Synthesis of Cell-Adhesive Anisotropic Multifunctional Particles by Stop Flow Lithography and Streptavidin-Biotin Interactions

Stopping Microfluidic Flow

A cross-comparison of three stop-flow configurations-such as low-pressure (LSF), high-pressure open-circuit (OC-HSF), and high-pressure short-circuit (SC-HSF) stop-flow-is presented to rapidly bring a high velocity flow O(m s-1) within a microchannel to a standstill O(µm s-1). The performance of three stop-flow configurations is assessed by measuring residual flow velocities within microchannels having three orders of magnitude different flow resistances. The LSF configuration outperforms the OC-HSF and SC-HSF configurations within a high flow resistance microchannel and results in a residual velocity of <10 µm s-1. The OC-HSF configuration results in a residual velocity of <150 µm s-1 within a low flow resistance microchannel. The SC-HSF configuration results in a residual velocity of <200 µm s-1 across the three orders-of-magnitude different flow resistance microchannels, and <100 µm s-1 for the low flow resistance channel. It is hypothesized that residual velocity results from compliance in fluidic circuits, which is further investigated by varying the elasticity of microchannel walls and connecting tubing. A numerical model is developed to estimate the expanded volumes of the compliant microchannel and connecting tubings under a pressure gradient and to calculate the distance traveled by the sample fluid. A comparison of the numerically and experimentally obtained traveling distances confirms the hypothesis that the residual velocities are an outcome of the compliance in the fluidic circuit.

CRISPR-Enhanced Hydrogel Microparticles for Multiplexed Detection of Nucleic Acids

CRISPR/Cas systems offer a powerful sensing mechanism to transduce sequence-specific information into amplified analytical signals. However, performing multiplexed CRISPR/Cas assays remains challenging and often requires complex approaches for multiplexed assays. Here, a hydrogel-based CRISPR/Cas12 system termed CLAMP (Cas-Loaded Annotated Micro-Particles) is described. The approach compartmentalizes the CRISPR/Cas reaction in spatially-encoded hydrogel microparticles (HMPs). Each HMP is identifiable by its face code and becomes fluorescent when target DNA is present. The assay is further streamlined by capturing HMPs inside a microfluidic device; the captured particles are then automatically recognized by a machine-learning algorithm. The CLAMP assay is fast, highly sensitive (attomolar detection limits with preamplification), and capable of multiplexing in a single-pot assay. As a proof-of-concept clinical application, CLAMP is applied to detect nucleic acid targets of human papillomavirus in cervical brushing samples.

Flow lithography for structured microparticles: fundamentals, methods and applications

Structured microparticles, with unique shapes, customizable sizes, multiple materials, and spatially-defined chemistries, are leading the way for emerging 'lab on a particle' technologies. These microparticles with engineered designs find applications in multiplexed diagnostics, drug delivery, single-cell secretion assays, single-molecule detection assays, high throughput cytometry, micro-robotics, self-assembly, and tissue engineering. In this article we review state-of-the-art particle manufacturing technologies based on flow-assisted photolithography performed inside microfluidic channels. Important physicochemical concepts are discussed to provide a basis for understanding the fabrication technologies. These photolithography technologies are compared based on the structural as well as compositional complexity of the fabricated particles. Particles are categorized, from 1D to 3D particles, based on the number of dimensions that can be independently controlled during the fabrication process. After discussing the advantages of the individual techniques, important applications of the fabricated particles are reviewed. Lastly, a future perspective is provided with potential directions to improve the throughput of particle fabrication, realize new particle shapes, measure particles in an automated manner, and adopt the 'lab on a particle' technologies to other areas of research.

