Bioengineering Microgels and Hydrogel Microparticles for Sensing Biomolecular Targets

Injectable liposome-containing click hydrogel microparticles for release of macromolecular cargos

Hydrogel microparticles ranging from 0.1-100 μm, referred to as microgels, are attractive for biological applications afforded by their injectability and modularity, which allows facile delivery of mixed populations for tailored combinations of therapeutics. Significant efforts have been made to broaden methods for microgel production including via the materials and chemistries by which they are made. Via droplet-based-microfluidics we have established a method for producing click poly-(ethylene)-glycol (PEG)-based microgels with or without chemically crosslinked liposomes (lipo-microgels) through the Michael-type addition reaction between thiol and either vinyl-sulfone or maleimide groups. Unifom spherical microgels and lipo-microgels were generated with sizes of 74 ± 16 μm and 82 ± 25 μm, respectively, suggesting injectability that was further supported by rheological analyses. Super-resolution confocal microscopy was used to further verify the presence of liposomes within the lipo-microgels and determine their distribution. Atomic force microscopy (AFM) was conducted to compare the mechanical properties and network architecture of bulk hydrogels, microgels, and lipo-microgels. Further, encapsulation and release of model cargo (FITC-Dextran 5 kDa) and protein (equine myoglobin) showed sustained release for up to 3 weeks and retention of protein composition and secondary structure, indicating their ability to both protect and release cargos of interest.

Multiplex genotyping of SNPs in genomic DNA via hydrogel-based assay mediated with MutS and polyethylene glycol

The simultaneous genotyping of multiple single nucleotide polymorphisms (SNPs) in genomic DNA derived from organisms holds significant potential for applications such as precision medicine and food product authentication. However, conventional assay technologies including qPCR-based techniques, microarrays, and hydrogel-based assays face limitations in efficient multiplexing of SNPs, particularly for large-size DNA beyond kilobase scales, due to constraints in multiplex capability, specificity, or sensitivity. In this study, a hydrogel-based multiplex SNP genotyping platform specifically designed for genomic DNA is presented. This platform integrates the ligation detection reaction (LDR) and rolling circle amplification (RCA) techniques within a hydrogel-based multiplex sensing system, enabling adaptable and sensitive SNP genotyping for genomic DNA. To enhance the specificity of the assay, MutS protein and polyethylene glycol are introduced into the protocol, reducing the non-specific ligation and RCA reactions synergistically. With significant specificity improvement of over 10-fold, three types of SNPs within an artificially constructed ∼1000 bp double-stranded DNA (dsDNA) are successfully genotyped with double-digit picomolar sensitivity. Furthermore, the practical applicability of the developed process for the origin identification of raw materials is demonstrated by genotyping three types of SNPs within genomic DNA obtained from two closely related plant species, Korean ginseng (Panax ginseng) and American ginseng (Panax quinquefolius), containing ca. 3.5 gigabase genome size. Of notable significance, this study marks the premiere achievement in PCR-free multiplex genotyping of SNPs in genomic DNA using a single fluorophore.

Hydrogel particles-on-chip (HyPoC): a fluorescence micro-sensor array for IgG immunoassay

Novel microparticles have generated growing interest in diagnostics for potential sensitivity and specificity in biomolecule detection and for the possibility to be integrated in a micro-system array as a lab-on-chip. Indeed, bead-based technologies integrated in microfluidics could speed up incubation steps, reduce reagent consumption and improve accessibility of diagnostic devices to non-expert users. To limit non-specific interactions with interfering molecules and to exploit the whole particle volume for bioconjugation, hydrogel microparticles, particularly polyethylene glycol-based, have emerged as promising materials to develop high-performing biosensors since their network can be functionalized to concentrate the target and improve detection. However, the limitations in positioning, trapping and mainly fine manipulation of a precise number of particles in microfluidics have largely impaired point-of-care applications. Herein, we developed an on-chip sandwich immunoassay for the detection of human immunoglobulin G in biological fluids. The detection system is based on finely engineered cleavable PEG-based microparticles, functionalized with specific monoclonal antibodies. By changing the particle number, we demonstrated tuneable specificity and sensitivity (down to 3 pM) in serum and urine. Therefore, a controlled number of hydrogel particles have been integrated in a microfluidic device for on-chip detection (HyPoC) allowing for their precise positioning and fluid exchange for incubation, washing and target detection. HyPoC dramatically decreases incubation time from 180 minutes to one minute and reduces washing volumes from 3.5 ml to 90 μL, achieving a limit of detection of 0.07 nM (with a dynamic range of 0.07-1 nM). Thus, the developed approach represents a versatile, fast and easy point-of-care testing platform for immunoassays.

