Synthesis of microgel sensors for spatial and temporal monitoring of protease activity

FRET Sensor-Modified Synthetic Hydrogels for Real-Time Monitoring of Cell-Derived Matrix Metalloproteinase Activity using Fluorescence Lifetime Imaging

Matrix remodeling plays central roles in a range of physiological and pathological processes and is driven predominantly by the activity of matrix metalloproteinases (MMPs), which degrade extracellular matrix (ECM) proteins. Our understanding of how MMPs regulate cell and tissue dynamics is often incomplete as in vivo approaches are lacking and many in vitro strategies cannot provide high-resolution, quantitative measures of enzyme activity in situ within tissue-like 3D microenvironments. Here, we incorporate a Förster resonance energy transfer (FRET) sensor of MMP activity into fully synthetic hydrogels that mimic many properties of the native ECM. We then use fluorescence lifetime imaging to provide a real-time, fluorophore concentration-independent quantification of MMP activity, establishing a highly accurate, readily adaptable platform for studying MMP dynamics in situ. MCF7 human breast cancer cells encapsulated within hydrogels highlight the detection of MMP activity both locally, at the sub-micron level, and within the bulk hydrogel. Our versatile platform may find use in a range of biological studies to explore questions in the dynamics of cancer metastasis, development, and tissue repair by providing high-resolution, quantitative and in situ readouts of local MMP activity within native tissue-like environments.

Synthetic and biopolymeric microgels: Review of similarities and difference in behaviour in bulk phases and at interfaces

This review discusses the current knowledge of interfacial and bulk interactions of biopolymeric microgels in relation to the well-established properties of synthetic microgels for applications as viscosity modifiers and Pickering stabilisers. We present a timeline showing the key milestones in designing microgels and their bulk/ interfacial performance. Poly(N-isopropylacrylamide) (pNIPAM) microgels have remained as the protagonist in the synthetic microgel domain whilst proteins or polysaccharides have been primarily used to fabricate biopolymeric microgels. Bulk properties of microgel dispersions are dominated by the volume fraction (ϕ) of the microgel particles, but ϕ is difficult to pinpoint, as addressed by many theoretical models. By evaluating recent experimental studies over the last five years, we find an increasing focus on the analysis of microgel elasticity as a key parameter in modulating their packing at the interfaces, within the provinces of both synthetic and biopolymeric systems. Production methods and physiochemical factors shown to influence microgel swelling in the aqueous phase can have a significant impact on their bulk as well as interfacial performance. Compared to synthetic microgels, biopolymer microgels show a greater tendency for polydispersity and aggregation and do not appear to have a core-corona structure. Comprehensive studies of biopolymeric microgels are still lacking, for example, to accurately determine their inter- and intra- particle interactions, whilst a wider variety of techniques need to be applied in order to allow comparisons to real systems of practical usage.

Protease-Responsive Hydrogel Microparticles for Intradermal Drug Delivery

Protease-responsive multi-arm polyethylene glycol-based microparticles with biscysteine peptide crosslinkers (CGPGG↓LAGGC) were obtained for intradermal drug delivery through inverse suspension photopolymerization. The average size of the spherical hydrated microparticles was ∼40 μm after crosslinking, making them attractive as a skin depot and suitable for intradermal injections, as they are readily dispensable through 27G needles. The effects of exposure to matrix metalloproteinase 9 (MMP-9) on the microparticles were evaluated by scanning electron microscopy and atomic force microscopy, demonstrating partial network destruction and decrease in elastic moduli. Given the recurring course of many skin diseases, the microparticles were exposed to MMP-9 in a flare-up mimicking fashion (multiple-time exposure), showing a significant increase in release of tofacitinib citrate (TC) from the MMP-responsive microparticles, which was not seen for the non-responsive microparticles (polyethylene glycol dithiol crosslinker). It was found that the degree of multi-arm complexity of the polyethylene glycol building blocks can be utilized to tune not only the release profile of TC but also the elastic moduli of the hydrogel microparticles, with Young's moduli ranging from 14 to 140 kPa going from 4-arm to 8-arm MMP-responsive microparticles. Finally, cytotoxicity studies conducted with skin fibroblasts showed no reduction in metabolic activity after 24 h exposure to the microparticles. Overall, these findings demonstrate that protease-responsive microparticles exhibit the properties of interest for intradermal drug delivery.

