Published: February 4th, 2013
We describe experimental details of the synthesis of patterned and reconfigurable particles from two dimensional (2D) precursors. This methodology can be used to create particles in a variety of shapes including polyhedra and grasping devices at length scales ranging from the micro to centimeter scale.
There are numerous techniques such as photolithography, electron-beam lithography and soft-lithography that can be used to precisely pattern two dimensional (2D) structures. These technologies are mature, offer high precision and many of them can be implemented in a high-throughput manner. We leverage the advantages of planar lithography and combine them with self-folding methods1-20 wherein physical forces derived from surface tension or residual stress, are used to curve or fold planar structures into three dimensional (3D) structures. In doing so, we make it possible to mass produce precisely patterned static and reconfigurable particles that are challenging to synthesize.
In this paper, we detail visualized experimental protocols to create patterned particles, notably, (a) permanently bonded, hollow, polyhedra that self-assemble and self-seal due to the minimization of surface energy of liquefied hinges21-23 and (b) grippers that self-fold due to residual stress powered hinges24,25. The specific protocol described can be used to create particles with overall sizes ranging from the micrometer to the centimeter length scales. Further, arbitrary patterns can be defined on the surfaces of the particles of importance in colloidal science, electronics, optics and medicine. More generally, the concept of self-assembling mechanically rigid particles with self-sealing hinges is applicable, with some process modifications, to the creation of particles at even smaller, 100 nm length scales22, 26 and with a range of materials including metals21, semiconductors9 and polymers27. With respect to residual stress powered actuation of reconfigurable grasping devices, our specific protocol utilizes chromium hinges of relevance to devices with sizes ranging from 100 μm to 2.5 mm. However, more generally, the concept of such tether-free residual stress powered actuation can be used with alternate high-stress materials such as heteroepitaxially deposited semiconductor films5,7 to possibly create even smaller nanoscale grasping devices.
We first describe a general protocol that can be used to fabricate patterned, sealed particles and reconfigurable grasping devices. Along with the general protocol, we provide one specific, visualized example for both the fabrication of sealed dodecahedral particles and reconfigurable microgrippers.
1. Mask Preparation and Design Rules
2. Substrate Preparation
3. Deposition of the Sacrificial Layer
In order to release the templates from the substrate after patterning, a sacrificial layer is required. A variety of films composed of either metals (e.g., copper), dielectrics (e.g., alumina) or polymers (e.g., PMMA, PVA, CYTOP etc.) can be utilized. When choosing a sacrificial film, important considerations are the ease of deposition and dissolution of the material and the etch selectivity.
4. Patterning the Panels
5. Patterning the Hinges
Similar to the patterning of the panels, in order to pattern hinges, a second round of photolithography needs to be done using the hinge mask (Figure 2b-c). The registry marks on the panel and hinge masks need to be overlaid to ensure proper alignment.
6. Releasing the Templates from the Substrate and Folding
Example 1. Protocol for the fabrication of surface tension driven self-assembled, permanently bonded, 300 μm size hollow dodecahedra (schematic representation in Figure 3):
Example 2. Protocol for fabrication of reconfigurable, thin film stress driven self-folding thermo-sensitive microgrippers (schematic representation in Figure 4):
7. Representative Results
Representative results in Figure 5 show self-assembled polyhedral particles in a variety of shapes as well as folding microgrippers. 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 in all three dimensions, and on selected faces if needed. The microgrippers can be closed under biologically benign conditions so that they can be used to excise tissue or loaded with biological cargo. Additionally, since the microgrippers can be made with a ferromagnetic material, they can be moved from afar using magnetic fields.
Figure 1. Design rules for the synthesis of patterned particles. (a-c) Mask design rules for the assembly of patterned polyhedral particles; (a) Schematic of the panel mask for a polyhedron of side length L, (b) schematic of the hinge mask featuring folding (0.2 L x 0.8 L) and locking or sealing (0.1 L x 0.8 L) hinges, and (c) schematic of the overlaid 2D precursor or net. (d-f) Mask design rules for the self-folding microgripper; (d) schematic of the hinge mask for a microgripper with tip to tip length D, (e) schematic of the panel mask with hinge gap g, and (f) schematic of the overlaid 2D precursor. Click here to view larger figure.
