Biomedical research using polymer microfluidics is actively conducted by researchers. The small devices could enable quick and simple diagnostics at patients’ home without visiting doctors at hospitals in the near future. The target of the diagnostics using microfluidics exists not only in developed countries where reduction of medical cost is highly required due to ageing society. It can be a promising solution to deliver medical service at low cost in developing countries where the number of hospitals and doctors is limited due to insufficient government budget and unstable supply-chain of expensive medical devices.
However, there are some challenges in polymer microfluidics fabrication process and actual use. Samco offers plasma treatment solutions to solve the process challenges for low-cost microfluidics fabrication. In this page, we will describe how we tackle the challenges using our plasma treatment technologies.
Polymer Microfluidics Fabrication Process
Conventional polymer microfluidics fabrication procedure is as follows.
1. Micro-channel Molding
2. VUV Irradiation (Surface Activation)
While there are several materials used in polymer microfluidics fabrication such as PDMS, PMMA and COC, Cyclic Olefin Polymer (COP) is one of the popular polymer materials due to several reasons.
1. low cost (easily processed using molding)
2. low fluorescence intensity (required for optical molecule detection methods)
Process Challenges in COP Microfluidics Fabrication
Even though COP microfluidics are promising for next-generation diagnostics, there are several challenges in fabrication process.
Auto-fluorescence degradation of COP
COP is originally hydrophobic as well as general polymer materials. Therefore, it requires surface modification to obtain hydrophilic surfaces. However, VUV irradiation deteriorates low auto-fluorescence of COP. The aggregated auto-fluorescence is noise for optical detection of molecules by fluorescence techniques.
Micro-channel collapse by substrate pressing
Oxygen plasma is another approach for surface activation and substrate bonding. A drawback of oxygen plasma treatment is low surface adhesion between substrates, and pressing is required to assure the substrate adhesion. However, the excessive pressing sometimes destroys micro-channel pattern with fine pitch which is a key in device design.
A new COP surface activation and bonding technology is needed with the process requirements below.
1. maintain low fluorescence intensity of COP
2. low-cost, safe and expandable for device production
Using H2O for Plasma Treatment
Aqua Plasma is plasma treatment process which uses H2O (water vapor) as process gas.
Water vapor generates reactive oxygen species and hydroxyl radicals.
There are several process features of Aqua Plasma.
1. Inexpensive and safe
Water as process gas source is inexpensive and safe. Easy to install compared to H2 which is used for gentle plasma cleaning.
2. Exclusive for Samco Plasma Cleaner AQ-2000
Aqua Plasma® is specially developed by Samco to provide plasma treatment solutions for microfluidics research customers.
COP/COP Direct Bonding
COP substrates were processed in Aqua Plasma. Immidiately after the process, direct COP/COP bonding was conducted. The table shows relationship between RF power and COP/COP bonding strength in bending test. Room-temperature direct substrate bonding between COP/COP was achieved at RF power of 10 W or more.
RF Power vs. COP/COP Bonding Strength
COP/Glass Direct Bonding
COP/Glass room-temperature bonding was achieved after 20-second Aqua Plasma treatment. Note that the bonding was performed without mechanical pressing. This means the process does not cause micro-channel pattern collapse which can be seen on O2 plasma treatment and subsequent mechanical pressing/heating. Maximum bonding strength was 1.612 N/cm2 and glass plate was destroyed.
COP/Glass Plates after Aqua Plasma® Treatment and Bonding
COP Surface Activation (Aqua Plasma vs. O2 Plasma vs. VUV)
Superhydrophilic (lower than 5°) surface was achieved after Aqua Plasma treatment. The contact angle was lower than VUV treatment.
COP Contact Angle after Aqua Plasma® or O2 Plasma Treatment
COP Contact Angle after VUV Treatment
COP Auto-fluorescence Intensity (VUV vs. O2 Plasma vs. Aqua Plasma)
The table on the right shows COP auto-fluorescence intensity before and after VUV, O2 plasma, and Aqua Plasma. Aqua Plasma treatment did not change COP’s fluorescence intensity unlike VUV. Note that Aqua Plasma enabled bonding without pressing/heating unlike VUV and O2 plasma. This means in Aqua Plasma treatment, there is no risk of breaking fine micro-channel pattern.
COP Auto-fluorescence intensity after VUV, O2 and Aqua Plasma® Treatment
Clear data w/o auto-fluorescence noise
No deterioration of auto-fluorescence intensity compared to VUV
Low-cost & safe
Water vapor as plasma gas source is low cost and safe.
Expandable to production
Batch substrate processing on multiple shelves allows high throughput.