Spatially Defined Cell-Secreted Protein Detection Using Granular Hydrogels: μGeLISA

Intercellular communication through secreted proteins is necessary in essential processes such as embryo and limb development, disease progression, and immune responses. There exist many techniques to study bulk solution protein concentrations, but there is a limited set of tools to study the concentrations of cell-secreted proteins in situ within diverse cell platforms while retaining spatial information. In this study, we have developed a microgel system that is able to quantitatively measure the cell-secreted protein concentration within defined three-dimensional culture configurations with single-cell spatial resolution, called μGeLISA (microgel-linked immunosorbent assay). This system, which is based on the surface modification of polyethylene glycol microgels, was able to detect interleukin 6 (IL-6) concentrations of 2.21-21.86 ng/mL. Microgels were also able to detect cell spheroid-secreted IL-6 and distinguish between low- and high-secreting single cells. The system was also adapted to measure the concentration of cell-secreted matrix metalloproteinase-2 (MMP-2). μGeLISA represents a highly versatile system with a straightforward fabrication process that can be adapted toward the detection of secreted proteins within a diverse range of cell culture configurations.

Design and characterization of matrix metalloproteinase-responsive hydrogels for the treatment of inflammatory skin diseases

Enzyme-responsive hydrogels, formed by step growth photopolymerization of biscysteine peptide linkers with alkene functionalized polyethylene glycol, provide interesting opportunities as biomaterials and drug delivery systems. In this study, we developed stimuli-responsive, specific, and cytocompatible hydrogels for delivery of anti-inflammatory drugs for the treatment of inflammatory skin diseases. We designed peptide linkers with optimized sensitivity towards matrix metalloproteinases, a family of proteolytic enzymes overexpressed in the extracellular matrix of the skin during inflammation. The peptide linkers were crosslinked with branched 4-arm and 8-arm polyethylene glycols by thiol-norbornene photopolymerization, leading to the formation of a hydrogel network, in which the anti-inflammatory Janus kinase inhibitor tofacitinib citrate was incorporated. The hydrogels were extensively characterized by physical properties, in vitro release studies, cytocompatibility with fibroblasts, and anti-inflammatory efficacy testing in both an atopic dermatitis-like keratinocyte assay and an activated T-cell assay. The drug release was studied after single and multiple-time exposure to matrix metalloproteinase 9 to mimic inflammatory flare-ups. Drug release was found to be triggered by matrix metalloproteinase 9 and to depend on type of crosslinker and of the polyethylene glycol polymer, due to differences in architecture and swelling behavior. Moreover, swollen hydrogels showed elastic properties similar to those of extracellular matrix proteins in the dermis. Cell studies revealed limited cytotoxicity when fibroblasts and keratinocytes were exposed to the hydrogels or their enzymatic cleavage products. Taken together, our results suggest multi-arm polyethylene glycol hydrogels as promising matrix metalloproteinase-responsive drug delivery systems, with potential in the treatment of inflammatory skin disease. STATEMENT OF SIGNIFICANCE: Smart responsive drug delivery systems such as matrix metalloproteinase-responsive hydrogels are excellent candidates for the treatment of inflammatory skin diseases including psoriasis. Their release profile can be optimized to correspond to the patient's individual disease state by tuning formulation parameters and disease-related stimuli, providing personalized treatment solutions. However, insufficient cross-linking efficiency, low matrix metalloproteinase sensitivity, and undesirable drug release kinetics remain major challenges in the development of such drug delivery systems. In this study, we address shortcomings of previous work by designing peptide linkers with optimized sensitivity towards matrix metalloproteinases and high cross-linking efficiencies. We further provide a proof-of-concept for the usability of the hydrogels in inflammatory skin conditions by employing a drug release set-up simulating inflammatory flare-ups.

Engineered hydrogels for mechanobiology

Cells' local mechanical environment can be as important in guiding cellular responses as many well-characterized biochemical cues. Hydrogels that mimic the native extracellular matrix can provide these mechanical cues to encapsulated cells, allowing for the study of their impact on cellular behaviours. Moreover, by harnessing cellular responses to mechanical cues, hydrogels can be used to create tissues in vitro for regenerative medicine applications and for disease modelling. This Primer outlines the importance and challenges of creating hydrogels that mimic the mechanical and biological properties of the native extracellular matrix. The design of hydrogels for mechanobiology studies is discussed, including appropriate choice of cross-linking chemistry and strategies to tailor hydrogel mechanical cues. Techniques for characterizing hydrogels are explained, highlighting methods used to analyze cell behaviour. Example applications for studying fundamental mechanobiological processes and regenerative therapies are provided, along with a discussion of the limitations of hydrogels as mimetics of the native extracellular matrix. The article ends with an outlook for the field, focusing on emerging technologies that will enable new insights into mechanobiology and its role in tissue homeostasis and disease.

