Free-form Light Actuators — Fabrication and Control of Actuation in Microscopic Scale

Here, we fabricate 3D polymeric micro/nano structures in which both the shape and the molecular alignment can be engineered with nanometer scale accuracy by the use of direct laser writing. Light induced deformation of several types of liquid crystalline elastomer microstructures can be controlled in the microscopic scale.

The over all goal of this two step laser writing procedure, is to obtain elastomer based micrometer sized light actuating structures. The method can help to answer key questions in the fields of microbiotics and microfluidics. Such as how to create a single micro-structure with complex movement.

The main advantage of this technique is that the molecular orientation can be manually patterned in the micro-scale. Thus, the specific form of actuation can be aperturely controlled. Some of these methods can provide inside microbiotics, but it also can be applied to different systems like tuning in photonic devices or materials responsive to different stimuli.

Demonstrating part of the procedure will be Daniele Martella a post doc from my laboratory. To begin clean a 3cm diameter glass cover slide with acetone and lens tissue. Then using forceps or tweezers place some glass micro-spheres on the slide about a half centimeter from the center point.

These function as spacers. Place a 1cm diameter microscope slide on the spacers and gently press it into position. Next add about two microliters of UV curing glue into the gap between the slides at the three different points where the spacers are located.

Before the glue penetrates to far into the gap, use UV light to cure the glue. This forms the cell for the experiment. Now along the edge of the cell, drop about 10 microliters of IPL resin.

Wait for a few minutes until the liquid has infiltrated into the entire area of the cell. Then using Fixo gum glue, fix the cell on the sample holder. Place the assembly into the Direct Laser Writing system and using a 100X objective, find the interface at the upper inner surface.

Next make a tilt correction, as the slide is not perfectly horizontal. Now write the structures of designed IPL grading patterns using 6 milliwatts of laser power at 60 microns per second. The grading patterns are made with curves or straight lines.

Then find the interface and tilt correct to the lower inner surface. Write the pattern there too. Now transfer the cell to an isopropanol bath for 12 to 24 hours.

Don't open the cell yet. The next day, dry the cell on a 50 degree Celsius hotplate for 10 to 20 minutes. Start this procedure by measuring out about 300mg of monomer mixture.

Load the mixture into a glass bottle and put it on a hotplate set between 70 and 80 degrees Celsius to melt the powder. Once melted stir it for an hour, between 90 and 150 RPM. Once the mixture is homogeneous, place the prepared cell on the hotplate at 60 degrees Celsius, and drop 20 microliters of mixture along the edge of the smaller glass slide.

Let the liquid move into the cell. Then shield the slide from light and transfer it to the microscope with a crossed polarizer and a temperature controller. Before illuminating the cell, attach an orange filter to the lamp to filter out the UV.Then using the heated stage, heat the cell to above 60 degrees Celsius.

Now decrease the temperature at a rate of two to 10 degrees Celsius per minute to measure the temperature range for liquid crystal phase. Observe the sample with image contrast invertion, while rotating it 45 degrees with respect to the polarizer axis. Eventually, a good homogeneous nematic liquid crystal phase should be noted.

Next fix the cell onto a sample holder for the DLW system. Load the sample and set the temperature to get to the liquid crystal phase. Then find the interface of the lower inner surface and perform the tilt correction using a 100X objective.

Then write the LCE structures with the laser power of four milliwatts and at 60 microns per second. If a 10X objective is in use, adjust the laser to 14 milliwatts and do not worry about finding the interface. In this case, the structure is fabricated to the whole thickness of the sample.

Then unload the cell and use a blade to remove the upper glass slide. With the upper slide removed, immerse the structures in a Toluene bath for five minutes and then air dry them for 10 minutes. To characterize the LCE microstructures place the sample in the homemade microscope and focus a laser on the structures using a 10X objective.

Now using a CMOS camera, observe the light induced deformation. Using the manual control of the micro-manipulation system, put the glass micro-tips that are on the manipulator arms close to the LCE microstructures. Now switch the laser on to low power, around 20 milliwatts to increase the temperature of the LCE, and thus soften the structure.

Then use the glass tips to pick up one LCE microstructure and hold it in the air. This process is needed to avoid adhesion from the glass surface. Next increase the laser power in excess of 100 milliwatts.

Then record the light induced deformation of the LCE structure for analysis. Four LCE cylindrical structures, 60 microns in diameter and 20 microns tall, were fabricated on four differently orientated IPL grading regions with a one micron period. Under light excitation, the dyes in the LCE absorbed energy and transferred it into the network.

The structures transitioned from nematic to isotropic phase, contracting along the original LC alignment director and expanding in the perpendicular direction. This technique enables the creation of compound actuators which contain more than one type of alignment in one single structure. For example, a 400 by 40 by 20 micron LCE stripe, was fabricated with two sections of alignment patterns which contained a 90 degree twist.

Under light illumination, the surface with parallel alignment contracted, while the one with perpendicular alignment expanded. Thus the structure is bent in two different directions. After watching this video, you should have a good understanding of how to align liquid crystalline elastomers using a Direct Laser Writing setup and how to make micro-actuators move using light.