Branched networks by directed assembly of shape anisotropic magnetic particles.

The directed assembly of shape anisotropic magnetic particles into targeted macrostructures requires judicious particle design. We present a framework to understand the self-assembly of magnetic non-Brownian H-shaped particles and the formation of branched networks under an applied magnetic field. A finite element integration (FEI) method is developed to identify the preferred particle orientation (relative to the applied field) at different values of the geometric parameters defining H shapes, and used to construct a phase diagram to generalize the results. Theoretical predictions are validated by comparing with experiments performed using magnetic hydrogels synthesized using stop-flow lithography (SFL). We demonstrate the ability of H-shaped particles to form chains parallel to the field that can thicken in a direction orthogonal to the field, and in some cases with branching. The assembly of a suspension containing H-shaped particles, or rods, or a combination of both, is reported.

Tuning curvature in flow lithography: a new class of concave/convex particles.

Polymeric particles of complex shapes and chemistry have been used for a wide variety of applications in the materials and bioengineering fields. An interesting means of introducing complexity is through curvature. In this work, stop-flow lithography is used to generate concave/convex particles at high throughputs of 3x10(4) particles/h. These particles have finely tuned curvature in the plane orthogonal to the plane of projection of light. The shape in the plane of projection of light is determined by the mask shape. The chemical programmability of this technique is further demonstrated by creating multifunctional particles, i.e., patchy and capped particles. The directed assembly of these particles can find potential application in a variety of fields like biology, photonics, and liquid crystals.

Stop-flow lithography to generate cell-laden microgel particles.

Encapsulating cells within hydrogels is important for generating three-dimensional (3D) tissue constructs for drug delivery and tissue engineering. This paper describes, for the first time, the fabrication of large numbers of cell-laden microgel particles using a continuous microfluidic process called stop-flow lithography (SFL). Prepolymer solution containing cells was flowed through a microfluidic device and arrays of individual particles were repeatedly defined using pulses of UV light through a transparency mask. Unlike photolithography, SFL can be used to synthesize microgel particles continuously while maintaining control over particle size, shape and anisotropy. Therefore, SFL may become a useful tool for generating cell-laden microgels for various biomedical applications.

Charge transfer absorption for pi-conjugated polymers and oligomers mixed with electron acceptors.

Pi-conjugated polymers and oligomers show charge transfer (CT) absorption bands when mixed with electron acceptors in chloroform solution. This is attributed to the formation of (ground state) donor-acceptor complexes in solution. By varying the concentration of the donor and acceptor, the extinction coefficient for the CT absorption and the association constant of donor and acceptor are estimated. The spectral position of the CT bands correlates with the electrochemical oxidation potential of the pi-conjugated donor and the reduction potential of the acceptor.