Recent Advances in Microgels: From Biomolecules to Functionality

The emerging applications of hydrogel materials at different length scales, in areas ranging from sustainability to health, have driven the progress in the design and manufacturing of microgels. Microgels can provide miniaturized, monodisperse, and regulatable compartments, which can be spatially separated or interconnected. These microscopic materials provide novel opportunities for generating biomimetic cell culture environments and are thus key to the advances of modern biomedical research. The evolution of the physical and chemical properties has, furthermore, highlighted the potentials of microgels in the context of materials science and bioengineering. This review describes the recent research progress in the fabrication, characterization, and applications of microgels generated from biomolecular building blocks. A key enabling technology allowing the tailoring of the properties of microgels is their synthesis through microfluidic technologies, and this paper highlights recent advances in these areas and their impact on expanding the physicochemical parameter space accessible using microgels. This review finally discusses the emerging roles that microgels play in liquid-liquid phase separation, micromechanics, biosensors, and regenerative medicine.

In-Line Characterization of the Temperature-Responsive Behavior of Surface-Bound Microgel Coatings by QCM-D: A Novel Strategy for Protein Repellence Evaluation

In this work, quartz crystal microbalance with dissipation monitoring (QCM-D) was used to develop a new method to evaluate the protein repellency of microgel coatings. Compared to traditional protocols for surface analysis, QCM has the advantage of a real-time quantitative approach with high sensitivity, allowing us to describe variations of the adsorbed mass with unprecedented accuracy. To enable the detectability of the film throughout the whole operational temperature interval, a poly(N-isopropylacrylamide-co-glycidyl methacrylate) p(NIPAm-co-GMA) microgel monolayer with defined thickness and rigidity was designed. Covalent adhesion of the film to the silica surface was achieved by epoxy-thiol click chemistry and tested for repeated temperature cycles, showing substantial reproducibility. Further functionalization of microgel surfaces by grafting polyzwitterionic chains remarkably improved the protein repellence leaving the strong surface adhesion unaltered. Before and after exposure to fluorescein-tagged bovine serum albumin (FITC-BSA), the coatings showed identical responsive behavior, proving the absence of protein deposition. In nonrepellent coatings, QCM monitoring instead displayed a characteristic shift in the volume phase transition (VPT), pointing out the effect of adsorbed proteins on the swelling behavior of pNIPAm. The combination of QCM-D and UV-visible (UV-vis) was used to evaluate the effect of increasing surface coverage, enabling to distinguish between the protein deposition occurring over the coated and the uncoated portion of the sensor.

Rapid colorimetric analysis of multiple microRNAs using encoded hydrogel microparticles

microRNAs (miRNAs) have attracted much attention as potential biomarkers for the diagnosis of various fatal diseases. With increasing interest in miRNA detection at practical sites, colorimetric bead-based assays have garnered much attention, because these allow for simple analysis with cheap and portable devices. Among them, the encoded hydrogel microparticle-based colorimetric miRNA assay is considered as one of the promising techniques, due to its strengths, such as large multiplex capacity, acceptable sensitivity, and simple analysis. However, it still imposes a limitation in terms of the assay time, particularly the colorimetric reaction time, which is too long, making the practical application of the assay difficult and undermining its detection accuracy. In this work, we present a rapid colorimetric assay based on encoded hydrogel microparticles, which exhibits a significant decrease in the colorimetric reaction time due to two factors: (1) an increase in the number of enzymes bound to hydrogel microparticles via a post-synthesis functionalization method, and (2) an elevation in the enzyme reaction temperature during colorimetric labeling. We obtained a comparable sensitivity of the colorimetric assay with three different miRNA targets, even with a shortened colorimetric reaction time. Furthermore, we validated that our colorimetric detection method is suitable for multiplex miRNA detection, owing to its low cross-reactivity.

