Tapomoy Bhattacharjee

Writing in the granular gel medium

The reversible fluid-solid transition in granular gels enables the three-dimensional writing of soft, delicate, macroscopic structures with microscopic detail. Gels made from soft microscale particles smoothly transition between the fluid and solid states, making them an ideal medium in which to create macroscopic structures with microscopic precision. While tracing out spatial paths with an injection tip, the granular gel fluidizes at the point of injection and then rapidly solidifies, trapping injected material in place. This physical approach to creating three-dimensional (3D) structures negates the effects of surface tension, gravity, and particle diffusion, allowing a limitless breadth of materials to be written. With this method, we used silicones, hydrogels, colloids, and living cells to create complex large aspect ratio 3D objects, thin closed shells, and hierarchically branched tubular networks. We crosslinked polymeric materials and removed them from the granular gel, whereas uncrosslinked particulate systems were left supported within the medium for long times. This approach can be immediately used in diverse areas, contributing to tissue engineering, flexible electronics, particle engineering, smart materials, and encapsulation technologies.

Self-assembled micro-organogels for 3D printing silicone structures

High-precision 3D printing of liquid silicone is achieved using a new oil-based microgel as a support medium. The widespread prevalence of commercial products made from microgels illustrates the immense practical value of harnessing the jamming transition; there are countless ways to use soft, solid materials that fluidize and become solid again with small variations in applied stress. The traditional routes of microgel synthesis produce materials that predominantly swell in aqueous solvents or, less often, in aggressive organic solvents, constraining ways that these exceptionally useful materials can be used. For example, aqueous microgels have been used as the foundation of three-dimensional (3D) bioprinting applications, yet the incompatibility of available microgels with nonpolar liquids, such as oils, limits their use in 3D printing with oil-based materials, such as silicone. We present a method to make micro-organogels swollen in mineral oil, using block copolymer self-assembly. The rheological properties of this micro-organogel material can be tuned, leveraging the jamming transition to facilitate its use in 3D printing of silicone structures. We find that the minimum printed feature size can be controlled by the yield stress of the micro-organogel medium, enabling the fabrication of numerous complex silicone structures, including branched perfusable networks and functional fluid pumps.

Elastic modulus and hydraulic permeability of MDCK monolayers

The critical role of cell mechanics in tissue health has led to the development of many in vitro methods that measure the elasticity of the cytoskeleton and whole cells, yet the connection between these local cell properties and bulk measurements of tissue mechanics remains unclear. To help bridge this gap, we have developed a monolayer indentation technique for measuring multi-cellular mechanics in vitro. Here, we measure the elasticity of cell monolayers and uncover the role of fluid permeability in these multi-cellular systems, finding that the resistance of fluid transport through cells controls their force-response at long times.

Lubricity from Entangled Polymer Networks on Hydrogels

Structural hydrogel materials are being considered and investigated for a wide variety of biotribological applications. Unfortunately, most of the mechanical strength and rigidity of these materials comes from high polymer concentrations and correspondingly low polymer mesh size, which results in high friction coefficients in aqueous environments. Recent measurements have revealed that soft, flexible, and large mesh size hydrogels can provide ultra low friction, but this comes at the expense of mechanical strength. In this paper, we have prepared a low friction structural hydrogel sample of polyhydroxyethyl-methacrylate (pHEMA) by polymerizing an entangled polymer network on the surface through a solution polymerization route. The entangled polymer network was made entirely from uncrosslinked polyacrylamide (pAAm) that was polymerized from an aqueous solution and had integral entanglement with the pHEMA surface. Measurements revealed that these entangled polymer networks could extend up to similar to 200 mu m from the surface, and these entangled polymer networks can provide reductions in friction coefficient of almost two orders of magnitude (mu > 0.7 to mu < 0.01).

Eliminating the surface location from soft matter contact mechanics measurements

Abstract The material properties of soft materials can be measured with rheometers and tensile testing instruments whenever there exist few limitations on sample volume, fixturing and general sample preparation, where samples often need to be prepared specifically to work with the hardware of a given instrument. By contrast, indentation methods are well suited for measuring material properties when sample preparation and geometry are highly constrained, as is the case with living cells, confluent cell layers, tissue samples, hydrogel coatings or soft objects with defined shapes like contact lenses. For example, indentation can be performed directly on cells grown in a Petri dish, without modifying typical cell culture protocols or materials. However, the low elastic modulus of these soft materials make it extremely difficult to determine when an indentation instrument first makes contact with a sample, which is critically important to know if material properties are to be determined with confidence. Here, we present an analysis method that eliminates the need to identify when an instrument makes contact with a sample. The method recasts the traditional force–displacement models of contact mechanics in terms of the first derivative of applied normal force with respect to indenter position, which automatically removes the unknown point of contact. This approach enables the selection of appropriate theoretical models for a given data-set and allows the measurement of sample material properties with the only fitting parameter being the elastic modulus.