Reversible control of biomaterial properties for dynamically tuning cell behavior

Using dynamic biomaterials to study the temporal role of bioactive peptides during osteogenesis

The extracellular matrix is a highly dynamic environment, and the precise temporal presentation of biochemical signals is critical for regulating cell behavior during development, healing, and disease progression. To mimic this behavior, we developed a modular DNA-based hydrogel platform to enable independent and reversible control over the immobilization of multiple biomolecules during in vitro cell culture. We combined reversible DNA handles with a norbornene-modified hyaluronic acid hydrogel to orthogonally add and remove multiple biomolecule-DNA conjugates at user-defined timepoints. We demonstrated that the persistent presentation of the cell adhesion peptide RGD was required to maintain cell spreading on hyaluronic acid hydrogels. Further, we discovered the delayed presentation of osteogenic growth peptide (OGP) increased alkaline phosphatase activity compared to other temporal variations. This finding is critically important when considering the design of OGP delivery approaches for bone repair. More broadly, this platform provides a unique approach to tease apart the temporal role of multiple biomolecules during development, regeneration, and disease progression.

Using dynamic biomaterials to study the temporal role of osteogenic growth peptide during osteogenesis

The extracellular matrix is a highly dynamic environment, and the precise temporal presentation of biochemical signals is critical for regulating cell behavior during development, healing, and disease progression. To mimic this behavior, we developed a modular DNA-based hydrogel platform to enable independent and reversible control over the immobilization of multiple biomolecules during in vitro cell culture. We combined reversible DNA handles with a norbornene-modified hyaluronic acid hydrogel to orthogonally add and remove multiple biomolecule-DNA conjugates at user-defined timepoints. We demonstrated that the persistent presentation of the cell adhesion peptide RGD was required to maintain cell spreading on hyaluronic acid hydrogels. Further, we discovered the delayed presentation of osteogenic growth peptide (OGP) increased alkaline phosphatase activity compared to other temporal variations. This finding is critically important when considering the design of OGP delivery approaches for bone repair. More broadly, this platform provides a unique approach to tease apart the temporal role of multiple biomolecules during development, regeneration, and disease progression.

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

Engineered hydrogels are increasingly used as extracellular matrix (ECM) surrogates for probing cell function in response to ECM remodeling events related to injury or disease (e.g., degradation followed by deposition/crosslinking). Inspired by these events, this work establishes an approach for pseudo-reversible mechanical property modulation in synthetic hydrogels by integrating orthogonal, enzymatically triggered crosslink degradation, and light-triggered photopolymerization stiffening. Hydrogels are formed by a photo-initiated thiol-ene reaction between multiarm polyethylene glycol and a dually enzymatically degradable peptide linker, which incorporates a thrombin-degradable sequence for triggered softening and a matrix metalloproteinase (MMP)-degradable sequence for cell-driven remodeling. Hydrogels are stiffened by photopolymerization using a flexible, MMP-degradable polymer-peptide conjugate and multiarm macromers, increasing the synthetic matrix crosslink density while retaining degradability. Integration of these tools enables sequential softening and stiffening inspired by matrix remodeling events within loose connective tissues (Young's modulus (E) ≈5 to 1.5 to 6 kPa with >3x ΔE). The cytocompatibility and utility of this approach is examined with breast cancer cells, where cell proliferation shows a dependence on the timing of triggered softening. This work provides innovative tools for 3D dynamic property modulation that are synthetically accessible and cell compatible.

Emerging Roles of Microfluidics in Brain Research: From Cerebral Fluids Manipulation to Brain-on-a-Chip and Neuroelectronic Devices Engineering

Remarkable progress made in the past few decades in brain research enables the manipulation of neuronal activity in single neurons and neural circuits and thus allows the decipherment of relations between nervous systems and behavior. The discovery of glymphatic and lymphatic systems in the brain and the recently unveiled tight relations between the gastrointestinal (GI) tract and the central nervous system (CNS) further revolutionize our understanding of brain structures and functions. Fundamental questions about how neurons conduct two-way communications with the gut to establish the gut-brain axis (GBA) and interact with essential brain components such as glial cells and blood vessels to regulate cerebral blood flow (CBF) and cerebrospinal fluid (CSF) in health and disease, however, remain. Microfluidics with unparalleled advantages in the control of fluids at microscale has emerged recently as an effective approach to address these critical questions in brain research. The dynamics of cerebral fluids (i.e., blood and CSF) and novel in vitro brain-on-a-chip models and microfluidic-integrated multifunctional neuroelectronic devices, for example, have been investigated. This review starts with a critical discussion of the current understanding of several key topics in brain research such as neurovascular coupling (NVC), glymphatic pathway, and GBA and then interrogates a wide range of microfluidic-based approaches that have been developed or can be improved to advance our fundamental understanding of brain functions. Last, emerging technologies for structuring microfluidic devices and their implications and future directions in brain research are discussed.