Fluorescent Peptomer Substrates for Differential Degradation by Metalloproteases

Proteases, especially MMPs, are attractive biomarkers given their central role in both physiological and pathological processes. Distinguishing MMP activity with degradable substrates, however, is a difficult task due to overlapping substrate specificity profiles. Here, we developed a system of peptomers (peptide-peptoid hybrids) to probe the impact of non-natural residues on MMP specificity for an MMP peptide consensus sequence. Peptoids are non-natural, N-substituted glycines with a large side-chain diversity. Given the presence of a hallmark proline residue in the P3 position of MMP consensus sequences, we hypothesized that peptoids may offer N-substituted alternatives to generate differential interactions with MMPs. To investigate this hypothesis, peptomer substrates were exposed to five different MMPs, as well as bacterial collagenase, and monitored by fluorescence resonance energy transfer and liquid chromatography-mass spectrometry to determine the rate of cleavage and the composition of degraded fragments, respectively. We found that peptoid residues are well tolerated in the P3 and P3' substrate sites and that the identity of the peptoid in these sites displays a moderate influence on the rate of cleavage. However, peptoid residues were even better tolerated in the P1 substrate site where activity was more strongly correlated with side-chain identity than side-chain position. All MMPs explored demonstrated similar trends in specificity for the peptomers but exhibited different degrees of variability in proteolytic rate. These kinetic profiles served as "fingerprints" for the proteases and yielded separation by multivariate data analysis. To further demonstrate the practical application of this tunability in degradation kinetics, peptomer substrates were tethered into hydrogels and released over distinct timescales. Overall, this work represents a significant step toward the design of probes that maximize differential MMP behavior and presents design rules to tune degradation kinetics with peptoid substitutions, which has promising implications for diagnostic and prognostic applications using array-based sensors.

Boosting the Clinical Translation of Organ-on-a-Chip Technology

Organ-on-a-chip devices have become a viable option for investigating critical physiological events and responses; this technology has matured substantially, and many systems have been reported for disease modeling or drug screening over the last decade. Despite the wide acceptance in the academic community, their adoption by clinical end-users is still a non-accomplished promise. The reasons behind this difficulty can be very diverse but most likely are related to the lack of predictive power, physiological relevance, and reliability necessary for being utilized in the clinical area. In this Perspective, we briefly discuss the main attributes of organ-on-a-chip platforms in academia and how these characteristics impede their easy translation to the clinic. We also discuss how academia, in conjunction with the industry, can contribute to boosting their adoption by proposing novel design concepts, fabrication methods, processes, and manufacturing materials, improving their standardization and versatility, and simplifying their manipulation and reusability.

Carbon Dots Embedded Hybrid Microgel with Phenylboronic Acid as Monomer for Fluorescent Glucose Sensing and Glucose-Triggered Insulin Release at Physiological pH

A multifunctional and biocompatible hybrid microgel (poly(VPBA-AAm)-CD) using N, S-doped carbon dots (CDs) and ethylene glycol dimethacrylate (EGDMA) as cross-linking agents, and 4-vinylbenzene boronic acid (VPBA) and acrylamide (AAm) as monomers, was designed in this work. This microgel can be easily prepared by a simple one-pot radical dispersion polymerization of the reactants using a rationally designed hydrogen-bonded complex method. The hybrid microgels were spherical particles with a smooth surface and an average particle size of 234 ± 8 nm. The poly(VPBA-AAm)-CD microgel displayed the glucose-responsive swelling within a clinically concerned range at a physiological pH and could realize the controllable release of insulin. In addition, the release rate of insulin in the hybrid microgel (poly(VPBA-AAm)-CD) could be triggered by glucose concentrations in the solution, and the increasing glucose concentrations can accelerate the insulin release. Further in vitro cytotoxicity studies showed that the microgel had good biocompatibility and no obvious toxicity to the cells. These indicate that the prepared microgel (poly(VPBA-AAm)-CD) may supply a new pattern for the self-regulating therapy of insulin deficiency in diabetes.

