Slippery and magnetically responsive micropillared surfaces for manipulation of droplets and beads

Abstract

Stimuli-responsive surfaces are of practical importance for applications ranging from enhanced mixing of reagents in lab-on-a-chip systems until probing cellular traction forces. Non-destructive reversible bending of cilia-inspired magnetic pillars can be used for controlled transportation of non-magnetic objects and bio-inspired sensing. Magnetic actuation of micropillars suspended in liquids allows controlled mixing, propelling, and stirring of fluids as well as droplet manipulation, which are important for various applications including generation of cell spheroids and droplet coalescence in microfluidic systems. In order to expand their practical applications, fabrication processes capable of rapid prototyping have to be developed. Inspired by biological cilia and their functionalities, actuating hairy surfaces are herein fabricated and implemented to manipulate both microbeads and droplets. The artificial cilia are based on microscale magnetic pillar arrays made of flexible polydimethylsiloxane functionalized with magnetic microparticles. The arrays are fabricated by a new method using patterned molds that relies on cryogenic separation to produce transparent cilia-inspired arrays without requiring manual interference to clean the templates during the process. Magnetic actuation of the pillar arrays is demonstrated in isopropanol and silicone oil. Filling with oil yields magnetically responsive slippery lubricated surfaces allowing directional motion of droplets by repetitive bending and recovery of the flexible magnetic pillars. The achieved structures allow manipulation of microbeads and droplets which is uncommon even at the sub-mm scale; directional motion is demonstrated for 250 μm–550 μm sized droplets. Droplet transportation is facilitated by extremely low hysteresis and a high degree of omnidirectional bending of the pillar array.

Digital Microfluidics─How Magnetically Driven Orientation of Pillars Influences Droplet Positioning

“…However, using a gradient of orientation of the pillars (Marangoni effect) with a low surface fraction or an asymmetric movement of the pillar could help to limit the use of vibration or tilted surfaces. Alternatively, the preparation of magnetic slippery surface could decrease drastically the contact angle hysteresis and help direct droplets on such surfaces …”

“…Alternatively, the preparation of magnetic slippery surface could decrease drastically the contact angle hysteresis and help direct droplets on such surfaces. 31 ■ CONCLUSIONS…”

Magneto-Responsive Actuating Surfaces with Controlled Wettability and Optical Transmittance

“…Furthermore, if the residual layer is not removed in this process, then the iron particles are aligned in a vertical shape because all the iron particles in PDMS are connected to each other by the magnetic flux. This alignment and high concentration of the iron particles cause an increment in Young’s modulus, and the actuating range of microstructures could decrease. , To avoid this issue, the iron particles could be moved to the end of microstructures by placing the master mold filling the PDMS/iron particle composite on the magnet after removing the residual layer. Thus, the middle of the microstructure was maintained as a pristine PDMS layer without the iron particle, and the sample with a large deflection could be achieved despite the low aspect ratio of the microstructures (Figure S1).…”

Microscopic artificial cilia – a review

“…[147][148][149][150][151][152][153] In recent years, increasingly experimental studies have been published exploring the particle manipulation capabilities of artificial cilia, where especially magnetically actuated artificial cilia have been used. 24,25,27,28,30,120,[144][145][146][154][155][156][157][158][159][160][161] The reported artificial cilia can not only remove particles, grains of sand, and microalgae 27,28 (see Fig. 11(a)) to create self-cleaning and anti-biofouling surfaces, but they can also transport particles (see Fig.…”