Rational Design of Hydrogel Networks with Dynamic Mechanical Properties to Mimic Matrix Remodeling

Engineering complex tissue-like microenvironments with biomaterials and biofabrication

Advances in tissue engineering for both system modeling and organ regeneration depend on embracing and recapitulating the target tissue's functional and structural complexity. Microenvironmental features such as anisotropy, heterogeneity, and other biochemical and mechanical spatiotemporal cues are essential in regulating tissue development and function. Novel biofabrication strategies and innovative biomaterial design have emerged as promising tools to better reproduce such features. These facilitate a transition towards high-fidelity biomimetic structures, offering opportunities for a deeper understanding of tissue function and the development of superior therapies. In this review, we explore some of the key structural and compositional aspects of tissues, lay out how to achieve similar outcomes with current fabrication strategies, and identify the main challenges and promising avenues for future research.

Structurally decoupled hyaluronic acid hydrogels for studying matrix metalloproteinase-mediated invasion of metastatic breast cancer cells

In recent years, polymeric hydrogels have been employed to investigate cancer cell-extracellular matrix (ECM) interactions in vitro. In the context of breast cancer, cancer cells are known to degrade the ECM using matrix-metalloproteinases (MMPs) to support invasion resulting in disease progression. Polymeric hydrogels incorporating MMP-cleavable peptides have been employed to study cancer cell invasion, however, the approaches employed to incorporate these peptides often change other hydrogel properties. This underscores the need for decoupling hydrogel properties while incorporating MMP-cleavable peptides. Herein, we report structurally decoupled hyaluronic acid (HA) hydrogels formulated using varying ratios of a biologically sensitive MMP-cleavable peptide and an insensitive counterpart (Dithiothreitol (DTT) or polyethylene glycol dithiol (PEGDT)) to study MMP-mediated metastatic breast cancer cell invasion. Rheological, swelling ratio, estimated mesh size, and permeability measurements showed similar mechanical and physical properties for hydrogels crosslinked with different DTT (or PEGDT)/MMP ratios. However, their degradation rate in the presence of collagenase correlated with the ratio of MMP-cleavable peptide. Encapsulated metastatic breast cancer spheroids in HA hydrogels with MMP sensitivity exhibited increased invasiveness compared to those without MMP sensitivity after 14 days of culture. Overall, such structurally decoupled HA hydrogels provide a platform to study MMP-mediated breast cancer cell invasion in vitro.

Stepwise Stiffening/Softening of and Cell Recovery from Reversibly Formulated Hydrogel Interpenetrating Networks

Biomechanical contributions of the extracellular matrix underpin cell growth and proliferation, differentiation, signal transduction, and other fate decisions. As such, biomaterials whose mechanics can be spatiotemporally altered- particularly in a reversible manner- are extremely valuable for studying these mechanobiological phenomena. Herein, a poly(ethylene glycol) (PEG)-based hydrogel model consisting of two interpenetrating step-growth networks is introduced that are independently formed via largely orthogonal bioorthogonal chemistries and sequentially degraded with distinct recombinant sortases, affording reversibly tunable stiffness ranges that span healthy and diseased soft tissues (e.g., 500 Pa-6 kPa) alongside terminal cell recovery for pooled and/or single-cell analysis in a near "biologically invisible" manner. Spatiotemporal control of gelation within the primary supporting network is achieved via mask-based and two-photon lithography; these stiffened patterned regions can be subsequently returned to the original soft state following sortase-based secondary network degradation. Using this approach, the effects of 4D-triggered network mechanical changes on human mesenchymal stem cell morphology and Hippo signaling, as well as Caco-2 colorectal cancer cell mechanomemory using transcriptomics and metabolic assays are investigated. This platform is expected to be of broad utility for studying and directing mechanobiological phenomena, patterned cell fate, and disease resolution in softer matrices.

Engineered Protein Hydrogels as Biomimetic Cellular Scaffolds

The biochemical and biophysical properties of the extracellular matrix (ECM) play a pivotal role in regulating cellular behaviors such as proliferation, migration, and differentiation. Engineered protein-based hydrogels, with highly tunable multifunctional properties, have the potential to replicate key features of the native ECM. Formed by self-assembly or crosslinking, engineered protein-based hydrogels can induce a range of cell behaviors through bioactive and functional domains incorporated into the polymer backbone. Using recombinant techniques, the amino acid sequence of the protein backbone can be designed with precise control over the chain-length, folded structure, and cell-interaction sites. In this review, the modular design of engineered protein-based hydrogels from both a molecular- and network-level perspective are discussed, and summarize recent progress and case studies to highlight the diverse strategies used to construct biomimetic scaffolds. This review focuses on amino acid sequences that form structural blocks, bioactive blocks, and stimuli-responsive blocks designed into the protein backbone for highly precise and tunable control of scaffold properties. Both physical and chemical methods to stabilize dynamic protein networks with defined structure and bioactivity for cell culture applications are discussed. Finally, a discussion of future directions of engineered protein-based hydrogels as biomimetic cellular scaffolds is concluded.

