Computer-generated hologram tailored for dielectrophoretic PDMS patterning

Dielectrophoretic clustering is obtained both for liquid and solid matter thanks to light shaping performed by phase only Spatial Light Modulator (SLM). We present a procedure able to perform two functions: design polymeric stable structures usable as microfluidic channels and trapping micro objects. These two tasks are combined to realize a single device. The liquid matter is Polydimethylsiloxane (PDMS) and its patterning in microstructures is developed by means of photorefractive effect in a functionalized substrate. X-cut Iron-doped Lithium Niobate (LN) crystal is used as substrate while a thin film of PDMS is spin on it. When LN, covered by PDMS, is exposed to structured laser light, a space charge field arise that is able to induce self-patterning of the PDMS liquid film. The rearrangement of PDMS is due to the dielectrophoretic effect. Light structuring is achieved by a SLM positioned in the conjugated plane of the LN crystals. PDMS devices we realized are microfluidic channels. The first step of our procedure is the computing of a suitable Computer Generated Hologram (CGH) to be displayed by the SLM. An ideal target is designed and given as input to an Iterative Fourier Transform Algorithm (IFTA) to calculate the CGH. The IFTA used has been implemented for this particular application and it's tailored to generate a continuous light intensity profile in the LN plane. Then PDMS microstructures are cured to induce solidification. Such PDMS channels are then used to trap particles floating inside. Trapping is realized exploiting again dielectrophoresis induced by photorefractive effect. LN with PDMS channel is exposed to laser light which present, now, a periodic two-dimensional intensity profile. The charge distribution due to this second exposure is able to trap particle in the previously built channels. We realize a device with high degree of flexibility avoiding the need of moulds fabrication.

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