Modulation of hydrogel stiffness by external stimuli: soft materials for mechanotransduction studies

Mechanotransduction is an important process in determining cell survival, proliferation, migration and differentiation. The extracellular matrix (ECM) is the component of natural tissue that provides structural support and biochemical signals to adhering cells. The ECM is dynamic and undergoes physical and biochemical changes in response to various stimuli and there is an interest in understanding the effect of dynamic changes in stiffness on cell behaviour and fate. Therefore, stimuli-responsive hydrogels have been developed to mimic the cells' microenvironment in a controlled fashion. Herein, we review strategies for dynamic modulation of stiffness using various stimuli, such as light, temperature and pH. Special emphasis is placed on conducting polymer (CP) hydrogels and their fabrication procedures. We believe that the redox properties of CPs and hydrogels' biological properties make CPs hydrogels a promising substrate to investigate the effect of dynamic stiffness changes and mechanical actuation on cell fate in future studies.

Design of azobenzene-bearing hydrogel with photoswitchable mechanics driven by photo-induced phase transition for in vitro disease modeling

Mechanics of the extracellular matrix (ECM) exhibit changes during many biological events. During disease progression, such as cancer, matrix stiffening or softening occurs due to crosslinking of the collagen matrix or matrix degradation through cell-secreted enzymes. Engineered hydrogels have emerged as a prime in vitro model to mimic such dynamic mechanics during disease progression. Although there have been a variety of engineered hydrogels, few can offer both stiffening and softening properties under the same working principle. In addition, to model individual disease progression, it is desirable to control the kinetics of mechanical changes. To this end, we describe a photoresponsive hydrogel that undergoes stiffness changes by the photo-induced phase transition. The hydrogel was composed of a copolymer of azobenzene acrylate monomer (AzoAA) and N,N-dimethyl acrylamide (DMA). By tuning the amount of azobenzene, the phase transition behavior of this polymer occurs solely by light irradiation, because of the photoisomerization of azobenzene. This phase behavior was confirmed at 37 ^°C by turbidity measurements. In addition, the crosslinked poly(AzoAA-r-DMA) gel undergoes reversible swelling-deswelling upon photoisomerization by ultraviolet or visible light. Furthermore, the poly(AzoAA-r-DMA) sheet gels exhibited modulus changes at different isomerization states of azobenzene. When MCF-7 cells were cultured on the gels, stiffening at different timepoints induced varied responses in the gene expression levels of E-cadherin. Not only did this suggest an adaptive behavior of the cells against changes in mechanics during disease progression, this also demonstrated our material's potential towards in vitro disease modeling. STATEMENT OF SIGNIFICANCE: During disease progression such as cancer, cellular microenvironment called extracellular matrix (ECM) undergoes stiffness changes. Hydrogels, which are swollen network of crosslinked polymers, have been used to model such dynamic mechanical environment of the ECM. However, few could offer both stiffening and softening properties under the same working principle. Herein, we fabricated a novel photoresponsive hydrogel with switchable mechanics, activated by photo-induced structural change of the polymer chains within the hydrogel. When breast cancer cells were cultured on our dynamic hydrogels, gene expression and morphological observation suggested that cells react to changes in stiffness by a transient response, as opposed to a sustained one. The photoresponsive hydrogel offers possibility for use as a patient-specific model of diseases.

User-defined, temporal presentation of bioactive molecules on hydrogel substrates using supramolecular coiled coil complexes

The ability to spatiotemporally control the presentation of relevant biomolecules in synthetic culture systems has gained significant attention as researchers strive to recapitulate the endogenous extracellular matrix (ECM) in vitro. With the biochemical composition of the ECM constantly in flux, the development of platforms that allow for user-defined control of bioactivity is desired. Here, we reversibly conjugate bioactive molecules to hydrogel-based substrates through supramolecular coiled coil complexes that form between complementary peptides. Our system employs a thiolated peptide for tethering to hydrogel surfaces (T-peptide) through a spatially-controlled photomediated click reaction. The complementary association peptide (A-peptide), containing the bioactive domain, forms a heterodimeric coiled coil complex with the T-peptide. Addition of a disruptor peptide (D-peptide) engineered specifically to target the A-peptide outcompetes the T-peptide for binding, and removes the A-peptide and the attached bioactive motif from the scaffold. We use this platform to demonstrate spatiotemporal control of biomolecule presentation within hydrogel systems in a repeatable process that can be extended to adhesive motifs for cell culture. NIH 3T3 fibroblasts seeded on hyaluronic acid hydrogels and polyethylene glycol-based fibrous substrates supramolecularly functionalized with an RGD motif demonstrated significant cell spreading over their nonfunctionalized counterparts. Upon displacement of the RGD motif, fibroblasts occupied less area and clustured on the substrates. Taken together, this platform enables facile user-defined incorporation and removal of biomolecules in a repeatable process for controlled presentation of bioactivity in engineered culture systems.