Scalable Fabrication and Use of 3D Structured Microparticles Spatially Functionalized with Biomolecules

Microparticles with defined shapes and spatial chemical modification can interface with cells and tissues at the cellular scale. However, conventional methods to fabricate shaped microparticles have trade-offs between the throughput of manufacture and the precision of particle shape and chemical functionalization. Here, we achieved scalable production of hydrogel microparticles at rates of greater than 40 million/hour with localized surface chemistry using a parallelized step emulsification device and temperature-induced phase-separation. The approach harnesses a polymerizable polyethylene glycol (PEG) and gelatin aqueous two-phase system (ATPS) which conditionally phase separates within microfluidically generated droplets. Following droplet formation, phase separation is induced and phase separated droplets are subsequently cross-linked to form uniform crescent and hollow shell particles with gelatin functionalization on the boundary of the cavity. The gelatin localization enabled deterministic cell loading in subnanoliter-sized crescent-shaped particles, which we refer to as nanovials, with cavity dimensions tuned to the size of cells. Loading on nanovials also imparted improved cell viability during analysis and sorting using standard fluorescence activated cell sorters, presumably by protecting cells from shear stress. This localization effect was further exploited to selectively functionalize capture antibodies to nanovial cavities enabling single-cell secretion assays with reduced cross-talk in a simplified format.

Specific detection of Staphylococcus aureus infection and marker for Alzheimer disease by surface enhanced Raman spectroscopy using silver and gold nanoparticle-coated magnetic polystyrene beads

Targeted and effective therapy of diseases demands utilization of rapid methods of identification of the given markers. Surface enhanced Raman spectroscopy (SERS) in conjunction with streptavidin-biotin complex is a promising alternative to culture or PCR based methods used for such purposes. Many biotinylated antibodies are available on the market and so this system offers a powerful tool for many analytical applications. Here, we present a very fast and easy-to-use procedure for preparation of streptavidin coated magnetic polystyrene-Au (or Ag) nanocomposite particles as efficient substrate for surface SERS purposes. As a precursor for the preparation of SERS active and magnetically separable composite, commercially available streptavidin coated polystyrene (PS) microparticles with a magnetic core were utilized. These composites of PS particles with silver or gold nanoparticles were prepared by reducing Au(III) or Ag(I) ions using ascorbic acid or dopamine. The choice of the reducing agent influences the morphology and the size of the prepared Ag or Au particles (15-100 nm). The prepare composites were also characterized by HR-TEM images, mapping of elements and also magnetization measurements. The content of Au and Ag was determined by AAS analysis. The synthesized composites have a significantly lower density against magnetic composites based on iron oxides, which considerably decreases the tendency to sedimentation. The polystyrene shell on a magnetic iron oxide core also pronouncedly reduces the inclination to particle aggregation. Moreover, the preparation and purification of this SERS substrate takes only a few minutes. The PS composite with thorny Au particles with the size of approximately 100 nm prepared was utilized for specific and selective detection of Staphylococcus aureus infection in joint knee fluid (PJI) and tau protein (marker for Alzheimer disease).

Elimination of Unreacted Acrylate Double Bonds in the Polymer Networks of Microparticles Synthesized via Flow Lithography

Flow lithography (FL), a versatile technique used to synthesize anisotropic multifunctional microparticles, has attracted substantial interest, given that the resulting particles with complex geometries and multilayered biochemical functionalities can be used in a wide variety of applications. However, after this process, there are double bonds remaining from the cross-linkable groups of monomers. The unreacted cross-linkable groups can affect the particles' biochemical properties. Here, we verify that the microparticles produced by FL contain a significant number of unreacted acrylate double bonds (UADBs), which could cause irreversible biochemical changes in the particle and pernicious effects to biological systems. We also confirm that the particles contain a considerable number of UADBs, regardless of the various synthetic (lithographic) conditions that can be used in a typical FL process. We present an effective way to eliminate a substantial amount of UADBs after synthesis by linking biochemically inert poly(ethylene glycol) based on click chemistry. We verify that eliminating UADBs by using this click chemistry approach can efficiently resolve problems, such as the occurrence of random reactions and the cytotoxicity of UADBs.

