Synthesis and active sorting of magnetic liquid beads

doi:10.21203/rs.3.rs-3933599/v1

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Synthesis and active sorting of magnetic liquid beads

https://doi.org/10.21203/rs.3.rs-3933599/v1

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published 06 May, 2024

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This paper reports the fabrication and characterization of magnetic liquid beads using microfluidic techniques. The liquid beads consist of a fluorinated oil core and a polymer shell embedded with magnetite particles. The beads are generated in a flow-focusing PDMS device and cured by photopolymerisation. The mobility response of the beads to an external magnetic field was investigated by characterising their motion towards a permanent magnet. Magnetic sorting of liquid beads with 90% efficiency was achieved due to the unique magnetic property of the shell. The results show that the liquid beads can be controlled magnetically and have potential applications in drug delivery, bioimaging, sensing, and tissue engineering. The present paper also discusses the effects of parameters such as magnetite concentration, bead size, and mass on the magnetic force and sorting efficiency.

Magnetic liquid beads

Active sorting

Microfluidic device

Drug delivery

Liquid beads are core-shell particles with a solid shell and a liquid core. Liquid beads differ from liquid marbles (Aussillous and Quéré 2006) based on their shell composition; liquid beads have solid shells whereas liquid marbles have a soft powdered coating as shells. The shell material of the liquid beads can be either organic polymers or inorganic substances such as metal, metal oxides, and silica or some combination of organic–inorganic materials (Chong et al. 2015; Yadav et al. 2023). Liquid beads have gained increasing attention due to their distinctive core and shell structure leading to promising applications. This unique composition provides features and properties that cannot be achieved by the shell or the core alone. With a liquid core encapsulated by a solid shell, liquid beads find application in various fields such as food processing (Gumustas et al. 2019), cosmetics (Van Tran et al. 2019), and digital microfluidics (Bian et al. 2018; Brassard et al. 2008; Ooi et al. 2021). Specific examples are transportation and controlled release of substances like pharmaceuticals (Mahdavi et al. 2020), nutrients (Castro-Ceseña et al. 2016), and cells (He et al. 2015; Jenjob et al. 2020). These applications necessitate the use of a particle shell to protect the core from its surroundings, preventing contamination. The shell provides physical support to the core while ensuring there is no unintentional release to its surroundings. With these constraints in consideration, the shells are designed to respond to a particular stimulus in one or more stages (Jing et al. 2011). The reported stimuli are electric field (Gao et al. 2021), a magnetic field (Spoială et al. 2021), light (Amoli-Diva et al. 2018), heat (Kurzhals et al. 2015), pH (Su et al. 2018), acoustic waves (Leibacher et al. 2014), and surface tension (Singha et al. 2019).

Microfluidic technique has become a flexible and contemporary method for generating core–shell droplets, acting as a template to produce the liquid beads. The compact and cost-effective nature of microfluidics offers substantial advantages over traditional methods (Galogahi et al. 2020). This technology allows for precise control of small fluid volumes in microchannels, facilitating accurate modifications in core droplet size and shell thickness (Murshed et al. 2009; Tan et al. 2008). Additionally, it yields droplets with a narrow size distribution at regular intervals (Yamagishi et al. 1996). Figure 1(a) depicts the production of liquid beads using a microfluidic device.

To date, numerous studies have been focusing on encapsulating analytes as cores in shells of liquid beads. The reported methods often include diverse undesirable components, necessitating subsequent purification (Yoon et al. 2006). Therefore, sorting based on particle properties becomes crucial for their subsequent use (Jackson and Lu 2013). Moreover, sorting minimises waste by ensuring only particles meeting specific criteria are selected, enhancing resource efficiency and cost-effectiveness (Iranmanesh and Hulliger 2017). Active sorting allows for a more precise and specific selection of liquid beads based on their tailorable properties. In addition, isolated core-shell structure enables enhanced sorting efficiency ensuring minimum contamination. Solid polymer shells are well suited as they can embed various materials for specific stimuli responses, required for active sorting. For example magnetic particles are widely used for assembly, trigger release and active sorting applications (Peng et al. 2008). Gold nanoparticles embedded in polymeric shell respond to infrared irradiation for sorting and drug delivery applications (Cheng et al. 2020). Furthermore, active sorting is easily automated and integrated into larger systems, advantageous for high-throughput applications.