Figure 2. Experimental images and conceptual animations of important steps in the fabrication and assembly process. (a) Screenshot of an AutoCAD panel mask for dodecahedral precursors. (b-c) Optical images of 2D precursors for, (b) dodecahedra, and (c) microgrippers on a silicon substrate. (d) Released dodecahedral nets. Scale bars: 200 μm. (e-n) Conceptual animation of, (e-i) the surface tension driven assembly of a dodecahedron, and (j-n) thin film stress driven folding of a microgripper around a bead (Animation by David Filipiak).
Figure 3. Schematic illustration of the important fabrication steps for the surface tension driven assembly of a cubic particle.
Figure 4. Schematic illustration of the important fabrication steps for the residual stress driven folding of a six-digit grasping device.
Figure 5. Images of origami inspired self-assembled patterned and reconfigurable particles. (a) Optical image of self-assembled particles in a variety of shapes. (b-e) SEM images of a (b) self-assembled porous cube, (c) pyramid, (d) truncated octahedron and (e) dodecahedron. Scale bars: 100 μm. (f-h) Optical snapshots of self-folding microgrippers, and (i) SEM image of a folded microgripper (Image by Timothy Leong). Scale bars: 200 μm.
Our origami-inspired assembly process is versatile and can be used for synthesizing a variety of 3D static and reconfigurable particles with a wide range of materials, shapes and sizes. Further, the ability to precisely pattern sensors and electronic modules on these particles is important for optics and electronics. In contrast to patchy particles formed by alternate methods, where patterns are relatively imprecise, this methodology provides a means to synthesize precisely patterned particles. In surface tension based assembly, the use of liquefying sealing hinges ensures that the particles are well sealed and mechanically rigid after assembly (on cooling). Previously, we have observed that the seams are leak-proof even for small molecules39,40. Electrodeposition of a thin layer of Au after assembly can provide additional strength and enhance the leak proof nature of the seams. The thin film stress based folding is useful for applications in which stimuli responsive folding is required such as in microgrippers that have been used to perform in vitro and in vivo biological sampling and in pick-and-place operations in robotics. While the specific methodology described here can be used to create reconfigurable microgrippers that only close once, the appropriate choice of materials and methods to manipulate stress in bilayers can be utilized to also create grasping devices that can be reconfigured over multiple-cycles37, 41. The highlight of the use of residual stress to power these devices is that they do not require any tethers or wires and so have excellent maneuverability to enable actuation in hard to reach places. Further, by an appropriate choice of polymeric triggers, stimuli responsive behavior can be enabled with a range of stimuli including enzymes42 to enable autonomous function of relevance to robotics and surgery.
No conflicts of interest declared.
We acknowledge funding from the NSF through grants CMMI 0854881 and CBET 1066898. The authors thank Matthew Mullens for helpful suggestions.
|Name of the reagent
|950 Poly methyl methacrylate A11
|Chromium-plated tungsten rods
|R. D. Mathis Company
|Evaporation source for Cr
|Evaporation source for Cu
|Evaporation source for Au
|SPR 220 7.0
|Rohm and Haas
|S 1800 series photoresists
|Rohm and Hass
|Megaposit MF- 26 A developer
|Rohm and Haas
|Developer for SPR 220 7.0 photoresist
|Microposit 351 developer
|Rohm and Hass
|Developer for S 1800 series photoresists
|Plating solution for Ni
|Techni Solder Mate NF 820 60/40 RTU
|Plating solution for Pb-Sn hinges
|APS 100 Copper etchant
|Transene Company Inc.
|CRE 473 Chromium etchant
|Transene Company Inc.
|High boiling point organic solvent for Pb-Sn hinge based self-folding
|Indalloy 5RMA flux
|Indium Corporation of America
|Chemical that cleans the solder surface and inhibits oxidation for good Pb-Sn reflow
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