Direct Conjugation of Streptavidin to Encoded Hydrogel Microparticles for Multiplex Biomolecule Detection with Rapid Probe-Set Modification

Encoded hydrogel microparticles synthesized via flow lithography have drawn attention for multiplex biomarker detection due to their high multiplex capability and solution-like hybridization kinetics. However, the current methods for preparing particles cannot achieve a flexible, rapid probe-set modification, which is necessary for the production of various combinations of target panels in clinical diagnosis. In order to accomplish the unmet needs, streptavidin was incorporated into the encoded hydrogel microparticles to take advantage of the rapid streptavidin–biotin interactions that can be used in probe-set modification. However, the existing methods suffer from low efficiency of streptavidin conjugation, cause undesirable deformation of particles, and impair the assay capability. Here, we present a simple and powerful method to conjugate streptavidin to the encoded hydrogel microparticles for better assay performance and rapid probe-set modification. Streptavidin was directly conjugated to the encoded hydrogel microparticles using the aza-Michael addition click reaction, which can proceed in mild, aqueous condition without catalysts. A highly flexible and sensitive assay was developed to quantify DNA and proteins using streptavidin-conjugated encoded hydrogel microparticles. We also validated the potential applications of our particles conducting multiplex detection of cancer-related miRNAs.

Soft temperature-responsive microgels of complex shape in stop-flow lithography

Stop-flow lithography (SFL) has emerged as a facile high-throughput fabrication method for μm-sized anisometric particles; yet, the fabrication of soft, anisometric microgels has not frequently been addressed in the literature. Furthermore, and to the best of the authors' knowledge, no soft, complex-shaped microgels with temperature-responsive behavior have been fabricated with this technology before. However, such microgels have tremendous potential as building blocks and actuating elements in rapidly developing fields, such as tissue engineering and additive manufacturing of soft polymeric building blocks, bio-hybrid materials, or soft micro-robotics. Given their great potential, we prove in this work that SFL is a viable method for the fabrication of soft, temperature-responsive, and complex-shaped microgels. The microgels, fabricated in this work, consist of poly(N-isopropylacrylamide) (pNIPAm), which is crosslinked with N,N'-methylenebis(acrylamide). The results confirm that the shape of the pNIPAm microgels is determined by the transparency mask, used in SFL. Furthermore, it is shown that, in order to realize stable microgels, a minimum threshold of crosslinker concentration of 2 wt% is required. Above this threshold, the stiffness of pNIPAm microgels can be deliberately altered by adjusting the concentration of the crosslinker. The fabricated pNIPAm microgels show the targeted temperature-responsive behavior. Within this context, temperature-dependent reversible swelling is confirmed, even for fractal-like geometries, such as micro snowflakes. Thus, these microgels provide the targeted unique combination of softness, shape complexity, and temperature responsiveness and increase the freedom of design for actuated building blocks.

One-Step Photochemical Generation of Biofunctionalized Hydrogel Particles via Two-Phase Flow

Biofunctional hydrogel particles have become increasingly popular in medical diagnostics; however, their generation is time-consuming and typically requires several process steps. We report on a new method for the simple, fast, and reproducible one-step generation of monodisperse hydrogel particles equipped with biofunctional molecules such as proteins or DNA. Key to the approach is the simultaneous photo cross-linking of the polymer chains and covalent binding of proteins or DNA through a C,H insertion reaction inside aqueous plug compartments that are produced via microfluidics. The strong performance in biological binding assays of the functionalized particles is demonstrated.

Nanoengineering of Poly(ethylene glycol) Particles for Stealth and Targeting

The assembly of particles composed solely or mainly of poly(ethylene glycol) (PEG) is an emerging area that is gaining increasing interest within bio-nano science. PEG, widely considered to be the "gold standard" among polymers for drug delivery, is providing a platform for exploring fundamental questions and phenomena at the interface between particle engineering and biomedicine. These include the targeting and stealth behaviors of synthetic nanomaterials in biological environments. In this feature article, we discuss recent work in the nanoengineering of PEG particles and explore how they are enabling improved targeting and stealth performance. Specific examples include PEG particles prepared through surface-initiated polymerization, mesoporous silica replication via postinfiltration, and particle assembly through metal-phenolic coordination. This particle class exhibits unique in vivo behavior (e.g., biodistribution and immune cell interactions) and has recently been explored for drug delivery applications.