Use of photosensitive molecules in the crosslinking of biopolymers: applications and considerations in biomaterials development

The development of diverse types of biomaterials has significantly contributed to bringing new biomedical strategies to treat clinical conditions. Applications of these biomaterials can range from mechanical support and protection of injured tissues to joint replacement, tissue implants, and drug delivery systems. Among the strategies commonly used to prepare biomaterials, the use of electromagnetic radiation to initiate crosslinking stands out. The predominance of photo-induced polymerization methods relies on a fast, efficient, and straightforward process that can be easily adjusted to clinical needs. This strategy consists of irradiating the components that form the material with photons in the near ultraviolet-visible wavelength range (i.e., ∼310 to 750 nm) in the presence of a photoactive molecule. Upon photon absorption, photosensitive molecules can generate excited species that initiate photopolymerization through different reaction mechanisms. However, this process could promote undesired side reactions depending on the target zone or treatment type (e.g., oxidative stress and modification of biomolecules such as proteins and lipids). This review explores the basic concepts behind the photopolymerization process of ex situ and in situ biomaterials. Particular emphasis was put on the photosensitization initiated by the most employed photosensitizers and the photoreactions that they mediate in aqueous media. Finally, the undesired oxidation reactions at the bio-interface and potential solutions are presented.

Outlook and opportunities for engineered environments of breast cancer dormancy

Dormant, disseminated breast cancer cells resist treatment and may relapse into malignant metastases after decades of quiescence. Identifying how and why these dormant breast cancer cells are triggered into outgrowth is a key unsolved step in treating latent, metastatic breast cancer. However, our understanding of breast cancer dormancy in vivo is limited by technical challenges and ethical concerns with triggering the activation of dormant breast cancer. In vitro models avoid many of these challenges by simulating breast cancer dormancy and activation in well-controlled, bench-top conditions, creating opportunities for fundamental insights into breast cancer biology that complement what can be achieved through animal and clinical studies. In this review, we address clinical and preclinical approaches to treating breast cancer dormancy, how precisely controlled artificial environments reveal key interactions that regulate breast cancer dormancy, and how future generations of biomaterials could answer further questions about breast cancer dormancy.

Neurogenic Cell Behavior in 3D Culture Enhanced Within a Highly Compliant Synthetic Hydrogel Platform Formed via Competitive Crosslinking

Purpose

Scaffold materials that better support neurogenesis are still needed to improve cell therapy outcomes for neural tissue damage. We have used a modularly tunable, highly compliant, degradable hydrogel to explore the impacts of hydrogel compliance stiffness on neural differentiation. Here we implemented competitive matrix crosslinking mechanics to finely tune synthetic hydrogel moduli within soft tissue stiffnesses, a range much softer than typically achievable in synthetic crosslinked hydrogels, providing a modularly controlled and ultrasoft 3D culture model which supports and enhances neurogenic cell behavior.

Methods

Soluble competitive allyl monomers were mixed with proteolytically-degradable poly(ethylene glycol) diacrylate derivatives and crosslinked to form a matrix, and resultant hydrogel stiffness and diffusive properties were evaluated. Neural PC12 cells or primary rat fetal neural stem cells (NSCs) were encapsulated within the hydrogels, and cell morphology and phenotype were investigated to understand cell-matrix interactions and the effects of environmental stiffness on neural cell behavior within this model.

Results

Addition of allyl monomers caused a concentration-dependent decrease in hydrogel compressive modulus from 4.40 kPa to 0.26 kPa (natural neural tissue stiffness) without influencing soluble protein diffusion kinetics through the gel matrix. PC12 cells encapsulated in the softest hydrogels showed significantly enhanced neurite extension in comparison to PC12s in all other hydrogel stiffnesses tested. Encapsulated neural stem cells demonstrated significantly greater spreading and elongation in 0.26 kPa alloc hydrogels than in 4.4 kPa hydrogels. When soluble growth factor deprivation (for promotion of neural differentiation) was evaluated within the neural stiffness gels (0.26 kPa), NSCs showed increased neuronal marker expression, indicating early enhancement of neurogenic differentiation.

Conclusions

Implementing allyl-acrylate crosslinking competition reduced synthetic hydrogel stiffness to provide a supportive environment for 3D neural tissue culture, resulting in enhanced neurogenic behavior of encapsulated cells. These results indicate the potential suitability of this ultrasoft hydrogel system as a model platform for further investigating environmental factors on neural cell behavior.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12195-024-00794-2.