However, separation techniques for biological samples face many challenges. Sorting cells in biological samples based on their intrinsic properties may result in damage caused by shear stress during centrifugation, membrane pressure in filtration, or Joule heating in electrophoresis. An encapsulated magnetic shell offers several advantages, including high specificity, lower risk of cell damage, shorter sorting time. Magnetophoretic sorting is independent of pH and ionic concentration of the solution (Chung et al. 2016; Fulcrand et al. 2011). The experimental setup required in magnetic separation technique is economical compared to other active techniques, and scalable to industrial capacities. Other advantages include their non-invasive nature and compatibility with microfluidics.

2.1 Materials

Silicon wafers were acquired from IBD Technologies Ltd. (Wiltshire, UK). The photoresist SU-8 3050 was sourced from MicroChem Corp (Westborough, USA). PDMS prepolymer and the associated curing agent (Sylgard 184) were procured from Dow Corning (Midland, MI, USA). Poly (vinyl alcohol) (PVA) with an 87–90% hydrolysis rate and an average molecular weight ranging from 30,000 to 70,000 was obtained from Sigma-Aldrich.

Spherical magnetite particles (Sigma-Aldrich, Iron II/III oxide) had a diameter of less than 5 µm and density of approximately 5×10³ kg/m³. The shell liquid (polymer phase) was prepared by mixing 0.06 g of ethyl-4(dimethylamino)benzoate (Merck), 0.05 g of camphorquinone (Merck), 10 g of trimethylolpropane trimethacrylate (TMPTMA, Merck) and magnetite particles. The particles were added at in concentrations of 1% and 2% by weight. The surface tension, viscosity, and density of this polymer solution were measured at σ_polymer = 0.032 N/m, µ_polymer = 0.042 Pa.s, and ρ_polymer = 1.07×10³ kg/m³ respectively (Takei et al. 2019).

Fluorinated oil (HFE Novec 7500 from 3M) was chosen as the core liquid (oil phase). It has a surface tension of σ_HFE = 0.015 N/m, a viscosity of µ_HFE = 1.31×10^− 3 Pa.s, a density of ρ_HFE = 1.63×10³ kg/m³, and a boiling point of 128°C (Rausch et al. 2015). For the outer continuous phase, an aqueous solution containing 50% glycerol (Chemsupply) in water (Milli-Q) was employed. This solution has a surface tension of σ_sol = 0.067 N/m, a viscosity of µ_sol = 6×10^− 3 Pa.s, and a density of ρ_sol = 1.12×10³ kg/m³ (Gregory 1991).

2.2 Fabrication of the microfluidic device

The PDMS-based microfluidic device was fabricated following a procedure reported by Galogahi et al. (2021). To summarize, the process involved designing a mask with CleWin by WieWeb Software. The design was then printed on a transparent film. A layer of SU-8 3050 (MicroChem) was applied to a 4-inch silicon wafer using spin coating. This layer was later patterned to create the microchannel mould through photolithography followed by an annealing process. The microchannels were 100 µm wide and 120 µm deep with a constriction width of 30 µm.

The microfluidic device was fabricated using the soft lithography approach. A degassed mixture of polydimethylsiloxane (PDMS) base and the curing agent in a 10:1 ratio was poured onto the SU-8 mould and cured for 1 hour at 75°C. Once cured, the PDMS replica was carefully peeled off, and inlets and outlets were created using a biopsy puncher. Finally, the PDMS device was bonded onto a glass substrate after treating both the PDMS and the glass substrate in an oxygen plasma cleaner (PDC-32G-2, Harrick Plasma).

2.3 Preparation of Magnetic liquid bead

The microfluidic device featured had five inlets and one outlet. The flow rates of the inlet fluids were regulated by a syringe pump (NEM-B101-03 A, CETONI GmbH, Germany). The core and shell fluids were injected through the first and second inlets, while the third and fourth inlets delivered the continuous phase. The core liquid flow rate was 30 µL/h, while the flow rates of the shell and the continuous phase was 100 and 400 µL/h respectively. A spacer fluid was introduced through the last inlet at a flow rate of 500 µL/h to prevent unintended merging of core-shell droplets.