Implantable Photothermal Agents based on Gold Nanorods-Encapsulated Microcube

Gold nanorods (GNRs) are of great interest in cancer therapy given their ability to ablate tumor cells using deep tissue-penetrating near-infrared light. GNRs coated with tumor-specific moieties have the potential to target tumor tissue to minimize damage to normal tissue. However, perfect targeting is difficult to achieve given that nanoparticles could be broadly dispersed inside the body. Moreover, interaction between targeting groups and biological molecules could lower targeting abilities, resulting in off-target accumulation which might produce nanotoxicity. Here we introduce GNR-encapsulated microcubes (GNR@MCs) that can be utilized as implantable photothermal agents. GNR@MCs are created by encapsulating GNRs in polymeric networks via stop flow lithography (SFL), a one-phase synthesis technique which allows for creation of surfactant-free, uniform particles, and injection of GNR@MCs into the body after a simple rinse step. GNRs are highly packed and firmly encapsulated inside MCs, and entrapped GNRs exhibit optical properties comparable to that of unbound GNRs and photothermal efficiency (58%) in line with that of nano-sized agents (51–95%). Photothermal ablation in murine models is achieved using GNR@MCs stably implanted into the tumor tissue, which suggests that GNR@MCs can be a safe and effective platform for cancer therapy.

Compartmentalized Microhelices Prepared via Electrohydrodynamic Cojetting

Anisotropically compartmentalized microparticles have attracted increasing interest in areas ranging from sensing, drug delivery, and catalysis to microactuators. Herein, a facile method is reported for the preparation of helically decorated microbuilding blocks, using a modified electrohydrodynamic cojetting method. Bicompartmental microfibers are twisted in situ, during electrojetting, resulting in helical microfibers. Subsequent cryosectioning of aligned fiber bundles provides access to helically decorated microcylinders. The unique helical structure endows the microfibers/microcylinders with several novel functions such as translational motion in response to rotating magnetic fields. Finally, microspheres with helically patterned compartments are obtained after interfacially driven shape shifting of helically decorated microcylinders.

Microfluidic platform for selective microparticle parking and paired particle isolation in droplet arrays

Immobilizing microscale objects (e.g., cells, spheroids, and microparticles) in arrays for direct observation and analysis is a critical step of many biological and chemical assays; however, existing techniques are often limited in their ability to precisely capture, arrange, isolate, and recollect objects of interest. In this work, we present a microfluidic platform that selectively parks microparticles in hydrodynamic traps based on particle physical characteristics (size, stiffness, and internal structure). We present an accompanying scaling analysis for the particle parking process to enable rational design of microfluidic traps and selection of operating conditions for successful parking of desired particles with specific size and elastic modulus. Our platform also enables parking of encoded particle pairs in defined spatial arrangements and subsequent isolation of these pairs in aqueous droplets, creating distinct microenvironments with no cross-contamination. In addition, we demonstrate the ability to recollect objects of interest (i.e., one particle from each pair) after observation within the channel. This integrated device is ideal for multiplexed assays or microenvironment fabrication for controlled biological studies.

Edge smoothness enhancement in DMD scanning lithography system based on a wobulation technique

The resolution of digital micro-mirror device (DMD) scanning lithography is limited in the transverse direction (the scanning direction is vertical) as a result of the compacted size of the DMD micro-mirror and the low magnification of the projection lens. Above-stated restrictions lead to an unsatisfactory saw-tooth edge (size ~one DMD pixel) after the lithography process within all directions except for the scanning orientation. In order to smooth the edge, an optimized sub-pattern construction method, described as the combination of wobulation techniques and the continuous scanning lithography process, is proposed. Afterward, lithography experiments were implemented by introducing the wobulation techniques within the DMD scanning lithography system. The experimental results show that the saw-tooth edge is reduced to nearly 0.5 pixel size after 1/2 pixel dislocation superposition exposure, and is even scaled down to less than 0.1 pixel after 1/4 pixel dislocation superposition exposure. At this point, the edge of the lithography pattern is appropriately smoothed. The effectiveness of the above-mentioned method that improves the edge smoothness of the lithography pattern is demonstrated.