On the path to predicting immune responses in the lung: Modeling the pulmonary innate immune system at the air-liquid interface (ALI)

Chronic respiratory diseases and infections are among the largest contributors to death globally, many of which still have no cure, including chronic obstructive pulmonary disorder, idiopathic pulmonary fibrosis, and respiratory syncytial virus among others. Pulmonary therapeutics afford untapped potential for treating lung infection and disease through direct delivery to the site of action. However, the ability to innovate new therapeutic paradigms for respiratory diseases will rely on modeling the human lung microenvironment and including key cellular interactions that drive disease. One key feature of the lung microenvironment is the air-liquid interface (ALI). ALI interface modeling techniques, using cell-culture inserts, organoids, microfluidics, and precision lung slices (PCLS), are rapidly developing; however, one major component of these models is lacking-innate immune cell populations. Macrophages, neutrophils, and dendritic cells, among others, represent key lung cell populations, acting as the first responders during lung infection or injury. Innate immune cells respond to and modulate stromal cells and bridge the gap between the innate and adaptive immune system, controlling the bodies response to foreign pathogens and debris. In this article, we review the current state of ALI culture systems with a focus on innate immune cells and suggest ways to build on current models to add complexity and relevant immune cell populations.

Crossover of surface waves and capillary-viscous-elastic transition in soft biomaterials detected by resonant acoustic rheometry

Viscoelastic properties of hydrogels are important for their application in science and industry. However, rheological assessment of soft hydrogel biomaterials is challenging due to their complex, rapid, and often time-dependent behaviors. Resonant acoustic rheometry (RAR) is a newly developed technique capable of inducing and measuring resonant surface waves in samples in a non-contact fashion. By applying RAR at high temporal resolution during thrombin-induced fibrin gelation and ultraviolet-initiated polyethylene glycol (PEG) polymerization, we observed distinct changes in both frequency and amplitude of the resonant surface waves as the materials changed over time. RAR detected a series of capillary-elastic, capillary-viscous, and visco-elastic transitions that are uniquely manifested as crossover of different types of surface waves in the temporally evolving materials. These results reveal the dynamic interplay of surface tension, viscosity, and elasticity that is controlled by the kinetics of polymerization and crosslinking during hydrogel formation. RAR overcomes many limitations of conventional rheological approaches by offering a new way to comprehensively and longitudinally characterize soft materials during dynamic processes.

Structurally decoupled stiffness and solute transport in multi-arm poly(ethylene glycol) hydrogels

Synthetic hydrogels are widely used as artificial 3D environments for cell culture, facilitating the controlled study of cell-environment interactions. However, most hydrogels are limited in their ability to represent the physical properties of biological tissues because stiffness and solute transport properties in hydrogels are closely correlated. Resultingly, experimental investigations of cell-environment interactions in hydrogels are confounded by simultaneous changes in multiple physical properties. Here, we overcame this limitation by simultaneously manipulating four structural parameters to synthesize a library of multi-arm poly (ethylene glycol) (PEG) hydrogel formulations with robustly decoupled stiffness and solute transport. This structural design approach avoids chemical alterations or additions to the network that might have unanticipated effects on encapsulated cells. An algorithm created to statistically evaluate stiffness-transport decoupling within the dataset identified 46 of the 73 synthesized formulations as robustly decoupled. We show that the swollen polymer network model accurately predicts 11 out of 12 structure-property relationships, suggesting that this approach to decoupling stiffness and solute transport in hydrogels is fundamentally validated and potentially broadly applicable. Furthermore, the unprecedented control of hydrogel network structure provided by multi-arm PEG hydrogels confirmed several fundamental modeling assumptions. This study enables nuanced hydrogel design for uncompromised investigation of cell-environment interactions.

Dynamic Hydrogels with Viscoelasticity and Tunable Stiffness for the Regulation of Cell Behavior and Fate

The extracellular matrix (ECM) of natural cells typically exhibits dynamic mechanical properties (viscoelasticity and dynamic stiffness). The viscoelasticity and dynamic stiffness of the ECM play a crucial role in biological processes, such as tissue growth, development, physiology, and disease. Hydrogels with viscoelasticity and dynamic stiffness have recently been used to investigate the regulation of cell behavior and fate. This article first emphasizes the importance of tissue viscoelasticity and dynamic stiffness and provides an overview of characterization techniques at both macro- and microscale. Then, the viscoelastic hydrogels (crosslinked via ion bonding, hydrogen bonding, hydrophobic interactions, and supramolecular interactions) and dynamic stiffness hydrogels (softening, stiffening, and reversible stiffness) with different crosslinking strategies are summarized, along with the significant impact of viscoelasticity and dynamic stiffness on cell spreading, proliferation, migration, and differentiation in two-dimensional (2D) and three-dimensional (3D) cell cultures. Finally, the emerging trends in the development of dynamic mechanical hydrogels are discussed.

