Jatinder S. Randhawa

Biopsy with Thermally‐Responsive Untethered Microtools

Thermally activated, untethered microgrippers can reach narrow conduits in the body and be used to excise tissue for diagnostic analyses. As depicted in the figure, the feasibility of an in vivo biopsy of the porcine bile duct using untethered microgrippers is demonstrated.

Pick-and-Place Using Chemically Actuated Microgrippers

In this communication, we demonstrate the concept of single-use, chemically triggered, reversible tools in the form of mobile grippers that can be used to manipulate micro-objects. Both the closing and opening of the mobile grippers are triggered by chemicals, namely acetic acid (CH(3)COOH) and hydrogen peroxide (H(2)O(2)), respectively. The grippers close and open en masse based on chemically triggered, mechanical property changes within trilayer joints patterned within the gripper, and no external power is needed for operation. We describe the actuation of the gripper using a multilayer thin film model and demonstrate the utility of the gripper by picking-and-placing 200 microm diameter tubes and beads. Our pick-and-place microgripper is a first step toward the development of functional Micro Chemo-Mechanical Systems (MCMS), which are actuated by chemistry as opposed to electricity [as in Micro Electro-Mechanical Systems (MEMS)].

Voltage-gated ion transport through semiconducting conical nanopores formed by metal nanoparticle-assisted plasma etching.

Nanopores with conical geometries have been found to rectify ionic current in electrolytes. While nanopores in semiconducting membranes are known to modulate ionic transport through gated modification of pore surface charge, the fabrication of conical nanopores in silicon (Si) has proven challenging. Here, we report the discovery that gold (Au) nanoparticle (NP)-assisted plasma etching results in the formation of conical etch profiles in Si. These conical profiles result due to enhanced Si etch rates in the vicinity of the Au NPs. We show that this process provides a convenient and versatile means to fabricate conical nanopores in Si membranes and crystals with variable pore-diameters and cone-angles. We investigated ionic transport through these pores and observed that rectification ratios could be enhanced by a factor of over 100 by voltage gating alone, and that these pores could function as ionic switches with high on-off ratios of approximately 260. Further, we demonstrate voltage gated control over protein transport, which is of importance in lab-on-a-chip devices and biomolecular separations.

Reversible Actuation of Microstructures by Surface‐Chemical Modification of Thin‐Film Bilayers

Adv. Mater. 2010, 22, 407–41

Three-Dimensional Chemical Patterns for Cellular Self-Organization†

In nature, three dimensional (3D) chemical patterns are generated and sustained with precisely controlled spatial and temporal profiles, on a variety of length and time scales.[1,2] Several studies have outlined the need for the development of in vitro methodologies that replicate the 3D spatio-temporal chemical patterns associated with chemotaxis, cell signaling, angiogenesis, homeostasis and immune surveillance.[3–7] There are a number of in vitro microfluidic systems that have been developed to mimic in vivo chemical micro-environments such as the creation of interleukin-8 gradients to study neutrophil chemotaxis.[8] However, microfluidic systems are inherently planar (2D) and their overall size and dependency on external equipment to enable active flow restricts their applicability.[9–11] Hence, the development of passive systems that enable diffusion-based 3D chemical pattern formation is attractive since they can be readily utilized to generate and sustain patterns within cell culture, homogeneous gels and other stationary media. Existing microparticles and reservoirs[12] can be utilized to create chemical patterns in 3D environments, however, the pre-dominant spatial release profile is one that is spherically symmetric[13] (Figure 1a).

Importance of surface patterns for defect mitigation in three-dimensional self-assembly.

This article investigates the three-dimensional self-assembly of submillimeter scale polyhedra using surface forces. Using a combination of energy landscape calculations and experiments, we investigate the influence of patterns of hydrophobic surfaces on generating defect-free, closed-packed aggregates of polyhedra, with a focus on cubic units. Calculations show that surface patterning strongly affects the interaction between individual units as well as that of the unit with the growing assembly. As expected, an increase in the hydrophobic surface area on each face results in larger global minima. However, it is the distribution of hydrophobic surface area on each cubic face that is strongly correlated to the energetic parameters driving low-defect assembly. For patterns with the same overall area, minimizing the radius of gyration and maximizing the angular distribution leads to steep energy curves, with a lower propensity for entrapment in metastable states. Experimentally, 200-500 microm sized metallic polyhedra were fabricated using a self-folding process, and the exposed surfaces were coated with a hydrophobic polymer. Cubes with surface patterns were agitated to cause aggregative self-assembly. Experimental results were consistent with energy calculations and suggest that geometric patterns with large overall areas, low radii of gyration, and high angular distributions result in efficient and low-defect assembly.

Three-dimensional surface current loops in terahertz responsive microarrays

We investigated the effect of the third dimension in terahertz (THz) responsive microarrays using finite element simulations and by measuring their transmission spectra in the range of 0.2 to 1.1 THz. We observed that the presence of three-dimensional (3D) surface current loops in microarrays composed of cubic units resulted in enhanced transmission as compared to two-dimensional single and stacked layers, suggesting that the incorporation of geometries that enable truly 3D current loops is an important factor in optically responsive metamaterials.

Concentric ring pattern formation in heated chromium-gold thin films on silicon

Patterns that formed spontaneously upon heating thermally evaporated thin films of gold (Au) and chromium (Cr) on silicon (Si) substrates to 500°C were investigated. A variety of patterns were observed. The type of pattern formed correlated with the thickness ratio of Cr to Au. The most striking pattern consisted of concentric rings spaced approximately 10–30μm apart. Each ring was composed of pits 2–5μm in length, which extended 200–400nm into the silicon substrate. While concentric patterns are widely seen in chemical reaction-diffusion systems, pattern formation in the heated Au∕Cr∕Si system has not been previously observed.