Small Oligonucleotides Detection in Three-Dimensional Polymer Network of DNA-PEG Hydrogels

The control of the three-dimensional (3D) polymer network structure is important for permselective materials when specific biomolecule detection is needed. Here we investigate conditions to obtain a tailored hydrogel network that combines both molecular filtering and molecular capture capabilities for biosensing applications. Along this line, short oligonucleotide detection in a displacement assay is set within PEGDA hydrogels synthetized by UV radical photopolymerization. To provide insights on the molecular filter capability, diffusion studies of several probes (sulforhodamine G and dextrans) with different hydrodynamic radii were carried out using NMR technique. Moreover, fluorometric analyses of hybridization of DNA oligonucleotides inside PEGDA hydrogels shed light on the mechanisms of recognition in 3D, highlighting that mesh size and crowding effect greatly impact the hybridization mechanism on a polymer network. Finally, we found the best probe density and diffusion transport conditions to allow the specific oligonucleotide capture and detection inside PEGDA hydrogels for oligonucleotide detection and the filtering out of higher molecular weight molecules.

Polymer Composition Primarily Determines the Protein Recognition Characteristics of Molecularly Imprinted Hydrogels

Synthetic hydrogels with the ability to recognize and bind target proteins are useful for a number of applications, including biosensing and therapeutic agent delivery. One popular method for fabricating recognitive hydrogels is molecular imprinting. A long-standing hypothesis of the field is that these molecularly imprinted polymers (MIPs) retain the chemical and geometric profile of their protein template, resulting in subsequent ability to recognize the template in solution. Here, we systematically determined the influence of network composition, as well as the identity, amount, and extraction of imprinting templates, on the protein binding of MIPs. Network composition (i.e. the relative number of ionizable and hydrophobic groups) explained the extent of protein adsorption in all cases. The identity and amount of imprinting template, albeit a protein or synthetic polymer (PEG) of similar molecular weight, did not significantly influence the amount of protein bound. While the purification method influenced the extent of template adsorption, it did so by chemically modifying the network (acrylamide hydrolysis, increasing the acid content by up to 21%) and not by voiding occupied MIP pores. Therefore, our results indicate that material composition determines the extent to which MIPs bind template and non-template proteins.

Hydrogel-Based Technologies for the Diagnosis of Skin Pathology

Hydrogels, swellable hydrophilic polymer networks fabricated through chemical cross-linking or physical entanglement are increasingly utilized in various biomedical applications over the past few decades. Hydrogel-based microparticles, dressings and microneedle patches have been explored to achieve safe, sustained and on-demand therapeutic purposes toward numerous skin pathologies, through incorporation of stimuli-responsive moieties and therapeutic agents. More recently, these platforms are expanded to fulfill the diagnostic and monitoring role. Herein, the development of hydrogel technology to achieve diagnosis and monitoring of pathological skin conditions are highlighted, with proteins, nucleic acids, metabolites, and reactive species employed as target biomarkers, among others. The scope of this review includes the characteristics of hydrogel materials, its fabrication procedures, examples of diagnostic studies, as well as discussion pertaining clinical translation of hydrogel systems.

Recent Advances in Degradable Hybrids of Biomolecules and NGs for Targeted Delivery

Recently, the fast development of hybrid nanogels dedicated to various applications has been seen. In this context, nanogels incorporating biomolecules into their nanonetworks are promising innovative carriers that gain great potential in biomedical applications. Hybrid nanogels containing various types of biomolecules are exclusively designed for: improved and controlled release of drugs, targeted delivery, improvement of biocompatibility, and overcoming of immunological response and cell self-defense. This review provides recent advances in this rapidly developing field and concentrates on: (1) the key physical consequences of using hybrid nanogels and introduction of biomolecules; (2) the construction and functionalization of degradable hybrid nanogels; (3) the advantages of hybrid nanogels in controlled and targeted delivery; and (4) the analysis of the specificity of drug release mechanisms in hybrid nanogels. The limitations and future directions of hybrid nanogels in targeted specific- and real-time delivery are also discussed.

