​​Gunjan Agarwal​

Affiliation: Massachusetts Institute of Technology, Micropower and Nanoengineering Group

Funding Agency: National Science Foundation (NSF), Air Force Office of Scientific Research (AFOSR), Pappalardo Graduate Fellowship

Collaborators: Laboratory for Multiscale Regenerative Technologies (MIT), The Doyle Group (MIT)

Bottom-up fabrication techniques such as self-assembly are a very effective means of integrating small-scale building blocks into complex, more valuable architectures, which are difficult to create using conventional, top-down manufacturing methods. This research organizes small-scale components into shape- and size-matched sites on a template’s surface using a novel type of selective assembly: templated assembly by selective removal (TASR). The TASR approach utilizes surface interactions between the components and substrate to promote adhesion and thereby the assembly of the components on the substrate surface. The magnitude of these surface interactions is at its peak value when the shapes and sizes of the components match the shapes and sizes of the assembly sites, and the contact area between interacting surfaces is maximized. The incorporation of fluid forces created by externally-input, high frequency (MHz-range) acoustic excitations makes this technique highly selective in component placement, by dislodging components from any poorly-matched wells in which they may have assembled. Quasi-random assembly and deterministic selective removal proceed until the system is fully assembled. Selectively self-assembled systems are important for a wide range of applications, ranging from metamaterials, MEMS and microelectronics, optically-based chemical sensors, bio-medical diagnostics, to shape- and size-based chromatography.

      Successful assembly of silica microspheres using TASR on a silicon template carried out previously [1-2] established that the technique works effectively for assembly of hard materials on a rigid substrate. However, the situation for the assembly of soft materials is more challenging due to the larger deformation experienced in these systems, which in turn influences the degree of shape matching between the component and the template. This work presents the first experimental demonstration of successful, selective assembly of deformable microsystens using TASR. The TASR technique is demonstrated to achieve highly selective organization of deformable, inorganic micro-components (such as polymer colloids, hydrogels) [3] and biological cells [4] onto patterned rigid substrates, as well as with more economical, replicated polymer templates [5]. An extensive range of material combinations at diverse length scales have been successfully assembled using the TASR process, and the assembly of both isotropic (spherical) and anisotropic (cylindrical) components into corresponding patterned assembly sites has been achieved. Since the TASR system is particularly well matched to the length scales for typical optical and biological systems, the successful selective assembly of deformable microsystems using TASR has opened up a host of applications not previously possible using this technique.

        A quantitative mechanical model is also formulated for predicting the degree of success in self-assembly of deformable materials at small scales. Self-assembly depends on the tendency of systems to minimize their free energy. The introduction of either deformable substrate templates or deformable components introduces a new set of considerations, since deformation can change the degree of shape matching between component and substrate, and hence, may change the assembly yield depending on how well the undeformed component matches the undeformed assembly site. Here, an energy-based theory is introduced to model the self-assembly process that takes into account the mechanical behavior of the components and substrate. If the deformations of components and substrate are fully elastic upon interaction, any reduction in system free energy due to the increase in contact area upon deformation will equal the increase in system free energy due to the storage of elastic energy in the deformed structures. Therefore, for purely elastic deformations, the self-assembly is predicted to be successful. In contrast, if the deformation enters the plastic regime, some of the energy will be dissipated, and the assembly process may be marginally successful at best. In that case, the probability increases that deformable components will remain assembled in poorly matched sites from which identically shaped rigid components would have been removed by the selective removal process.
         

References:

1. S. Jung and C. Livermore, Achieving Selective Assembly with Template Topography and Ultrasonically Induced Fluid Forces, Nanoletters, 2005, 5(11), 2188-94.

2. F. Eid, S. Jung, C. Livermore, Templated assembly by selective removal: simultaneous, selective assembly and model verification, Nanotechnology, 2008, 19, 285602.

3. G. Agarwal, A. Servi, F. Eid, and C. Livermore, Selective Self-Assembly of Polymer Structures using Templated Assembly by Selective Removal, IEEE Transactions on Nanotechnology, 2011, 10(3), 617-625.

4. G. Agarwal, and C. Livermore, Chip-Based Size-Selective Sorting of Biological Cells using High Frequency Acoustic Excitation, Lab on a Chip, 2011, 11(13), 2204-2211.

5. G. Agarwal, and C. Livermore, Size-Selective, Biocompatible, Manufacturable Platform for Structuring Deformable Microsystems, Lab on a Chip, 2014, 14(17), 3385-3393.