Microfluidic flow assembly system with magnetic clamp for unlimited geometry in millimetric hydrogel film patterning

https://doi.org/10.1016/j.apmt.2021.101330Get rights and content

Highlights

  • Develop a new microfluidic assembly system for the fabrication of millimetric polymeric sheets with.

  • geometrically unlimited patterns.

  • Create a magnetic clamp in a microfluidic channel which enables the microfluidic flow assembly system.

  • Fabricate multifunctional millimetric hydrogel films for dual pH-temperature and shape-encoded sensing.

Abstract

Polymeric sheets with geometrically selective patterns and functions can be used for many labeling and sensing applications. Conventional fabrication techniques such as templated-based printing require cumbersome, multiple and intensive steps, pose difficulty towards automation, and thus are mostly applied for simple array patterned sheets. Alternative microfluidics-based methods, such as flow lithography is limited to creating parallel patterns. We develop a microfluidic assembly fabrication based on flow lithography to make millimetric polymeric sheets with geometrically complex and selective patterns. This new fabrication is empowered by integrating a magnetically actuated clamp in the microfluidic channel. The integration of this magnetic clamp in combination of flow lithography allows rapid flow-assisted exchanges of multiple compositions during sequence patterning. Resolution is limited only by the quality of the UV photolithography source and the microscope optics. Using our method, we demonstrate the synthesis of enclosed geometrically complex hydrogels film with the inclusion of choice chemical functionalizations for pH and temperature sensing, fluorescent markers and for biomolecule sensing, fluorescence and enzymatic activity.

Introduction

There is ongoing interest in the manufacturing of multifunctional polymer sheets with geometrically selective and sub millimeter scale patterns because of their myriad applications, which are in high demand and offer tremendous value. These polymers can be rationally designed, implementing chemical groups that can provide stimuli responses [1], intrinsic optical properties, such as colors [2] or a scaffold for cells or biomolecule binding [3]. Stimuli responsive polymers with functional patterns and shapes can be applied to environmental sensing [4], soft robotics [5], or polymer origami [6,7], those with optically tunable properties can serve as photonic devices [8], functionalized scaffolds can be used for complex cell cultures [9], while visible or fluorescent patterns can be applied to anticounterfeiting [10], even in medicines [11]. Manufacturing these polymer sheets with micrometer resolution and high throughput remains a challenge. Here we focus on hydrogel sheets with arbitrary geometries and various functionalizations, by demonstrating a unique magnetic clamping mechanism within a microfluidic device, providing an arm to hold the initial scaffold in place during the flow lithography of component polymers. Our microfluidic platform is scalable and lends itself to many applications, notably those that involve multiple measurements constrained to a polymer sheet for multiplexing.

Recent progress in 3D printing's finer resolution and higher speed can be adapted to print patterned functional sheets [12]. However, these single material creations are typically limited in resolution of approximately 100 μm, while incorporating multiple materials poses challenges and requires a complex such as parallelization with 128 nozzles [13]. Conventional microfabrication methods, for planar patterned polymeric sheets are photolithography [14], molds [15], self-forming, sacrificial and template masters [16] among many others. Photolithography-based patterning can achieve a feature resolution below 1 μm but requires extensive equipment to do so [5,14]. Soft lithographic template printing or molding is a popular choice because of less equipment dependence and it offers a resolution inherently limited by template feature resolution [15,16]. However, within all these conventional methods, implementing multiple-compositions with geometrical selective patterns is challenging and involves multiple cumbersome steps and labor-intensive procedures, which become difficult to automate. Thus, patterned polymeric sheets based on these conventional methods are mostly parallel or array type structures with two compositions [5,[14], [15], [16]]. A recent template method based on porous membranes with shape-patterned microwells shows the ability to fabricate complex patterned structures with multiple-compositions [10]. In this new method, complex functional patterns with a smallest feature size of 15 μm are created by flowing a suspension of selective non-spherical 2D particles into predetermined shape-patterned microwells of a porous substrate by applying negative pressure in a sequential manner. Because geometrical patterning is enabled by filling particles into microwells, the patterns are discrete and it is difficult to fabricate them as continuous and interconnected\. To reduce the sensitivity to defects and increase the yield in this particle assembly process, an excessive number of microparticles with well-defined size and shape are needed and repeated washing and assembly steps for each pattern are required.

Among many microfabrication approaches, over the last two decades, microfluidics-based synthesis techniques [17], [18], [19] have been developed with the goal of overcoming limitations inherent in other approaches where microfluidics can offer precise flow control, microscale geometry, and easy sample handing. Ideally, in order to have a simple and scalable device that can also achieve high throughput, the multifunctional or multiplexed hydrogel film patterning would be produced with minimal interventions by mechanical means. Slit channel lithography (SCL) [20] with multiple parallel prepolymeric solutions achieves this goal for functional sheets and can produce them at a faster rate with the inclusion of an air pulse within the stop flow lithography (SFL) configuration [21], while the implementations of polymer formation in microfluidic channels can facilitate through put in a continuous flow fashion, namely continuous flow lithography (CFL) [22]. Both SFL and CFL have the ability to produce microparticles with any 2D extruded shapes and curvatures for microparticles of single composition. The parallel flow of prepolymers can be used to create films with multifunctional layers. Moreover, films can also be made with tunable features and chemical anisotropy in microfluidic channels [20]. However, spatial patterns are mostly limited to parallel composition and the creation of geometrically-selective shapes with complex curvatures remains difficult to achieve. The patterning geometry produced with such a setup is typically limited to the parallel layer configuration. To circumvent this parallel patterning limitation in microfluidics, others have mechanically localized a particle of a few hundred microns during synthesis steps with either mechanically squeezing the channel ceiling down onto the particle [23] or a lock-release with a ceiling mount [24] in multi-layered or three-dimensional channel configurations. Although these have proven to be fruitful for the production of three dimensional microparticles with non-parallel multifunctional patterns in an additive process, especially the lock-release method based SFL that can form particles with complex curvatures of patterns, these approaches are not applicable to millimetric polymer sheet production.

