratio was found to be closer to 0. Ratio between design width and measured width of lines of various width. For low-resource setting labs that may not be able to afford a spin coater, we found that using vinyl stickers as spacers can be a good alternative method Figure 5. However, we noted that, as the layer of methacrylate became thicker, the microfluidics sidewall was no longer perpendicular to the bottom surface.
The recorded slope angle created by the sidewall and the bottom surface at 1. It is, thus, important to note that the thickness of the gel will influence the final geometry. The polymer did not adhere to the mold once cured and the patterns were reproduced accurately Figure 6. The channels were smooth and well-defined, which validates this method for fabrication of microfluidics molds.
Figure 6 illustrates several fabricated designs using the proposed method such as capillary pumps and gradient generators, which performed satisfactorily and the fluids flowed easily when tested shortly after plasma bonding. The methacrylate mold was successfully used to reproduce various shapes and design in two polymers adapted to microfluidic applications: PDMS and Flexdym. Due to a number of its shortcomings absorption of small molecules, manufacturing method incompatible with mass production, and reversible hydrophilicity , PDMS is often considered less than ideal for prototyping.
We, thus, also tested our master mold with an alternative material, Flexdym, which uses a molding method similar to hot embossing called soft embossing. Similar to the PDMS chips, we observed high-reproducibility of the pattern and smooth walls Figure 6.denapaga.tk
Low-Cost, Accessible Fabrication Methods for Microfluidics Research in Low-Resource Settings
One additional advantage of the Flexdym material is that bonding does not require plasma treatment, which reduces the initial capital cost by a significant amount. Alternative methods of bonding PDMS without plasma [ 10 , 29 , 30 ] can also be used, which either adds an adhesive layer to the PDMS or uses stickers while Flexdym present the advantage of an inherently adhesive nature [ 26 ].
Lastly, the chips made with Flexdym were more hydrophilic than their PDMS counterpart and, therefore, filled more easily with the reagents. In summary, we have developed an affordable, accessible process to fabricate microfluidic devices using resources available and reasonably-priced for low-resource settings.
We anticipate that the ability to prototype a wide range of devices with minimal setup and running costs will accelerate the development of microfluidic devices and, most importantly, enable researchers from low-resource settings to join the community of microfluidics engineering.
- Assassins Riddle: A Brother Athelstan Medieval Mystery 7?
- Microfluidics & Microfabrication Facility;
- Inimi Frânte.
- Robust Microfabrication of Highly Parallelized Three-Dimensional Microfluidics on Silicon.
- SearchWorks Catalog!
The authors are thankful to the Biomedical Engineering Department - International University, Ho Chi Minh city, Vietnam for providing the necessary resources required for this work. We thank BlackHole Lab for providing all information on pricing of equipment for microfabrication.