Microfluidic manipulation by spiral hollow-fibre actuators

Abstract

A microfluidic manipulation system that can sense a liquid and control its flow is highly desirable. However, conventional sensors and motors have difficulty fitting the limited space in microfluidic devices; moreover, fast sensing and actuation are required because of the fast liquid flow in the hollow fibre. In this study, fast torsional and tensile actuators were developed using hollow fibres employing spiral nonlinear stress, which can sense the fluid temperature and sort the fluid into the desired vessels. The fluid-driven actuation exhibited a highly increased response speed (27 times as fast as that of air-driven actuation) and increased power density (90 times that of an air-driven solid fibre actuator). A 0.5 K fluid temperature fluctuation produced a 20° rotation of the hollow fibre. These high performances originated from increments in both heat transfer and the average bias angle, which was understood through theoretical analysis. This work provides a new design strategy for intelligent microfluidics and inspiration for soft robots and smart devices for biological, optical, or magnetic applications.

Fig. 1. Hollow-fibre actuators for microfluidic manipulation.

a Schematic of a compact microfluidic manipulation device that can sense the liquid temperature and control the liquid flow by employing a twisted hollow-fibre actuator. b Schematic of the tensile hollow-fibre actuator used for sensing the liquid temperature and sorting the liquid into the desired cell. c Comparison of the heterochiral and homochiral PEHF_580-990 actuators driven by 95 °C water and air. d Images of the coiled and twisted hollow-fibre actuators.

Fig. 2. Actuation performance of torsional PEHF_580-990 actuators.

a Schematic and b serial images of a twisted PEHF_580-990 actuator during water flow. c Maximum rotation angle and maximum rotation speed as a function of inserted twist density. d Comparison of the torsional PEHF_580-990 actuator driven by 95 °C water and air. e, f Experimental and theoretical results of the rotation angle as a function of time for the twisted PEHF_580-990 actuator at different temperatures (e) and inserted twist densities (f). If not specified, in the following figure captions, the actuation was driven by 95 °C water, and the water flow rate was 0.5 g s⁻¹. If not specified, the error bars in c and in the following figures indicate standard deviations.

Fig. 3. Actuation performance of load-free tensile hollow-fibre actuators.

a Tensile stroke and contraction speed during 10,000 fluid-driven heating-cooling actuation cycles. Inset: time dependence of contraction driven by alternatively flowing 90 °C and 25 °C water at a flow rate of 1.72 g s⁻¹. b Infrared images of the homochiral PEHF_580-990 actuators during water flow. c Experimental and theoretical results of a homochiral PEHF_580-990 actuator driven by flowing water at different temperatures. d Actuation stroke as a function of water temperature for heterochiral and homochiral PEHF_580-990 actuators with different inserted twist densities. e Schematic illustration and photograph of the heterochiral PEHF_580-990 actuator. f Comparison of the contraction and response time of PEHF_580-990 actuators with those of typical polymer fibre actuators reported in previous studies.

Fig. 4. Actuation performance of fluid-driven homochiral hollow-fibre actuators.

a Optical and infrared images of a PEHF_580-990 actuator lifting a 10-g load in 1.2 s when flowing 90 °C water at a flow rate of 1.72 g s⁻¹. b Maximum contraction at different masses of the load for 10 consecutive heating-cooling cycles of homochiral PEHF_580-990 actuators with a spring index of 4.0 and an inserted twist of 300 turns m⁻¹ driven by alternatively flowing 90 °C and 25 °C water. c Work capacity of PEHF_580-990 actuators with different inserted twist densities and applied loads. d Work capacity of PEHF actuators with different sheath ratios at different loads. e Comparison of the work capacity for nylon 6 actuators made of hollow and solid fibres at different loads. The isobaric stress in d and e was calculated as the weight of the load divided by the cross-sectional area of the hollow fibre sheath. f Comparison of the specific work capacity of nylon hollow fibres in this work with that of single-filament pure-polymer fibre actuators in previous studies. The specific work capacity is defined as E/(m·Δθ), where E denotes the work output, m denotes the mass, and Δθ is the temperature change.

Fig. 5. Microfluidic sensing and manipulation by PEHF_580-990 actuators.

a Schematic of the microfluidic sensing and manipulation device. b Optical images of PEHF_580-990 actuators for sensing the liquid temperature and sorting the liquid into the desired vessels. c Comparison of the temperature resolution and specific torsional stroke of PEHF_580-990 with those in previous studies. The specific torsional stroke is defined as ΔT/(T·Δθ), where ΔT denotes the thermally driven torsional rotation, T denotes the twist density, and Δθ is the temperature change. The temperature resolution is the minimum temperature change used to trigger torsional actuation. d Torsional angle as a function of water temperature.