4D Printing of Extrudable and Degradable Poly(Ethylene Glycol) Microgel Scaffolds for Multidimensional Cell Culture

Granular synthetic hydrogels are useful bioinks for their compatibility with a variety of chemistries, affording printable, stimuli-responsive scaffolds with programmable structure and function. Additive manufacturing of microscale hydrogels, or microgels, allows for the fabrication of large cellularized constructs with percolating interstitial space, providing a platform for tissue engineering at length scales that are inaccessible by bulk encapsulation where transport of media and other biological factors are limited by scaffold density. Herein, synthetic microgels with varying degrees of degradability are prepared with diameters on the order of hundreds of microns by submerged electrospray and UV photopolymerization. Porous microgel scaffolds are assembled by particle jamming and extrusion printing, and semi-orthogonal chemical cues are utilized to tune the void fraction in printed scaffolds in a logic-gated manner. Scaffolds with different void fractions are easily cellularized post printing and microgels can be directly annealed into cell-laden structures. Finally, high-throughput direct encapsulation of cells within printable microgels is demonstrated, enabling large-scale 3D culture in a macroporous biomaterial. This approach provides unprecedented spatiotemporal control over the properties of printed microporous annealed particle scaffolds for 2.5D and 3D tissue culture.

Hydrogels as functional components in artificial cell systems

Recent years have seen substantial efforts aimed at constructing artificial cells from various molecular components with the aim of mimicking the processes, behaviours and architectures found in biological systems. Artificial cell development ultimately aims to produce model constructs that progress our understanding of biology, as well as forming the basis for functional bio-inspired devices that can be used in fields such as therapeutic delivery, biosensing, cell therapy and bioremediation. Typically, artificial cells rely on a bilayer membrane chassis and have fluid aqueous interiors to mimic biological cells. However, a desire to more accurately replicate the gel-like properties of intracellular and extracellular biological environments has driven increasing efforts to build cell mimics based on hydrogels. This has enabled researchers to exploit some of the unique functional properties of hydrogels that have seen them deployed in fields such as tissue engineering, biomaterials and drug delivery. In this Review, we explore how hydrogels can be leveraged in the context of artificial cell development. We also discuss how hydrogels can potentially be incorporated within the next generation of artificial cells to engineer improved biological mimics and functional microsystems.

Trends in Droplet Microfluidics: From Droplet Generation to Biomedical Applications

Over the past decade, droplet microfluidics has attracted growing interest in biology, medicine, and engineering. In this feature article, we review the advances in droplet microfluidics, primarily focusing on the research conducted by our group. Starting from the introduction to the mechanisms of microfluidic droplet formation and the strategies for cell encapsulation in droplets, we then focus on droplet transformation into microgels. Furthermore, we review three biomedical applications of droplet microfluidics, that is, 3D cell culture, single-cell analysis, and in vitro organ and disease modeling. We conclude with our perspective on future directions in the development of droplet microfluidics for biomedical applications.

Impact of Collagen Triple Helix Structure on Melanoma Cell Invadopodia Formation and Matrix Degradation upon BRAF Inhibitor Treatment

A collagen-rich tumor microenvironment (TME) is associated with worse outcomes in cancer patients and contributes to drug resistance in many cancer types. In melanoma, stiff and fibrillar collagen-abundant tissue is observed after failure of therapeutic treatments with BRAF inhibitors. Increased collagen in the TME can affect properties of the extracellular matrix (ECM), including stiffness, adhesiveness, and interaction of integrins with triple helix forming nanostructures. Decoupling these biochemical and biophysical properties of the ECM can lead to a better understanding of how each of these individual properties affect melanoma cancer behavior and drug efficacy. In addition, as drug treatment can induce cancer cell phenotypic switch, cancer cell responsiveness to the TME can be dynamically changed during therapeutic treatments. To investigate cancer cell phenotype changes and the role of the cancer TME, poly(ethylene glycol) (PEG) hydrogels functionalized with collagen mimetic peptides (CMPs) is utilized, or an interpenetrating network (IPN) of type І collagen within the PEG system to culture various melanoma cell lines in the presence or absence of Vemurafenib (PLX4032) drug treatment is prepared. Additionally, the potential of using CMP functionalized PEG hydrogels, which can provide better tunability is explored, to replace the existing invadopodia assay platform based on fluorescent gelatin.

