Negative dielectrophoretic patterning with colloidal particles and encapsulation into a hydrogel

Microparticle patterns have been fabricated on a nonconductive glass substrate and a conductive indium tin oxide (ITO) substrate using negative dielectrophoresis (n-DEP). The patterned microparticles on the substrate were immobilized by covalent bonding or embedded into polymer sheets or strings. The patterning device consisted of an ITO interdigitated microband array (IDA) electrode as the template, a glass or ITO substrate, and a polyester film (10-microm thickness) as the spacer. A suspension of 2-microm-diameter polystyrene particles was introduced into the device between the upper IDA and the bottom glass or ITO support. An ac electrical signal (typically 20 Vpp, 3 MHz) was then applied to the IDA, resulting in the formation of line patterns with low electric field gradient regions on the bottom support. When the glass substrate was used as the bottom support, the particles aligned under the microband electrodes of the IDA within 5 s because the aligned areas on the support were regions with the weakest electric field; however, for the ITO support, the particles were directed to the regions under the electrode gap and aligned on the support because these regions had the weakest electric field. The width of the particle lines could be roughly controlled by regulating the initial concentration of the suspended particles. The particles forming the line and grid patterns with single-particle widths were immobilized by using a cross-linking reaction between the amino groups on the aligned particles and N-hydroxysuccinimide-activated ester on the glass substrate activated by succinimidyl 4-(p-maleimidophenyl)-butyrate (SMPB). The patterned particles were also embedded in a photoreactive hydrogel polymer. A prepolymer solution of poly(ethylene glycol) diacrylate (PEG-DA) was used as the suspension medium to maintain the particle patterns in the polymerized hydrogel sheet and string following photopolymerization. The hydrogel sheets with particle patterns were fabricated by ultraviolet (UV) irradiation through the ITO-IDA template for 120 s. Hydrogel strings with the aligned particles were fabricated by using a conductive ITO support and a Pt-IDA template. Pt-IDA was used as a template as well as a photomask to block UV transmission. The present procedure affords extremely simple, rapid, and highly reproducible fabrication of particle arrays. The reusability of the template IDA electrode is also a substantial advantage over previous methods.