Flow lithography in ultraviolet-curable polydimethylsiloxane microfluidic chips

Flow Lithography (FL) is the technique used for the synthesis of hydrogel microparticles with various complex shapes and distinct chemical compositions by combining microfluidics with photolithography. Although polydimethylsiloxane (PDMS) has been used most widely as almost the sole material for FL, PDMS microfluidic chips have limitations: (1) undesired shrinkage due to the thermal expansion of masters used for replica molding and (2) interfacial delamination between two thermally cured PDMS layers. Here, we propose the utilization of ultraviolet (UV)-curable PDMS (X-34-4184) for FL as an excellent alternative material of the conventional PDMS. Our proposed utilization of the UV-curable PDMS offers three key advantages, observed in our study: (1) UV-curable PDMS exhibited almost the same oxygen permeability as the conventional PDMS. (2) The almost complete absence of shrinkage facilitated the fabrication of more precise reverse duplication of microstructures. (3) UV-cured PDMS microfluidic chips were capable of much stronger interfacial bonding so that the burst pressure increased to ∼0.9 MPa. Owing to these benefits, we demonstrated a substantial improvement of productivity in synthesizing polyethylene glycol diacrylate microparticles via stop flow lithography, by applying a flow time (40 ms) an order of magnitude shorter. Our results suggest that UV-cured PDMS chips can be used as a general platform for various types of flow lithography and also be employed readily in other applications where very precise replication of structures on micro- or sub-micrometer scales and/or strong interfacial bonding are desirable.

High-Throughput Fabrication and Modular Assembly of 3D Heterogeneous Microscale Tissues

3D hydrogel microstructures that encapsulate cells have been used in broad applications in microscale tissue engineering, personalized drug screening, and regenerative medicine. Recent technological advances in microstructure assembly, such as bioprinting, magnetic assembly, microfluidics, and acoustics, have enabled the construction of designed 3D tissue structures with spatially organized cells in vitro. However, a bottleneck exists that still hampers the application of microtissue structures, due to a lack of techniques that combined high-throughput fabrication and flexible assembly. Here, a versatile method for fabricating customized microstructures and reorganizing building blocks composed of functional components into a combined single geometric shape is demonstrated. The arbitrary microstructures are dynamically synthesized in a microfluidic device and then transferred to an optically induced electrokinetics chip for manipulation and assembly. Moreover, building blocks containing different cells can be arranged into a desired geometry with specific shape and size, which can be used for microscale tissue engineering.

Wrinkling Non-Spherical Particles and Its Application in Cell Attachment Promotion

Surface wrinkled particles are ubiquitous in nature and present in different sizes and shapes, such as plant pollens and peppercorn seeds. These natural wrinkles provide the particles with advanced functions to survive and thrive in nature. In this work, by combining flow lithography and plasma treatment, we have developed a simple method that can rapidly create wrinkled non-spherical particles, mimicking the surface textures in nature. Due to the oxygen inhibition in flow lithography, the non-spherical particles synthesized in a microfluidic channel are covered by a partially cured polymer (PCP) layer. When exposed to plasma treatment, this PCP layer rapidly buckles, forming surface-wrinkled particles. We designed and fabricated various particles with desired shapes and sizes. The surfaces of these shapes were tuned to created wrinkle morphologies by controlling UV exposure time and the washing process. We further demonstrated that wrinkles on the particles significantly promoted cell attachment without any chemical modification, potentially providing a new route for cell attachment for various biomedical applications.

Flexible Octopus-Shaped Hydrogel Particles for Specific Cell Capture

Multiarm hydrogel microparticles with varying geometry are fabricated to specifically capture cells expressing epithelial cell adhesion molecule. Results show that particle shape influences cell-capture efficiency due to differences in surface area, hydrodynamic effects, and steric constraints. These findings can lead to improved particle design for cell separation and diagnostic applications.