Here we report a microfluidic assembly fabrication technique based on flow lithography to create arbitrary shapes and complex patterns with multiple components in localized geometries without the limitation of parallel stripes much like SCL. This new assembly process is empowered by integrating a magnetically actuated clamp inside the microfluidic channel. This integration of a clamp in combination with flow lithography allows for the rapid flow-assisted exchanges of multi polymeric solutions in place for subsequent flow lithography patterning steps. Considering the hydrogels used in a wide range of applications such as in biochemistry featuring fluorescent reporters, given that the flexible nature and hydrophilicity of hydrogels can enhance detection kinetics [25] or other sensor multiplexing or encoding, all demonstrations in this study are shown in hydrogels. Using our microfluidic assembly platform, we fabricate hydrogel sheets of 1.5 × 1.5 mm2 with various patterns and compositions. We also demonstrate tandem pH-temperature sensing and shape-encoded biosensing enabled by multipatterned hydrogel sheets as a proof-of-concept. This technique can be scaled to create unique films with various patterns, functionalizations, or multiplexed sensing, among others. Our approach takes inspiration from the modern factory assembly line.

Section snippets

Preparation of microfluidic device and clamp

The photomasks for the microfluidics channels and the flow lithography hydrogel sheets were designed in AutoCAD (Autodesk), and printed at a resolution of ∼ 1 pixel per micron (25,000 dpi, 8 µm guaranteed resolution limit) by CAD/Art Services (OR, USA). The positive master mold for the microfluidic devices was made using SU-8 2050 photoresist (Kayaku Advanced Materials, formerly Microchem) spin-coated for a single layer of 70 μm in height onto a 100 mm diameter silicon wafer (ID 452, University

Design principle of the magnetic clamp

In our demonstrated magnetic clamping system (Fig. 1a,b), the clamp (Fig. 1:i) is positioned and operated within the clamp holder structures (Fig. 1 b: iii) for the hydrogel film frame (Fig. 1 a,b: ii). The microfluidic channel and clamp holder are constructed from polydimethylsiloxane (PDMS) and bonded to PDMS coated glass using thermal curing, and then the clamp is built inside the channel. We use photopolymerization via patterned photomasks and an objective to fabricate the hydrogel frame (

Assembly process workflow

With a working clamp protocol, we can make multifunctional hydrogel (see Fig. 2). First the magnetic bead and prepolymer mixture is introduced into a specifically designed microfluidic channel. The UV illumination pattern from the photomask is aligned to the channel, the clamp is formed by photopolymerization, and then the solution is washed away (Fig. 2a). Next the prepolymeric solution for the hydrogel frame is introduced, and the frame pattern is aligned to the clamp and channel. The

Complex shape generation

With the demonstration of simple, unique fills, we moved on to more complex geometries, as shown in the composite fluorescent images in Fig. 3 (with corresponding brightfield images in figure S2). The intensity of fluorescence can also be directly modulated by polymerization density, achieved here by increasing the UV power or exposure time. Alternatively, one could increase or decrease the concentration of fluorescent marker as a means to adjust the fluorescence intensity. From a production

Encoding

Next, we show repeated, sequential functionalized polymer fill in a nested concentric assembly, where the functionalization prepolymer is alternated for a bullseye pattern (Fig. 4a). This requires a wash in-between each color fill, with the photolithography being used to assemble the hydrogel structure from the outside in, with some feature overlap for structural support. Here we have assembled it with the manual use of appropriate photomasks, and with manual alignment and exposures, limiting

Multiplexed sensing

Our flow lithography clamping arrangement provides us the great opportunity to incorporate multi-sensing into our hydrogels. We could embed a response to many stimuli, such as heat, pH, ionic strength, external electromagnetic fields, etc. [4] into a single hydrogel. Here we have create a multi-sensing hydrogel by integrating the thermo-responsive polymer poly(N-isopropylacrylamide) (PNIPAM) [33], and a blend of acrylic acid into PEGDA for pH sensing [32]. By creating a sensing strip aligned

Outlook and future directions

Our magnetic clamp platform presented here permits one to make precise geometrical patterns with multiple components within a hydrogel. It can be made scalable by existing microfluidic techniques, namely multiple parallel channels [42] and in serial with many frames in a line. There are numerous proven polymeric materials and functionalities that can be introduced in a top-down design process by the photomasks and digital projection systems used for flow lithography.

This system is in the early

Author contributions

D.K.H conceptualized the project, both authors designed the devices and experiments, D.A.N.F. conducted the experiments and collected data, and both authors wrote the manuscript.

Data availability statement

The data that support the findings of this study are available from the corresponding author.

Supplementary Information

Supplementary figures to this study can be found online.

CRediT authorship contribution statement

Daniel A.N. Foster: Conceptualization, Methodology, Software, Validation, Investigation, Resources, Writing – original draft, Writing – review & editing, Visualization, Project administration. Dae Kun Hwang: Conceptualization, Methodology, Resources, Writing – original draft, Writing – review & editing, Visualization, Supervision, Project administration, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

D.K.H. acknowledges funding support from the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery grants program RGPIN-2017–04489, and Canada Research Chair program. We would like to thank our colleagues Morteza Jeyhani, Adrian T. Nash and Shyan I. Thompson for helpful discussions about experimental designs, and Steven Hayes of the Keenan Research centre for Biomedical Science Core Facilities at St. Michael's Hospital for helpful discussions regarding protein

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