Abstract
We here develop a three-dimensional DMF (3D DMF) platform with patterned electrodes submerged in an oil medium to provide fundamental solutions to the technical limitations of 2D DMF platforms and water–air systems. 3D droplet manipulation on patterned electrodes is demonstrated by programmably controlling electrical signals. We also demonstrate the formation of precipitates on the 3D DMF platform through the reaction of different chemical samples. A droplet containing precipitates, hanging on the top electrode, can be manipulated without adhesion of precipitates to the solid surface. This method could be a good alternative strategy to alleviate the existing problems of 2D DMF systems such as cross-contamination and solute adsorption. In addition, we ascertain the feasibility of temperature-controlled chemical reaction on the 3D DMF platform by introducing a simple heating process. To demonstrate applicability of the 3D DMF system to 3D biological process, we examine the 3D manipulation of droplets containing mouse fibroblasts in the 3D DMF platform. Finally, we show detachment of droplets wrapped by a flexible thin film by adopting the electro-elasto-capillarity (EEC). The employment of the EEC may offer a strong potential in the development of 3D DMF platforms for drug encapsulation and actuation of microelectromechanical devices.
Highlights
Methods have technical limitations because of the direct contact of the moving droplets with the solid surfaces, even though the droplets can be manipulated on 3D structures
The pulse width (Tp) of the applied square pulse signal is determined based on the results of a preliminary experiment on droplet spreading with varying DC voltages
To demonstrate the ability of droplet manipulation on the 3D Digital microfluidics (DMF) platform at high temperatures, we examine the detachment of 4 μ L droplet containing thermochromic ink (4% v/v), which changes color according to temperature at different temperatures that range from 23 °C to 60 °C (Fig. 5a)
Summary
Methods have technical limitations because of the direct contact of the moving droplets with the solid surfaces, even though the droplets can be manipulated on 3D structures. All the aforementioned droplet lift-off methods were performed in an air medium[24,25,26,27,28,29] They have several weak points, such as the requirement for high driving voltage and the difficulty in droplet manipulation at a high temperature because of droplet evaporation. The ambient oil prevents droplet evaporation, which allows the droplet to be manipulated at a high temperature[1,32] It likewise inhibits surface contamination caused by the adsorption of biomolecules[33,34]. These circumstances spontaneously require the development of a new droplet detachment method in immiscible fluids using EW actuations. The iodine–starch reaction process is examined as a proof-of-concept of the temperature-controlled chemical reaction on the 3D DMF platform
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