Precision biomaterials in cancer theranostics and modelling

Despite significant achievements in the understanding and treatment of cancer, it remains a major burden. Traditional therapeutic approaches based on the 'one-size-fits-all' paradigm are becoming obsolete, as demonstrated by the increasing number of patients failing to respond to treatments. In contrast, more precise approaches based on individualized genetic profiling of tumors have already demonstrated their potential. However, even more personalized treatments display shortcomings mainly associated with systemic delivery, such as low local drug efficacy or specificity. A large amount of effort is currently being invested in developing precision medicine-based strategies for improving the efficiency of cancer theranostics and modelling, which are envisioned to be more accurate, standardized, localized, and less expensive. To this end, interdisciplinary research fields, such as biomedicine, material sciences, pharmacology, chemistry, tissue engineering, and nanotechnology, must converge for boosting the precision cancer ecosystem. In this regard, precision biomaterials have emerged as a promising strategy to detect, model, and treat cancer more efficiently. These are defined as those biomaterials precisely engineered with specific theranostic functions and bioactive components, with the possibility to be tailored to the cancer patient needs, thus having a vast potential in the increasing demand for more efficient treatments. In this review, we discuss the latest advances in the field of precision biomaterials in cancer research, which are expected to revolutionize disease management, focusing on their uses for cancer modelling, detection, and therapeutic applications. We finally comment on the needed requirements to accelerate their application in the clinic to improve cancer patient prognosis.

Direct comparison of angiogenesis in natural and synthetic biomaterials reveals that matrix porosity regulates endothelial cell invasion speed and sprout diameter

Vascularization of large, diffusion-hindered biomaterial implants requires an understanding of how extracellular matrix (ECM) properties regulate angiogenesis. Sundry biomaterials assessed across many disparate angiogenesis assays have highlighted ECM determinants that influence this complex multicellular process. However, the abundance of material platforms, each with unique parameters to model endothelial cell (EC) sprouting presents additional challenges of interpretation and comparison between studies. In this work we directly compared the angiogenic potential of commonly utilized natural (collagen and fibrin) and synthetic dextran vinyl sulfone (DexVS) hydrogels in a multiplexed angiogenesis-on-a-chip platform. Modulating matrix density of collagen and fibrin hydrogels confirmed prior findings that increases in matrix density correspond to increased EC invasion as connected, multicellular sprouts, but with decreased invasion speeds. Angiogenesis in synthetic DexVS hydrogels, however, resulted in fewer multicellular sprouts. Characterizing hydrogel Young's modulus and permeability (a measure of matrix porosity), we identified matrix permeability to significantly correlate with EC invasion depth and sprout diameter. Although microporous collagen and fibrin hydrogels produced lumenized sprouts in vitro, they rapidly resorbed post-implantation into the murine epididymal fat pad. In contrast, DexVS hydrogels proved comparatively stable. To enhance angiogenesis within DexVS hydrogels, we incorporated sacrificial microgels to generate cell-scale pores throughout the hydrogel. Microporous DexVS hydrogels resulted in lumenized sprouts in vitro and enhanced cell invasion in vivo. Towards the design of vascularized biomaterials for long-term regenerative therapies, this work suggests that synthetic biomaterials offer improved size and shape control following implantation and that tuning matrix porosity may better support host angiogenesis. STATEMENT OF SIGNIFICANCE: Understanding how extracellular matrix properties govern angiogenesis will inform biomaterial design for engineering vascularized implantable grafts. Here, we utilized a multiplexed angiogenesis-on-a-chip platform to compare the angiogenic potential of natural (collagen and fibrin) and synthetic dextran vinyl sulfone (DexVS) hydrogels. Characterization of matrix properties and sprout morphometrics across these materials points to matrix porosity as a critical regulator of sprout invasion speed and diameter, supported by the observation that nanoporous DexVS hydrogels yielded endothelial cell sprouts that were not perfusable. To enhance angiogenesis into synthetic hydrogels, we incorporated sacrificial microgels to generate microporosity. We find that microporosity increased sprout diameter in vitro and cell invasion in vivo. This work establishes a composite materials approach to enhance the vascularization of synthetic hydrogels.

Two-step deswelling in the Volume Phase Transition of thermoresponsive microgels

Microgels, colloidal-scale polymer networks, are the prototype soft colloids. When the constituent polymers are thermoresponsive, they undergo a volume phase transition (VPT) from a swollen to a collapsed state at a characteristic temperature, close to ambient one, of great appeal for several applications. To describe this phenomenon, microgels are usually treated as neutral, but here we show that electrostatics needs to be taken into account. In particular, deswelling occurs via a two-step, rather than a homogeneous, particle collapse, mainly driven by peripheral charges located on the microgel corona, for which we also establish a unifying framework encompassing all studied microgels. Our work thus provides a change of perspective to describe these fascinating systems.