A Programmable Toolkit to Dynamically Signal Cells Using Peptide Strand Displacement

The native extracellular matrix communicates and interacts with cells by dynamically displaying signals to control their behavior. Mimicking this dynamic environment in vitro is essential in order to unravel how cell-matrix interactions guide cell fate. Here, we present a synthetic platform for the temporal display of cell-adhesive signals using coiled-coil peptides. By designing an integrin-engaging coiled-coil pair to have a toehold (unpaired domain), we were able to use a peptide strand displacement reaction to remove the cell cue from the surface. This allowed us to test how the user-defined display of RGDS ligands at variable duration and periodicity of ligand exposure influence cell spreading degree and kinetics. Transient display of α_Vβ₃-selective ligands instructed fibroblast cells to reversibly spread and contract in response to changes in ligand exposure over multiple cycles, exhibiting a universal kinetic response. Also, cells that were triggered to spread and contract repeatedly exhibited greater enrichment of integrins in focal adhesions versus cells cultured on persistent RGDS-displaying surfaces. This dynamic platform will allow us to uncover the molecular code by which cells sense and respond to changes in their environment and will provide insights into ways to program cellular behavior.

Reversible Control of Gelatin Hydrogel Stiffness by Using DNA Crosslinkers*

Biomaterials with dynamically tunable properties are critical for a range of applications in regenerative medicine and basic biology. In this work, we show the reversible control of gelatin methacrylate (GelMA) hydrogel stiffness through the use of DNA crosslinkers. We replaced some of the inter-GelMA crosslinks with double-stranded DNA, allowing for their removal through toehold-mediated strand displacement. The crosslinks could be restored by adding fresh dsDNA with complementary handles to those on the hydrogel. The elastic modulus (G') of the hydrogels could be tuned between 500 and 1000 Pa, reversibly, over two cycles without degradation of performance. By functionalizing the gels with a second DNA strand, it was possible to control the crosslink density and a model ligand in an orthogonal fashion with two different displacement strands. Our results demonstrate the potential for DNA to reversibly control both stiffness and ligand presentation in a protein-based hydrogel, and will be useful for teasing apart the spatiotemporal behavior of encapsulated cells.

Supramolecular Biopolymers for Tissue Engineering

Supramolecular biopolymers (SBPs) are those polymeric units derived from macromolecules that can assemble with each other by noncovalent interactions. Macromolecular structures are commonly found in living systems such as proteins, DNA/RNA, and polysaccharides. Bioorganic chemistry allows the generation of sequence-specific supramolecular units like SBPs that can be tailored for novel applications in tissue engineering (TE). SBPs hold advantages over other conventional polymers previously used for TE; these materials can be easily functionalized; they are self-healing, biodegradable, stimuli-responsive, and nonimmunogenic. These characteristics are vital for the further development of current trends in TE, such as the use of pluripotent cells for organoid generation, cell-free scaffolds for tissue regeneration, patient-derived organ models, and controlled delivery systems of small molecules. In this review, we will analyse the 3 subtypes of SBPs: peptide-, nucleic acid-, and oligosaccharide-derived. Then, we will discuss the role that SBPs will be playing in TE as dynamic scaffolds, therapeutic scaffolds, and bioinks. Finally, we will describe possible outlooks of SBPs for TE.

A Quantitative Description for Designing the Extrudability of Shear-Thinning Physical Hydrogels

Physically associated hydrogels (PHs) capable of reversible transitions between solid and liquid-like states have enabled novel strategies for 3D printing, therapeutic drug and cell delivery, and regenerative medicine. Among the many design criteria (e.g., viscoelasticity, cargo diffusivity, biocompatibility) for these applications, engineering PHs for extrudability is a necessary and critical design criterion for the successful application of these materials. As the development of many distinct PH material systems continues, a strategy to determine the extrudability of PHs a priori will be exceedingly useful for reducing costly and time-consuming trial-and-error experimentation. Here, a strategy to determine the property-function relationships for PHs in injectable drug delivery applications at clinically relevant flow rates is presented. This strategy-validated with two chemically and physically distinct PHs-reveals material design spaces in the form of Ashby-style plots that highlight acceptable, application-specific material properties. It is shown that the flow behavior of PHs does not obey a single shear-thinning power law and the implications for injectable drug delivery are discussed. This approach for generating design criteria has potential for streamlining the screening of PHs and their utility in applications with varying geometrical (i.e., needle diameter) and process (i.e., flow rate) constraints.

Hyaluronic Acid Biomaterials for Central Nervous System Regenerative Medicine

Hyaluronic acid (HA) is a primary component of the brain extracellular matrix and functions through cellular receptors to regulate cell behavior within the central nervous system (CNS). These behaviors, such as migration, proliferation, differentiation, and inflammation contribute to maintenance and homeostasis of the CNS. However, such equilibrium is disrupted following injury or disease leading to significantly altered extracellular matrix milieu and cell functions. This imbalance thereby inhibits inherent homeostatic processes that support critical tissue health and functionality in the CNS. To mitigate the damage sustained by injury/disease, HA-based tissue engineering constructs have been investigated for CNS regenerative medicine applications. HA's effectiveness in tissue healing and regeneration is primarily attributed to its impact on cell signaling and the ease of customizing chemical and mechanical properties. This review focuses on recent findings to highlight the applications of HA-based materials in CNS regenerative medicine.