When characterizing cells or any other microscopic objects it’s sometimes useful to guide them through tiny channels when observing or counting them under the microscope. The term is sometimes called as microfluidics. Therefore many different methods exist to create these devices. One method etches channels in glass or plastic, another additively procedure – sometimes called photolithography – cures a photoresist/resin to get a 3D-like structure after washing the uncured parts away. This usually costs lots of money. Further details here: http://pubs.rsc.org/en/content/articlehtml/2016/lc/c6lc00284f
One part of my research was to improve phase-contrast using a brightfield microscope without adding any special optics like the DIC-prism or phasering in Zernike phase-contrast. Defined phase objects are hard to get. Thinking about a light guide embedded in immersion oil could be one chance to have a phase object, but this extends the field of view (FOV) of a 20x/63x objective which was used in my setup.
Cells are usually amorphous and won’t have defined phase retardation. Therefore I was thinking about a way how I can get a defined phase on the cheap with off the shelf components.
Getting it the right way
Etching glass or plastic was too complicated for me as I’m hardly any farmiliar with chemistry. Buying the necessary parts felt expensive as well.. Ok. Trying the lithographic process seems like the route to go. I have an idea of how silicon wavers are processed to get the computer chips in electrical devices, I know about optics, so could be a good starting point.
3D Printing with UV curing resins
3D printers majorly follow two different techniques. The first melts a material and additively produces 3D structures on a substrate (FDM – fused disposed modelling). The other one images masks into a fluid/resin which cures where the intensity is high enough (SLA – Stereolithographiy). This is usually done with UV-light sources whose energy is high enough to start the concatenation of the photoactive polymer chains. Looking for professional SLA printers gives an idea how it works – and how the price says:
“No, choose for another option.” Even the pure resin is not really affordable. One liter costs between 80-200 Euro, even though I just need a few ml for the start.
When searching for alternatives I found this thing called LOCA (Liquid optically clear adhesive; https://en.wikipedia.org/wiki/Liquid_optically_clear_adhesive) which is usually used to make an optical bonding between an LCD and the touchscreen. Once the screen cracks and got exchanged, you have to rebind it. Therefore special repair-sets exist at ebay. It simply consists of this LOCA (http://www.loctite.com.au/light-cure-adhesives-loctite-loca-6025.htm) and a UV-LED flashlight. It costs 5-10 Euro. I haven’t found any research using this as a UV-resin, but it was definitely worth a try.
The lithographic process needs a negative amplitude-ask. So everywhere the light can pass, the resin gets cured. In this case – for a ring structure – I created a white ring on black substrate in PowerPoint.
This mask got printed with a Laser-printer on a sheet of plastic though for overhead projectors. After cutting out the printed area and sticking it to the flashlight the next step follows: Testing.
After applying a droplet of the LOCA fluid to a coverslip, which was laying around, the fluid got dispensed with the help of another cover slip. It would definitely better to spin-coat this, but the viscosity – so far – is too high. Diluting the fluid may help I guess?
Next step is to turn on the UV lamp and to place the coverslip on the flashlight. That’s it. Now wait! I have no idea how the time relates to the exposure-process, so this needs to be covered later on, but after 10 minutes you’re safe! The result can be seen after washing the slide and observing it from a side view. A clear phase-step is recognizable. And hey. It’s in a ring shape!
The process of washing the uncured LOCA definitely needs to get tuned. Acetate or Isopropanol should do it, but now I just had some hot water and papers..well.. not the matter of choice.
The phase-object gets sandwiched between the substrate cover glass and a second one. The space in between was filled with water to adjust the refractive index mismatch a bit (LOCA~1,5; Air~1; Water~1,3).
The result looks promising. Following the acquisitions taken with a microscope in Phase-contrast and differential phase contrast, the phase step is about 25 µm (that is highly insecure due to experimental errors and so on!) But anyway. This shows, that microstructures can be produced for a few cents each other than a few hundred euros..
Putting away the uncured glue?! Well – using a dilluter available from the hardware store seems to be a good solution. Removing it mechanically isn’t an option, cause there will be scratches on the phase object. Using ultrasound is another option, but I have no device lying around. Any suggestions?