Thermoresponsive microgels are one of the most investigated types of soft colloids, thanks to their ability to undergo a Volume Phase Transition (VPT) close to ambient temperature. However, this fundamental phenomenon still lacks a detailed microscopic understanding, particularly regarding the presence and the role of charges in the deswelling process. This is particularly important for the widely used poly(N-isopropylacrylamide)–based microgels, where the constituent monomers are neutral but charged groups arise due to the initiator molecules used in the synthesis. Here, we address this point combining experiments with state-of-the-art simulations to show that the microgel collapse does not happen in a homogeneous fashion, but through a two-step mechanism, entirely attributable to electrostatic effects. The signature of this phenomenon is the emergence of a minimum in the ratio between gyration and hydrodynamic radii at the VPT. Thanks to simulations of microgels with different cross-linker concentrations, charge contents, and charge distributions, we provide evidence that peripheral charges arising from the synthesis are responsible for this behavior and we further build a universal master curve able to predict the two-step deswelling. Our results have direct relevance on fundamental soft condensed matter science and on applications where microgels are involved, ranging from materials to biomedical technologies.

Direct monitoring of protease activity using an integrated microchip coated with multilayered fluorogenic nanofilms

Proteases play an essential role in the four sequential but overlapping phases of wound healing: hemostasis, inflammation, proliferation, and remodeling. In chronic wounds, excessive protease secretion damages the newly formed extracellular matrix, thereby delaying or preventing the normal healing process. Peptide-based fluorogenic sensors provide a visual platform to sense and analyze protease activity through changes in the fluorescence intensity. Here, we have developed an integrated microfluidic chip coated with multilayered fluorogenic nanofilms that can directly monitor protease activity. Fluorogenic protease sensors were chemically conjugated to polymer films coated on the surface of parallel microfluidic channels. Capillary flow layer-by-layer (CF-LbL) was used for film assembly and combined with subsequent sensor modification to establish a novel platform sensing technology. The benefits of our platform include facile fabrication and processing, controllable film nanostructure, small sample volume, and high sensitivity. We observed increased fluorescence of the LbL nanofilms when they were exposed to model recombinant proteases, confirming their responsiveness to protease activity. Increases in the nanofilms' fluorescence intensity were also observed during incubation with liquid extracted from murine infected wounds, demonstrating the potential of these films to provide real-time, in situ information about protease activity levels.

Dynamic Endothelial Stalk Cell-Matrix Interactions Regulate Angiogenic Sprout Diameter

Angiogenesis is a complex, multicellular process that involves bidirectional interactions between extracellular matrix (ECM) and collectively invading endothelial cell (EC) sprouts that extend the microvasculature during development, wound healing, and disease processes. While many aspects of angiogenesis have been well studied, the relationship between endothelial sprout morphology and subsequent neovessel function remains relatively unknown. Here, we investigated how various soluble and physical matrix cues that regulate endothelial sprouting speed and proliferation correspond to changes in sprout morphology, namely, sprout stalk diameter. We found that sprout stalk cells utilize a combination of cytoskeletal forces and proteolysis to physically compact and degrade the surrounding matrix, thus creating sufficient space in three-dimensional (3D) ECM for lateral expansion. As increasing sprout diameter precedes lumenization to generate perfusable neovessels, this work highlights how dynamic endothelial stalk cell-ECM interactions promote the generation of functional neovessels during sprouting angiogenesis to provide insight into the design of vascularized, implantable biomaterials.

Potential of Drug Efficacy Evaluation in Lung and Kidney Cancer Models Using Organ-on-a-Chip Technology

Organ-on-a-chip (OoC) is an exponential technology with the potential to revolutionize disease, toxicology research, and drug discovery. Recent advances in OoC could be utilized for drug screening in disease models to evaluate the efficacy of new therapies and support new tools for the understanding of disease mechanisms. Rigorous validation of this technology is required to determine whether OoC models may represent human-relevant physiology and predict clinical outcomes in target disease models. Achievements in the OoC field could reveal exciting new avenues for drug development and discovery. This review attempts to highlight the benefits of OoC as per our understanding of the cellular and molecular pathways in lung and kidney cancer models, and discusses the challenges in evaluating drug efficacy.