The core phase comprised HFE 7500 oil, while the shell phase consisted of TMPTMA mixed with ethyl 4-(dimethylamino) benzoate, camphorquinone, and magnetite particles (1 wt% and 2 wt%). Prior to the blending, particles were treated with sodium dodecyl sulphate (SDS) to minimise agglomeration. The excess surfactant was removed by rinsing with pH11 NaOH solution. Subsequently, the washed particles were dispersed into Isopropyl alcohol and mixed to 2% by weight with the polymer(Choi et al. 2015; Tierno et al. 2005). The mixture was stirred mechanically for uniformity. The continuous phase and spacer fluid were composed of an aqueous solution containing 50% v/v glycerol and 0.06 mM Tween 20.

To generate the core–shell droplets, the initial core phase was introduced via the first inlet, where it interfaces with the shell phase flowing through the second inlet at the first junction. Subsequently, the shell phase encapsulates the core droplets at the second junction to form the core-shell structure of the magnetic liquid beads. To disperse the shell phase, the continuous phase was introduced through the third and fourth inlets. The core–shell droplets then move to the third junction, where a spacer phase was added to prevent coalescence. Finally, the formed droplets exit the device, where they are collected in a Petri dish sitting on a gently oscillating platform. Following collection, the resulting core–shell droplets were exposed to a 24 W blue light (450–490 nm) source for a duration of 20 minutes to harden and form the liquid beads with oil core, Fig. 1(b). Occasionally, during bead collection or the curing process, some beads may lose their core, resulting in solid beads. The bead size closely resembles that of core-shell beads; however, these beads are 5% lighter than their core-shell counterparts.

2.4 Characterisation of magnetic response of liquid bead

Figure 2 illustrates the experimental setup. It mainly comprises of a permanent magnet and a container with water. A holder made from 2-mm thick PMMA to secure the cubic permanent magnet (Neodymium, 13.5 × 13.5 × 13.5 mm) beside the container. This assembly was positioned on a linear stage to easily adjust the magnet's position relative to the container. The magnet's height was set to match the water level in the container. The entire setup was mounted on a microscope (Nikon Eclipse Ti1) for real-time monitoring. Subsequently, a liquid bead was carefully positioned on the water surface, ensuring it remained sufficiently distant from the magnet (5 mm) to avoid interference. The magnet was then gradually moved closer to within 3 mm of the particle using the linear stage until the floating bead starts accelerating towards the magnet. The motion of the bead was monitored and recorded using a DSLR camera (Nikon D7500) attached to the microscope. The images were extracted from the recorded videos, and further analysis of the images was conducted using the TrackMate plugin within the ImageJ software (Ershov et al. 2022).

2.5 Sorting device

The PDMS sorting device was made of a 1-mm thick PMMA sheet as a mould with the design drawn using CoralDRAW. The channels were cut into the sheet using a laser cutter (Rayjet 300, Trotec). Subsequently, the cut design was affixed to a glass slide and positioned within a Petri dish. To make the device, a mixture of PDMS and the curing agent was poured onto PMMA mould and cured for 1 hour at 75°C. Once cured, the PDMS replica was delicately peeled off, perforated for inlets and outlets, and then bonded with a glass slide, completing the assembly.

This microfluidic device consists of two straight channel sections with dimensions of 35 mm×1 mm and 15 mm×0.5 mm (width × length), respectively. Additionally, a flow focusing design was introduced at the inlet, and the height of the channel was determined by 1-mm thick PMMA sheet. A neodymium magnet (13.5 × 13.5 × 13.5 mm) was placed on one side of the straight section at a lateral distance of 2 mm as depicted in Fig. 3. The device was not fabricated by the standard photolithography and soft lithography techniques due to the difficulty in obtaining the desirable channel height. The relatively large channel height of 1 mm was necessary to avoid channel blockage by liquid beads.

3.1 Forces acting on the floating magnetic liquid bead

When a liquid bead when is placed on the surface of a water-filled container, it remains buoyant at the water-air interface, as depicted in Fig. 2. This buoyancy results from the equilibrium between downward gravitational force, upward forces of buoyancy acting on the bead and surface tension of the water. Zhang et al. (2012) have extensively examined and discussed the characteristics of the vertical magnetic forces. In the current investigation, our focus is on forces acting in horizontal direction. Two horizontal forces act on the bead floating on water surface, and the equilibrium of these forces is mathematically described by Eq. 1

$${m}_{p}{a}_{p}{=F}_{m}+{F}_{d}$$

Where, ${m}_{p}$ and ${a}_{p}$ are mass and acceleration of the bead. ${F}_{m}$ is magnetic force and ${F}_{d}$ is drag force acting on the bead.

