This protocol synthesizes precisely patterned static and reconfigurable particles of different shapes and sizes. First, design masks for the two dimensional net that will eventually self-assemble to the desired patterned particles. Then deposit the sacrificial and conducting layers onto a flat substrate.
Fabricate the 2D precursors by patterning panels and hinges using photolithography thin film deposition, and etching then dissolve the sacrificial layer to release the 2D precursors from the substrate. Ultimately, the released 2D precursors are exposed to specific stimuli to trigger folding of the particles ranging from the micrometer to the centimeter scale. The implications of this technique extend towards the creation of miniaturized smart particles, including those that can do biopsies in hard to reach places in the body.
Further, the method can be used with metals, semiconductors, and even polymers so that we can create capsules that could be important in drug delivery, especially since we can create them in nons, spherical and multifunctional geometries. The main advantage of this technique over existing methods like emotion, polymerization, or molding of particles, is that this method transform the accuracy and precision of plenary lithography into three dimensional patterning of particles with different shapes and sizes. Visual demonstration of this method is critical as the patterning and self folding steps are difficult to learn because they require both lithography and self-assembly skills.
In this demonstration, we will fabricate static, permanently sealed 300 micron sized Dora, as well as reconfigurable thermo sensitive micro grippers. Start with a two dimensional vector graphic software program. First, determine the number of panels in the polyhedra.
Proceed to figure out the high yielding two dimensional arrangement of panels, also called nets. Nets that have the lowest red sub generation and the greatest number of secondary vertex connections will typically assemble with the highest tails For the panel mask, draw the panels of the polyhedra as nets space, the adjacent panels by a gap width. Then insert registry mark for subsequent alignment with the hinge mask.
Now for the hinge mask, define the folding hinges in between the panels. Also define the ceiling hinges at the edges of the panels. Then verify that the panel and hinge masks overlay with registry Ceiling hinges provide considerable error tolerance during cell folding.
Now, print the masks on transparency films using a high resolution printer. Begin the substrate preparation with a flat substrate. Clean the silicon wafers with methanol, acetone, and isopropyl alcohol.
Then dry them with nitrogen gas and heat at 150 degrees Celsius for five to 10 minutes. On the silicon wafers spin coat, a 5.5 micron thick layer of 950 P-M-M-A-A 11 at 1, 100 RPM. After three minutes, bake the substrate at 180 degrees Celsius for 60 seconds.
Using a thermal evaporator deposit, an adhesion promoter of 30 nanometers of chromium and the conducting layer of 150 nanometers of copper. Then add a spin coat of approximately 10 microns thick SPR 220 at 1, 700 RPM set aside for three minutes. Now expose the wafers to the panel mask using approximately 460 millijoules per squared centimeter of UV light and a mercury based mask aligner.
Develop in MF 26 developer for two minutes, replenish the developer solution and develop for another two minutes. Proceed to calculate the total panel area and compute the current required to electrode deposit nickel from a commercial nickel sulfate solution. Then dip the wafer in the nickel electroplating solution.
Repeat the spin coating and alignment steps for the hinge mask. Now dice the wafer into small pieces that contain 50 to 60 nets. Coat the edges of the pieces with nail polish.
Next, calculate the total exposed hinge area and use it to compute the current required to electrode deposit lead tin solder from a commercial solder plating solution. Then dip the wafer into the solder plating solution. Dissolve the photo resist in acetone.
Rinse the wafer pieces with IPA and dry with nitrogen gas immerse wafer piece in etching a PS 100 for 25 to 40 seconds to dissolve the surrounding copper layer. Rinse with distilled water and dry with nitrogen gas. Now immerse the wafer piece in etching CRE 473 for 30 to 50 seconds to dissolve the surrounding chromium layer.
Then rinse with distilled water and dry with nitrogen gas. Finally, immerse the wafer piece in two to three milliliters of NMP and heat at 100 degrees Celsius for three to five minutes until the templates are released from the substrate. Transfer 10 to 20 templates into a small Petri dish and distribute them uniformly.
Then add approximately three to five milliliters of NMP and five to seven drops of in Delo five RMA liquid flux heat at 100 degrees Celsius for five minutes. The flux cleans and dissolves any oxide layer formed on the saer and thereby ensures good solder reflow on heating. Increase the hot plate temperature to 150 degrees Celsius for five minutes.
Slowly increase to 200 degrees Celsius until folding occurs after the dish has cooled, rinse the dodecahedral twice in acetone and once in ethanol store the dodecahedral particles in ethanol for reconfigurable structures with metallic sacrificial layer, immerse the wafer piece in a PS 100 to etch the underlying copper sacrificial layer. Wait until the micro grippers are completely released from the substrate. Rinse the released micro grippers with deionized water and keep them in cold water.
Then trigger the folding by placing the micro grippers in 37 degrees Celsius water. The general protocol described in the manuscript can be used to fabricate pattern sealed particles and reconfigurable grasping devices. Also included our specific visualized examples for both fabrication of sealed toral particles and reconfigurable micro grips.
As a general rule, at least two mask sets are needed, one for rigid panels that do not bend or curve, and the other for hinge regions that bend curve or seal. This pattern uses the mask design rules for a self folding micro gripper. AutoCAD program is used to design the masks of the 2D precursors.
The 2D precursors are then fabricated a silicon substrate as seen in these optical images of dedra and micro grippers. This realization shows self-assembly of a Dedra particle as prepared in the protocol detailed. In this article here, the micro grippers are induced by heat to self fold at 37 degrees Celsius.
Here the images depict assembly of a thin film, stress driven folding of a micro gripper around a bead self-assembled polyhedral particles can be created in a variety of shapes as well as folding micro grippers. This conceptual animation by David Fallac shows surface tension driven assembly of a cubic particle. First, the silicon wafer and photo mask are aligned precisely.
Then the wafers are exposed to the panel mask using approximately 460 millijoules per squared centimeter of UV light and a mercury based mask aligner to induce folding of the cube. The temperature is raised to the melting point of the hinge material, causing the liquid hinges to ball up to minimize exposed surface area. The edges fuse to minimize their surface energy and seal the particles, thus forming a complete polyhedral particle.
The fabrication and actuation process is highly parallel, and 3D structures can be fabricated and triggered simultaneously. Additionally, precise patterns as exemplified by square or triangular pores can be defined all three dimensions and on selected faces if needed. Self folding micro grippers can be closed under biologically benign conditions so that they can be used to excise tissue or they can be loaded with biological cargo.
This example shows bladder tissue extraction using thermo sensitive micro grippers. Additionally, since the micro grippers can be made with nickel a ferromagnetic material, they can be moved from afar using magnetic fields. After watching this video, you should have a good understanding of how to utilize origami inspired approaches to synthesize precisely patterned and reconfigurable particles in a variety of sizes, shapes, surface patterns, and stimulate responsive reconfigurability.
Remember to follow the governing design rules for the self-assembly of the particles and to understand the working principle of the process. It is very important to choose the right materials for the sacrificial layer panels and hinges. For example, you should not choose a sacrificial layer requiring high dissolution temperature if your trigger layer dissolves at these high temperatures After its development.
This technique paved the way for researchers in the fields of micro and nanotechnology to explore the development and applications of precisely patterned polyhedra and reconfigurable particles in electronics, optics, and medicine.