Kinetics of the thermal response of poly(N-isopropylacrylamide co methacrylic acid) hydrogel microparticles under different environmental stimuli: A time-lapse NMR study

HYPOTHESIS

Hydrogels of N-isopropylacrylamide and methacrylic acid (P(NIPAm-co-MAA)) display pH sensitivity and complex positively charged molecules through carboxylate groups, while having a critical solution temperature at which they reduce in volume and dehydrate. We aimed to elucidate how the responsiveness of MAA to environmental changes alters PNIPAm hydrogels at the molecular level using nuclear magnetic resonance (NMR). Time-lapse NMR allows us to follow the evolution of NMR signal under a temperature stimulus, providing unique information on conformational freedom of the hydrogel polymers.

EXPERIMENTS

We used time-lapse NMR to follow the evolution of the NMR signal with time over a temperature change from 25 to 40°C and to study the swelling/deswelling kinetics of P(NIPAm-co-MAA) microgels at different pH values and ionic strengths, and in the presence of positively charged molecules complexing carboxylate groups.

FINDINGS

At acid pH, hydrogel collapse is favored over neutral pH, and at basic pH the carboxylates remain steadily hydrated during temperature increase. Increasing ionic strength results in a faster, more effective collapse than decreasing pH. Complexation of medium-sized molecules with several charges (spermine, spermidine) causes a faster collapse than complexation with large molecular weight poly(allylamine) hydrochloride, but similar to the collapse effected by large poly(diallyldimethylammonium) chloride. This work opens new perspectives to using time-lapse NMR to study thermoresponsive systems that respond to multiple stimuli, with particular relevance in designing hydrogels for drug delivery.

Organs-on-chips: into the next decade

Organs-on-chips (OoCs), also known as microphysiological systems or 'tissue chips' (the terms are synonymous), have attracted substantial interest in recent years owing to their potential to be informative at multiple stages of the drug discovery and development process. These innovative devices could provide insights into normal human organ function and disease pathophysiology, as well as more accurately predict the safety and efficacy of investigational drugs in humans. Therefore, they are likely to become useful additions to traditional preclinical cell culture methods and in vivo animal studies in the near term, and in some cases replacements for them in the longer term. In the past decade, the OoC field has seen dramatic advances in the sophistication of biology and engineering, in the demonstration of physiological relevance and in the range of applications. These advances have also revealed new challenges and opportunities, and expertise from multiple biomedical and engineering fields will be needed to fully realize the promise of OoCs for fundamental and translational applications. This Review provides a snapshot of this fast-evolving technology, discusses current applications and caveats for their implementation, and offers suggestions for directions in the next decade.

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.

Natural-based Hydrogels: A Journey from Simple to Smart Networks for Medical Examination

Natural hydrogels, due to their unique biological properties, have been used extensively for various medical and clinical examinations that are performed to investigate the signs of disease. Recently, complex-crosslinking strategies improved the mechanical properties and advanced approaches have resulted in the introduction of naturally derived hydrogels that exhibit high biocompatibility, with shape memory and self-healing characteristics. Moreover, the creation of self-assembled natural hydrogels under physiological conditions has provided the opportunity to engineer fine-tuning properties. To highlight recent studies of natural-based hydrogels and their applications for medical investigation, a critical review was undertaken using published papers from the Science Direct database. This review presents different natural-based hydrogels (natural, natural-synthetic hybrid and complex-crosslinked hydrogels), their historical evolution, and recent studies of medical examination applications. The application of natural-based hydrogels in the design and fabrication of biosensors, catheters and medical electrodes, detection of cancer, targeted delivery of imaging compounds (bioimaging) and fabrication of fluorescent bioprobes is summarised here. Without doubt, in future, more useful and practical concepts will be derived to identify natural-based hydrogels for a wide range of clinical examination applications.

Goosebumps-Inspired Microgel Patterns with Switchable Adhesion and Friction

A substrate mimicking the surface topography and temperature sensitivity of skin goosebumps is fabricated. Close-packed arrays of thermoresponsive microgel particles undergo topographical changes in response to temperature changes between 25 and 37 °C, resembling the goosebump structure that human skin develops in response to temperature changes or other circumstances. Specifically, positively charged poly[2-(methacryloyloxy)ethyltrimethylammonium chloride] (PMETAC) brushes serve as an anchoring substrate for negatively charged poly(NIPAm-co-AA) microgels. The packing density and particle morphology can be tuned by brush layer thickness and pH of the microgel suspension. For brush layer thickness below 50 nm, particle monolayers are observed, with slightly flattened particle morphology at pH 3 and highly collapsed particles at pH above 7. Polymer brush films with thickness above 50 nm lead to the formation of particle multilayers. The temperature responsiveness of the monolayer assemblies allows reversible changes in the film morphology, which in turn affects underwater adhesion and friction at 25 and 37 °C. These results are promising for the design of new functional materials and may also serve as a model for biological structures and processes.