Assuming the surrounding medium to be diamagnetic, the driving force acting on a floating bead is described as per previous studies (Nguyen et al. 2010; Shevkoplyas et al. 2007).

$${F}_{m}=\frac{V\chi }{{\mu }_{o}}B\frac{dB}{dx}$$

Here, ${F}_{m}$ represents the magnetic force, $V$denotes the volume of the magnetite, $\chi$ is the magnetic susceptibility of the floating bead, ${\mu }_{o}$ is the permeability of vacuum, and $B$ represents the magnetic flux density.

Drag force emerges when an object moves at a relative velocity with respect to its surrounding fluid. This force originates from the necessity to displace the fluid elements in the path of the moving object. The drag force acting on a partially submerged spherical particle is described by Stokes' law:

$${F}_{d}=6\pi \beta \mu {r}_{p}{\upsilon }_{p}$$

Where $\beta$ is correction factor for coefficient of friction, $\mu$ is the dynamic viscosity of fluid, ${r}_{p}$ and ${\upsilon }_{p}$ represents the radius and horizontal velocity of the bead respectively. As reported in our previous work, $\beta$ is dependent on the degree of perturbation in the liquid interface (Ooi et al. 2016). This perturbation, in turn, depends on both the meniscus angle (ψ) and the submerged depth of the liquid bead, Fig. 2. The bead was assumed to be a solid spherical body which maintains its shape during the motion.

As per Eq. 2, we learnt the magnetic force varies proportionally with volume of the magnetite, flux density and flux density gradient of the magnetic field and the concentration of the magnetite particles (Khaw et al. 2016). This relationship is corroborated by Fig. 4(a), where liquid beads with 2% (wt) magnetite particles experience more force compared with 1% (wt) liquid bead. Assuming a uniform distribution of magnetite particles throughout the TMPTMA, the overall volume remains constant for all core-shell beads. In the case of a solid bead without a core, the volume available to magnetite particles increases by 15%. Additionally, the solid bead is 5% lighter than the core-shell. Thus, the solid beads experience more force compared to core-shell for same magnetite concentration, Fig. 4(b).

The magnetic flux density (B) of the magnet was measured using a handheld commercial gaussmeter (GM07, measurement range: 0 to 3 T, Hirst Magnetic Instruments Ltd, UK). The magnet is vertically affixed to a PMMA holder, positioned on the linear stage. Initially, the probe makes contact with the magnet to obtain the maximum reading. Subsequently, the linear stage is programmed to incrementally move 0.5 mm away from the probe for each measurement. Figure 5(a) illustrates the magnetic flux densities recorded as the distance between the gaussmeter probe and the magnet increases.

As demonstrated in Fig. 5(b) the floating magnetic bead starts experiencing magnetic force at 3mm. This helped us in designing the sorting microfluidic device where beads flowing through the channels remained within the range of magnetic influence. The displacement and distance of influence for beads carrying of 2%(wt) magnetite particle is more than their 1%(wt) counterpart. This observation holds true for solid beads when compared with core-shell beads.

3.2 Sorting

The magnetic and non-magnetic beads were collected after curing and then redispersed in water for sorting. Hydrophobic nature of the beads enabled us to utilise a PVA deposition strategy to facilitate their collection into a glass syringe through PTFE tubing. A 10% PVA solution was flushed into the syringe and tubing for 10 minutes, followed by a 15-minute baking process at 100°C. This process was repeated three times to achieve a desired level of hydrophilicity and prevented adhering of the beads to the syringe and tubing walls.