Supramolecular Microgels with Molecular Beacons at the Interface for Ultrasensitive, Amplification-Free, and SNP-Selective miRNA Fluorescence Detection

In this study, a supramolecular structure with femtomolar biorecognition properties is proposed for use in analytical devices. It is obtained by an innovative interface between synthetic hydrogel polymers and molecular beacon (mb) probes. Supramolecularly structured microgels are synthetized with a core-shell architecture with specific dyes polymerized in a desired compartment. Mb probes are opportunely conjugated at the microgel interface so that their recognition mechanism is preserved and their spatial distribution is optimized to avoid crowding effects. The miR-21, a microRNA involved in various biological processes and usually used as a biomarker in early cancer diagnosis, has been selected as the target. The results demonstrate that by tuning the spatial distribution of molecular probes immobilized on the microgel and/or the amount of microgels, the assay shows scalable sensitivity reaching a limit of detection down to about 10 fM, without amplification steps and with detection time as short as 1 h. The assay results specific toward single mutated targets, and it is stable in the presence of high-interfering oligonucleotides concentrations. The miRNA target is also detected in human serum with performances similar to those observed in PBS buffer because of microgel antifouling properties without the need of any surface treatment. All tests were performed in a low sample volume (20 μL). As a result, mb-microgel represents an innovative biosensor to precisely quantify microRNAs in a direct (mix&read), scalable, and selective way. Such an approach paves the way for creating innovative biosensing interfaces with other probes, such as hairpins, aptamers, and PNA.

One-step scalable fluorescent microgel bioassay for the ultrasensitive detection of endogenous viral miR-US4-5p

Human cytomegalovirus (hCMV) infection is the leading cause of birth defects in newborns and death in immunosuppressed people. Traditional techniques require time-consuming and costly analyses, and sometimes result in false positive results; thus, a rapid and accurate detection for hCMV infection is necessary. Recently, hcmv-miR-US4-5p was selected as the biomarker for cytomegalovirus diagnosis and follow-up. Herein, we propose a bioassay based on microgels endowed with optical fluorescent oligonucleotide probes for the detection of circulating endogenous hcmv-microRNAs. In particular, a double strand probe, based on the fluorescence recovery after target capture, was conjugated on microgels and the probe density was opportunely optimised. Then, the microgels were directly mixed with the sample. The fluorescence read-out was measured as a function of target concentration at a fixed number of microgels per tube. As a bead-based assay, the performances of optical detection in terms of dynamic working range and limit of detection could be finely tuned by tuning the number of microgels per tube. The limit of detection of the assay could be tuned in the range from 39.1 fM to 156 aM by changing the microgel concentration from 50 μg mL^-1 to 0.5 μg mL^-1, respectively. The assay results specific for the selected target were stable over a one-year time span and they were not affected by the presence of human serum. Therefore, this bioassay based on microgels might represent a flexible platform that should be able to predict, identify and follow-up several diseases by monitoring freely circulating oligonucleotides in body fluids.

Functional Stimuli-Responsive Gels: Hydrogels and Microgels

One strategy that has gained much attention in the last decades is the understanding and further mimicking of structures and behaviours found in nature, as inspiration to develop materials with additional functionalities. This review presents recent advances in stimuli-responsive gels with emphasis on functional hydrogels and microgels. The first part of the review highlights the high impact of stimuli-responsive hydrogels in materials science. From macro to micro scale, the review also collects the most recent studies on the preparation of hybrid polymeric microgels composed of a nanoparticle (able to respond to external stimuli), encapsulated or grown into a stimuli-responsive matrix (microgel). This combination gave rise to interesting multi-responsive functional microgels and paved a new path for the preparation of multi-stimuli "smart" systems. Finally, special attention is focused on a new generation of functional stimuli-responsive polymer hydrogels able to self-shape (shape-memory) and/or self-repair. This last functionality could be considered as the closing loop for smart polymeric gels.