Matrix-metalloproteinase expression and gelatinase activity in the avian retina and their influence on Müller glia proliferation

Gelatinases are a class of matrix metalloproteinases (MMPs) that degrade the extracellular matrix (ECM) to regulate intercellular signaling and cell migration. Gelatinase activity is tightly regulated via proteolytic activation and through the expression of tissue inhibitors of matrix metalloproteinases (TIMPs). Gelatinase activity has been implicated in retinal pathophysiology in different animal models and human disease. However, the role of gelatinases in retinal regeneration remains uncertain. In this study we investigated the dynamic changes in gelatinase activity in response to excitotoxic damage and how this enzymatic activity influenced the formation of Müller glia progenitor cells (MGPCs) in the avian retina. This study used hydrogels containing a gelatinase-degradable fluorescent peptide to measure gelatinase activity in vitro and dye quenched gelatin to localize enzymatic activity in situ. These data were corroborated by using single cell RNA sequencing (scRNA-seq). Gelatinase mRNA, specifically MMP2, was detected in oligodendrocytes and Non-Astrocytic Inner Retinal Glia (NIRG). Total retinal gelatinase activity was reduced following NMDA-treatment, and sustained inhibition of MMP2 prior to damage or growth factor treatment increased the formation of proliferating MGPCs and c-fos signaling. We observed that microglia, Müller glia (MG), and NIRG cells were involved in regulating changes in gelatinase activity through TIMP2 and TIMP3. Collectively, these findings implicate MMP2 in reprogramming of Muller glia into MGPCs.

Microparticles in Contact with Cells: From Carriers to Multifunctional Tissue Modulators

For several decades microparticles have been exclusively and extensively explored as spherical drug delivery vehicles and large-scale cell expansion carriers. More recently, microparticulate structures gained interest in broader bioengineering fields, integrating myriad strategies that include bottom-up tissue engineering, 3D bioprinting, and the development of tissue/disease models. The concept of bulk spherical micrometric particles as adequate supports for cell cultivation has been challenged, and systems with finely tuned geometric designs and (bio)chemical/physical features are current key players in impacting technologies. Herein, we critically review the state of the art and future trends of biomaterial microparticles in contact with cells and tissues, excluding internalization studies, and with emphasis on innovative particle design and applications.

Enzymatic Cross-Linking of Dynamic Thiol-Norbornene Click Hydrogels

Enzyme-mediated in situ forming hydrogels are attractive for many biomedical applications because gelation afforded by the enzymatic reactions can be readily controlled not only by tuning macromer compositions, but also by adjusting enzyme kinetics. For example, horseradish peroxidase (HRP) has been used extensively for in situ crosslinking of macromers containing hydroxyl-phenol groups. The use of HRP on initiating thiol-allylether polymerization has also been reported, yet no prior study has demonstrated enzymatic initiation of thiol-norbornene gelation. In this study, we discovered that HRP can generate thiyl radicals needed for initiating thiol-norbornene hydrogelation, which has only been demonstrated previously using photopolymerization. Enzymatic thiol-norbornene gelation not only overcomes light attenuation issue commonly observed in photopolymerized hydrogels, but also preserves modularity of the crosslinking. In particular, we prepared modular hydrogels from two sets of norbornene-modified macromers, 8-arm poly(ethylene glycol)-norbornene (PEG8NB) and gelatin-norbornene (GelNB). Bis-cysteine-containing peptides or PEG-tetra-thiol (PEG4SH) were used as crosslinkers for forming enzymatically and orthogonally polymerized hydrogels. For HRP-initiated PEG-peptide hydrogel crosslinking, gelation efficiency was significantly improved via adding tyrosine residues on the peptide crosslinkers. Interestingly, these additional tyrosine residues did not form permanent dityrosine crosslinks following HRP-induced gelation. As a result, they remained available for tyrosinase-mediated secondary crosslinking, which dynamically increases hydrogel stiffness. In addition to material characterizations, we also found that both PEG- and gelatin-based hydrogels provide excellent cytocompatibility for dynamic 3D cell culture. The enzymatic thiol-norbornene gelation scheme presented here offers a new crosslinking mechanism for preparing modularly and dynamically crosslinked hydrogels.