Systematic d-Amino Acid Substitutions to Control Peptide and Hydrogel Degradation in Cellular Microenvironments

Enzymatically degradable peptides are commonly used as linkers within hydrogels for biological applications; however, controlling the degradation of these engineered peptides with different contexts and cell types can prove challenging. In this work, we systematically examined the substitution of d-amino acids (D-AAs) for different l-amino acids in a peptide sequence commonly utilized in enzymatically degradable hydrogels (VPMS↓MRGG) to create peptide linkers with a range of different degradation times, in solution and in hydrogels, and investigated the cytocompatibility of these materials. We found that increasing the number of D-AA substitutions increased the resistance to enzymatic degradation both for free peptide and peptide-linked hydrogels; yet, this trend also was accompanied by increased cytotoxicity in cell culture. This work demonstrates the utility of D-AA-modified peptide sequences to create tunable biomaterials platforms tempered by considerations of cytotoxicity, where careful selection and optimization of different peptide designs is needed for specific biological applications.

Precision Hydrogels for the Study of Cancer Cell Mechanobiology

Cancer progression is associated with extensive remodeling of the tumor microenvironment (TME), resulting in alterations of biochemical and biophysical cues that affect both cancer and stromal cells. In particular, the mechanical characteristics of the TME extracellular matrix undergo significant changes. Bioengineered polymer hydrogels can be instrumental to systematically explore how mechanically changed microenvironments impact cancer cell behavior, including proliferation, survival, drug resistance, and invasion. This article reviews studies that have explored the impact of different mechanical cues of the cells' 3D microenvironment on cancer cell behavior using hydrogel-based in vitro models. In particular, advanced engineering strategies are highlighted for tailored hydrogel matrices recapitulating the TME's micrometer- and sub-micrometer-scale architectural and mechanical features, while accounting for its intrinsically heterogenic and dynamic nature. It is anticipated that such precision hydrogel systems will further the understanding of cancer mechanobiology.

Click chemistry functionalization of self-assembling peptide hydrogels

Self-assembling peptide (SAP) hydrogels provide a fibrous microenvironment to cells while also giving users control of biochemical and mechanical cues. Previously, biochemical cues were introduced by physically mixing them with SAPs prior to hydrogel assembly, or by incorporating them into the SAP sequence during peptide synthesis, which limited flexibility and increased costs. To circumvent these limitations, we developed "Click SAPs," a novel formulation that can be easily functionalized via click chemistry thiol-ene reaction. Due to its high cytocompatibility, the thiol-ene click reaction is currently used to crosslink and functionalize other types of polymeric hydrogels. In this study, we developed a click chemistry compatible SAP platform by addition of a modified lysine (lysine-alloc) to the SAP sequence, enabling effective coupling of thiol-containing molecules to the SAP hydrogel network. We demonstrate the flexibility of this approach by incorporating a fluorescent dye, a cellular adhesion peptide, and a matrix metalloproteinase-sensitive biosensor using the thiol-ene reaction in 3D Click SAPs. Using atomic force microscopy, we demonstrate that Click SAPs retain the ability to self-assemble into fibers, similar to previous systems. Additionally, a range of physiologically relevant stiffnesses can be achieved by adjusting SAP concentration. Encapsulated cells maintain high viability in Click SAPs and can interact with adhesion peptides and a matrix metalloproteinase biosensor, demonstrating that incorporated molecules retain their biological activity. The Click SAP platform supports easier functionalization with a wider array of bioactive molecules and enables new investigations with temporal and spatial control of the cellular microenvironment.

Microgels Formed by Spontaneous Click Chemistries Utilizing Microfluidic Flow Focusing for Cargo Release in Response to Endogenous or Exogenous Stimuli

Protein therapeutics have become increasingly popular for the treatment of a variety of diseases owing to their specificity to targets of interest. However, challenges associated with them have limited their use for a range of ailments, including the limited options available for local controlled delivery. To address this challenge, degradable hydrogel microparticles, or microgels, loaded with model biocargoes were created with tunable release profiles or triggered burst release using chemistries responsive to endogenous or exogeneous stimuli, respectively. Specifically, microfluidic flow-focusing was utilized to form homogenous microgels with different spontaneous click chemistries that afforded degradation either in response to redox environments for sustained cargo release or light for on-demand cargo release. The resulting microgels were an appropriate size to remain localized within tissues upon injection and were easily passed through a needle relevant for injection, providing means for localized delivery. Release of a model biopolymer was observed over the course of several weeks for redox-responsive formulations or triggered for immediate release from the light-responsive formulation. Overall, we demonstrate the ability of microgels to be formulated with different materials chemistries to achieve various therapeutic release modalities, providing new tools for creation of more complex protein release profiles to improve therapeutic regimens.