Surface fabrication methods for patterned deposition of nanometer thick brushes or micron thick, crosslinked films of an azlactone block co-polymer are reported. Critical experimental steps, representative results, and limitations of each method are discussed. These methods are useful for creating functional interfaces with tailored physical features and tunable surface reactivity.
These presented methods utilize parylene liftoff interface directed assembly and custom microcontact printing techniques. The methods are designed to provide high uniformity and control over the pattern polymer thickness. These methods are also designed to deposit PGMA block PVDMA films while completely preserving versatile azlactone functionality since azlactone groups can rapidly couple with nucleophiles in ring opening addition reactions.
Azlactone-based polymer films are used in many different environmental and biological applications, including analyte capturing, cell culturing, and antifouling and antiadhesive coatings. To begin the procedure, prepare five milliliters of a one percent by weight solution of PGMA b PVDMA in anhydrous chloroform. Then, turn on an oxygen plasma cleaner and place a two centimeter by two centimeter silicon substrate patterned with an 80 nanometer thick or a one nanometer thick layer of parylene N in the chamber.
Pump down the chamber to less than 400 milli torr vacuum gauge pressure. Slightly open the metering valve and allow air to enter the chamber. Wait until the gauge pressure reaches 800 to 1000 milli torr.
Set the plasma cleaner to high RF power such as 30 watts and treat the substrate with plasma for three minutes. Turn off the RF power in the vacuum pump when finished. Within 10 minutes of finishing plasma cleaning, fix the plasma-treated substrate on a spin coater chuck.
Set the spin coater to run at 1, 500 RPM for 15 seconds. Then, apply 100 microliters of the PGMA block PVDMA solution to the substrate and begin spin coating the substrate within one to two seconds of applying the solution. Anneal the film in a vacuum oven at one atmosphere vacuum and 110 degrees celsius for 18 hours.
Allow the sample to cool in ambient air. Finally, sonicate the sample for 10 minutes in 20 milliliters of acetone or chloroform to remove the parylene. Dry the substrate under a stream of nitrogen gas before storing it.
To begin preparing a chemically or biologically inert silicon substrate, plasma clean a bare silicon substrate for three minutes as previously described. Then, in a fume hood, pipette 100 microliters of TPS into a petri dish. Place the plasma-treated substrate in the dish of TPS in a vacuum desiccator within five to ten minutes of finishing plasma cleaning.
Evacuate the desiccator to 750 torrs of vacuum to begin TPS deposition. Allow TPS to deposit on the substrate for one hour to obtain a chemically inert substrate. If preparing a biologically inert substrate, then soak the sylonized substrate in zero point seven percent weight to volume solution of poloxamer 407 in ultra-pure water for 18 hours to generate a PEG layer on the TPS-coated substrate.
After that, rinse the biologically inert substrate with 30 milliliters of ultra pure water three times over the course of five minutes. Next, pattern the chemically or biologically inert substrate with positive photo resist and etch the exposed areas to the silicon surface. Then sonicate the sample in acetone for 10 minutes to remove the parylene layer.
Dry the sample under nitrogen gas. Fix the substrate on a spin coater chuck and set the spin coater to 1, 500 RPM for 15 seconds. Apply 100 microliters of one percent by weight PGMA block PVDMA in anhydrous chloroform to the substrate and start spin coating within one to two seconds of application.
Anneal the resulting polymer film in a vacuum oven at one atmosphere of vacuum and 110 degrees celsius for 18 hours. Remove the phys absorbed polymers from the background regions by 10 minutes of sonication in acetone or chloroform. Dry the sample under nitrogen gas afterwards.
To begin the microcontact printing process, sylonize a silicon master for micropillar arrays with TPS. Then, mix together 40 to 50 grams of PDMS base in curing agent in a ten-to-one ratio. Use soft lithography to pattern a slab of PDMS with micropillar arrays.
Cut one one point five by one point five centimeter stamp from the slab. Clean and dry the stamp and sylonize the stamp with TPS. Next, apply double-sided tape to the stage of a drill press stand.
Then, prepare five milliliters of a zero point two five to one percent by weight solution of PGMA block PVDMA in anhydrous chloroform. Immerse the sylonized stamp in this solution for at least three minutes. Promptly plasma treat a two centimeter by two centimeter bare silicon substrate for three minutes.
Within five minutes of finishing the plasma cleaning, remove the stamp from the polymer solution. Then, bring the substrate and stamp to the drill press stand. Place the substrate and the stamp on the double-sided tape on the stage with the substrate on the bottom.
Apply conformal contact between the stamp and substrate with an estimated pressure of 75 grams force per square centimeter, or seven point three five kilopascals for one minute. Then, carefully release the pressure and gently separate the stamp from the silicon substrate by hand. Within five minutes of separation, anneal the printed substrate in the vacuum oven for 18 hours at 110 degrees celsius.
Lastly, sonicate the printed substrate in acetone or chloroform for ten minutes to remove phys absorbed PGMA block PVDMA. Dry the printed substrate with nitrogen gas and store it in the vacuum desiccator until ready for characterization. The parylene liftoff technique produced brush structures of PGMA block PVDMA corresponding to a polymer film thickness of about 90 nanometers.
Annealing was essential to the technique as sonication and chloroform removed most of the non-annealed polymer. 80 nanometer thick parylene stencils showed greater film uniformity than one micron thick stencils on bare silicon substrates. Interface directed assembly on chemically or biologically inert substrates produced 90 to 100 nanometer thick structures without the edge defects observed in the parylene liftoff method.
Micro contact printing produced highly crosslinked polymer films, about three to nine microns thick, by using different concentrations of the polymer. In addition to treating the substrate with TPS and oxygen plasma, annealing was essential for this technique, as significant damage was observed to non-annealed films after sonication. The azlactone functionality was preserved by microcontact printing as indicated by the carbonyl stretching vibration at 1818 reciprocal centimeters.
Microcontact printing also allowed control of the film thickness by varying the concentration of the PGMA block PVDMA solution used to ink the stamp. Here we have shown that parylene liftoff procedure is a versatile method for patterning block hole polymers at microscope resolution. The crucial factor that affected the film uniformity was parylene stencil thickness.
The interface directed assembly patterning method uses parylene stencils to generate oxide patterns that guide the self assembly of PGMA block PVDMA polymers on surfaces which are chemically or biologically inert. The customized microcontact printing protocol presented here basically generates thicker PGMA block PVDMA polymer providing more surface-to-volume ratios that may enhance the loading of chemical or biological analytes in capture application, improve cell attachment, viability and proliferation in cell culture applications. The methods and results presented here describe multiple approaches that generates pattern interfaces with PGMA block PVDMA polymer.
The methods can be employed to create surfaces with brush or crosslinked structure of this polymer depending on the applications.