In this work, we showcase the sorting of magnetic and non-magnetic beads based on magnetic forces. We introduced two flow streams into a central channel: one containing a mix of magnetic and non-magnetic beads dispersed into water, and the other containing a buffer. The streams are separated into two outlet streams downstream, Fig. 3. The optimised flow rates for the buffer and the beads dispersed medium were set at 8 ml/hr and 4 ml/hr, respectively. The trajectories of 100 µm-sized magnetic and non-magnetic beads were observed within the microfluidic device. The deflection of beads depends on the balance between the magnetic force and the hydrodynamic drag force. Outlet 1 (0.5×1mm) branches out at a 30° angle from the straight outlet 2 (1×1mm). This design ensures that magnetic beads, under influence of the magnetic force deviate from straight path and exit from outlet 1, while non-magnetic beads continue along the straight path and exit from outlet 2, Video 1.

To evaluate sorting efficiency, bead motion was monitored using the ImageJ plugin TrackMate. The trajectories of beads over the last 10 frames are illustrated by the tails in Fig. 6(a). All the Beads depicted in Fig. 6(a[1]) maintain straight trajectories and emerge solely from outlet 2 in the absence of the magnet. In Fig. 6(a[2]), beads exhibit deviation from a straight path, adhere to the channel wall, and travel along the wall under magnetic influence. The beads exiting from outlet 1 are classified as magnetic, while those exiting from outlet 2 are categorised as non-magnetic. Additionally, about 10% beads exit from outlet 1 without adhering to the channel wall, leading to sorting error. Conversely, none of the beads after adhering to the channel wall exited from outlet 2. Figure 6(b) presents the outcomes, indicating that 90% of beads collected at outlet 1 were magnetic, while all beads collected at outlet 2 were non-magnetic.

Initially, this paper investigated the generation of magnetic beads within a flow-focusing PDMS microfluidic device. Through the surface treatment of magnetite particles, we successfully achieved a stable and uniform blending with TMPTMA, resulting in homogenous beads with smooth surfaces and spherical shape. However, some of the beads lack cores and we termed them as solid beads. Subsequently, we conducted an experiment to learn the dynamics of partially submerged magnetic beads and utilise the involved forces for bead sorting. In the presence of a permanent magnet, magnetic bead’s motion was observed. While the shape and size of the permanent magnet were fixed, the concentration of magnetite particles, its volume distribution and mass of the beads were varied. We demonstrated sorting of magnetic and non-magnetic beads based on difference in their magnetic responses. This difference is based on particle's magnetic susceptibility and its volume available in the bead. In addition, our experiment demonstrated potential sorting of magnetic core-shell beads from magnetic solid beads. However, this would require fine tuning the magnetic forces by incorporating more sophisticated magnet designs into microfluidic devices (Pamme et al. 2006; Watarai and Namba 2002). We achieved sorting efficiency of 90% that can also be improved by incorporating better flow focusing channel designs. The reduction in efficiency is attributed to the inhomogeneous distribution of magnetite particles in the beads. We achieved a throughput of 150–200 bead/min.

Author Contributions

A.S.Y.: Experiment, validation, writing—original draft. F.M.G.: Experiments and review(supporting). A.V.: Writing and review (supporting). D.T.T.: Writing and review (supporting). G.R.K: Writing and review (supporting). H.C.: Writing and review (supporting). K.R.S.: Review and supervision (supporting). N.-T.N.: Conceptualization, review, editing and supervision (lead).

Acknowledgement

N.T.N. acknowledges funding support from the Australian Research Council (ARC) through the ARC Discovery Project DP220100261 and the Australian Laureate Fellowship FL230100023. This work was partly performed at the Queensland node at Griffith University of the Australian National Fabrication Facility, a company established under the National Collaborative Research Infrastructure Strategy to provide nano and microfabrication facilities for Australian researchers. A.S.Y., F.M.G., A.V D.T.T. and H.C. acknowledge funding support from the Griffith University International Postgraduate Research Scholarship and an ARC Postgraduate Research Scholarship.

Competing Interests

Authors have no conflict of interest to disclose.

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No competing interests reported.

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You are reading this latest preprint version

Synthesis and active sorting of magnetic liquid beads

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

2. Experiments

2.1 Materials

2.2 Fabrication of the microfluidic device

2.3 Preparation of Magnetic liquid bead

2.4 Characterisation of magnetic response of liquid bead

2.5 Sorting device

3. Results and Discussions

3.1 Forces acting on the floating magnetic liquid bead

3.2 Sorting

4. Conclusion

Declarations

Author Contributions

Acknowledgement

Competing Interests

References

Additional Declarations

Supplementary Files

Status:

Journal Publication

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