To allow reliable predictions of the softening of polymeric substrates for neural implants in an in vivo environment, it is important to have a reliable in vitro method. Here, the use of dynamic mechanical analysis in phosphate buffered saline at body temperature is presented.
This method helps in monitoring how the material properties of implantable devices may change upon insertion into the body, particularly the softening of the materials. The main advantage of this technique is that it offers a simple in vitro method that mimics in vivo conditions and therefore minimizes the need for animal testing. This method is especially interesting for testing the thermomechanical properties of the material that will be used in implantable devices in general and is not limited to bioelectronics.
The loading of the sample and the measurement of the sample under the wet conditions are a little bit tricky and may cause unreliable test results if not performed properly. Begin by mixing quantitative amounts of thiol to alkene monomers with a total of 1 weight percent photoinitiator. Cover a 20-milliliter glass vial in aluminum foil to prevent incident light from contacting the monomer solution, and use a disposal plastic pipette to add 50 mole percent TATATO, 45 molar percent TMTMP, and five molar percent TMICN to the vial.
Then add 1 weight percent of the photoinitiator DMPA to the vial, and use planetary speed mixing to mix the contents of the vial without exposing the solution to light. When the contents have been thoroughly mixed, spin coat the resulting thiol-ene prepolymer mixture into glass microscope slides in five 50-micrometer-thick films, and immediately transfer the polymer films on the carrier substrate in a crosslinking chamber. Then photo-polymerize the films for 60 minutes under 365-nanometer ultraviolet bulbs, followed by post-curing in a vacuum oven for 24 hours at 120 degrees Celsius to further complete the conversion.
When the polymers have fully cured, use a carbon dioxide laser to cut the films into 4.5-millimeter-wide by 50-millimeter-long rectangles for dynamic mechanical testing. To set up the dynamic mechanical analyzer, equip the machine with the immersion fixture in tension mode. Connect the liquid nitrogen to the machine, and enable liquid nitrogen in air as a gas source for the furnace.
Write the method for the dry measurement with the machine software, including the conditioning, oscillation temperature ramp, and conditioning end of the test steps. Then write the method for the immersion testing with the machine software, including the conditioning, oscillation time, oscillation temperature ramp, and conditioning end of test steps. When the analyzer is ready, use calipers with a 001-millimeter precision to measure the actual thickness of the polymer specimen for dry in air testing, and enter the sample name, description, and sample geometry into the software.
Set the loading gap to 15 millimeters, and load the sample. Make sure to center and align the specimen before tightening the clamps. Then close the furnace, and start the dry measurement.
When the measurement is over, open the furnace, and remove the polymer sample from the machine. To measure the actual thickness of the polymer specimen for immersion testing, first measure the sample with calipers with a 001-millimeter precision, and enter the sample name, description, and sample geometry into the software. Prepare the setup with the immersion beaker fixed with a clamp at the upper grip, and set the loading gap to 15 millimeters.
Load the sample, making sure to center and align the specimen, and tighten the clamps. Place the immersion bath on the bottom fixture, and secure the bath tightly. Then fill the bath with room-temperature PBS.
Place the lid on top of the bath, close the furnace, and immediately start the immersion measurement, confirming that the drain is closed. It's important to start the measurement as soon as possible after the beaker is filled with the PBS to ensure that the full range of the softening is captured. When the measurement is over, open the drain to remove the PBS from the immersion bath, and open the furnace.
Then remove the lid from the beaker, unscrew and lift the immersion beaker, and remove the polymer sample from the machine. Using the temperature-time measuring mode of the protocol allows the softening profiles of different polymer formulations to be compared. Combining dry dynamic mechanical analysis measurements and immersion measurements in PBS allows the assessment of water-induced plasticization of different polymer formulations, as illustrated by the depression of the glass transition temperature and overall downshift of the modulus curves.
The softening of the polymers for in vivo applications works most effectively when the dry polymer has a glass transition temperature above body temperature but below that in the wet state. Thus, the modulus of the polymer drops from the glassy to rubbery modulus upon immersion under physiological conditions. When the glass transition temperature of both the dry and wet states of the polymer are well above body temperature, the polymer will not soften under physiological conditions.
This method allows you to replace the PBS with other relevant solutions to mimic the behavior of biomaterials in other environments.
