In situ control of cell substrate microtopographies using photolabile hydrogels

Photoresponsive Chemistries for User-Directed Hydrogel Network Modulation to Investigate Cell-Matrix Interactions

ConspectusSynthetic extracellular matrix (ECM) engineering is a highly interdisciplinary field integrating materials and polymer science and engineering, chemistry, cell biology, and medicine to develop innovative strategies to investigate and control cell-matrix interactions. Cellular microenvironments are complex and highly dynamic, changing in response to injury and disease. To capture some of these critical dynamics in vitro, biomaterial matrices have been developed with tailorable properties that can be modulated in situ in the presence of cells. While numerous macromolecules can serve as a basis in the design of a synthetic ECM, our group has exploited multi-arm poly(ethylene glycol) (PEG) macromolecules because of the ease of functionalization, many complementary bio-click reactions to conjugate biological signals, and ultimately, the ability to create well-defined systems to investigate cell-matrix interactions. To date, significant strides have been made in developing bio-responsive and transient synthetic ECM materials that degrade, relax stress, or strain-stiffen in response to cell-mediated stimuli through ECM-cleaving enzymes or integrin-mediated ECM adhesions. However, our group has also designed hydrogels incorporating different photoresponsive moieties, and these moieties facilitate user-defined spatiotemporal modulation of the extracellular microenvironment in vitro. The application of light allows one to break, form, and rearrange network bonds in the presence of cells to alter the biomechanical and biochemical microenvironment to investigate cell-matrix interactions in real-time. Such photoresponsive materials have facilitated fundamental discoveries in the biological pathways related to outside-in signaling, which guide important processes related to tissue development, homeostasis, disease progression, and regeneration.This review focuses on the phototunable chemical toolbox that has been used by Anseth and co-workers to modulate hydrogel properties post-network formation through: bond-breaking chemistries, such as o-nitrobenzyl and coumarin methyl ester photolysis; bond-forming chemistries, such as azadibenzocyclooctyne photo-oligomerization and anthracene dimerization; and bond-rearranging chemistries, such as allyl sulfide addition-fragmentation chain transfer and reversible ring opening polymerization of 1,2-dithiolanes. By using light to modulate the cellular microenvironment (in 2D, 3D, and even 4D), innovative experiments can be designed to study mechanosensing of single cells or multicellular constructs, pattern adhesive ligands to spatially control cell-integrin binding or modulate on-demand the surrounding cell niche to alter outside-in signaling in a temporally controlled manner. To date, these photochemically defined materials have been used for the culture, differentiation, and directed morphogenesis of primary cells and stem cells, co-cultured cells, and even multicellular constructs (e.g., organoids).Herein, we present examples of how this photochemical toolbox has been used under physiological reaction conditions with spatiotemporal control to answer important biological questions and address medical needs. Specifically, our group has exploited these materials to study mesenchymal stem cell mechanosensing and differentiation, the activation of fibroblasts in the context of valve and cardiac fibrosis, muscle stem cell response to matrix changes during injury and aging, and predictable symmetry breaking during intestinal organoid development. The materials and reactions described herein are diverse and enable the design and implementation of an array of hydrogels that can serve as cell delivery systems, tissue engineering scaffolds, or even in vitro models for studying disease or screening for new drug treatments.

Photoinduced Dithiolane Crosslinking for Multiresponsive Dynamic Hydrogels

While many hydrogels are elastic networks crosslinked by covalent bonds, viscoelastic hydrogels with adaptable crosslinks are increasingly being developed to better recapitulate time and position-dependent processes found in many tissues. In this work, 1,2-dithiolanes are presented as dynamic covalent photocrosslinkers of hydrogels, resulting in disulfide bonds throughout the hydrogel that respond to multiple stimuli. Using lipoic acid as a model dithiolane, disulfide crosslinks are formed under physiological conditions, enabling cell encapsulation via an initiator-free light-induced dithiolane ring-opening photopolymerization. The resulting hydrogels allow for multiple photoinduced dynamic responses including stress relaxation, stiffening, softening, and network functionalization using a single chemistry, which can be supplemented by permanent reaction with alkenes to further control network properties and connectivity using irreversible thioether crosslinks. Moreover, complementary photochemical approaches are used to achieve rapid and complete sample degradation via radical scission and post-gelation network stiffening when irradiated in the presence of reactive gel precursor. The results herein demonstrate the versatility of this material chemistry to study and direct 2D and 3D cell-material interactions. This work highlights dithiolane-based hydrogel photocrosslinking as a robust method for generating adaptable hydrogels with a range of biologically relevant mechanical and chemical properties that are varied on demand.

High-Resolution Nanotransfer Printing of Porous Crossbar Array Using Patterned Metal Molds by Extreme-Pressure Imprint Lithography

High-resolution nanotransfer printing (nTP) technologies have attracted a tremendous amount of attention due to their excellent patternability, high productivity, and cost-effectiveness. However, there is still a need to develop low-cost mold manufacturing methods, because most nTP techniques generally require the use of patterned molds fabricated by high-cost lithography technology. Here, we introduce a novel nTP strategy that uses imprinted metal molds to serve as an alternative to a Si stamp in the transfer printing process. We present a method by which to fabricate rigid surface-patterned metallic molds (Zn, Al, and Ni) based on the process of direct extreme-pressure imprint lithography (EPIL). We also demonstrate the nanoscale pattern formation of functional materials, in this case Au, TiO2, and GST, onto diverse surfaces of SiO2/Si, polished metal, and slippery glass by the versatile nTP method using the imprinted metallic molds with nanopatterns. Furthermore, we show the patterning results of nanoporous crossbar arrays on colorless polyimide (CPI) by a repeated nTP process. We expect that this combined nanopatterning method of EPIL and nTP processes will be extendable to the fabrication of various nanodevices with complex circuits based on micro/nanostructures.

Chemical Modification of Human Decellularized Extracellular Matrix for Incorporation into Phototunable Hybrid-Hydrogel Models of Tissue Fibrosis

Tissue fibrosis remains a serious health condition with high morbidity and mortality rates. There is a critical need to engineer model systems that better recapitulate the spatial and temporal changes in the fibrotic extracellular microenvironment and enable study of the cellular and molecular alterations that occur during pathogenesis. Here, we present a process for chemically modifying human decellularized extracellular matrix (dECM) and incorporating it into a dynamically tunable hybrid-hydrogel system containing a poly(ethylene glycol)-α methacrylate (PEGαMA) backbone. Following modification and characterization, an off-stoichiometry thiol-ene Michael addition reaction resulted in hybrid-hydrogels with mechanical properties that could be tuned to recapitulate many healthy tissue types. Next, photoinitiated, free-radical homopolymerization of excess α-methacrylates increased crosslinking density and hybrid-hydrogel elastic modulus to mimic a fibrotic microenvironment. The incorporation of dECM into the PEGαMA hydrogel decreased the elastic modulus and, relative to fully synthetic hydrogels, increased the swelling ratio, the average molecular weight between crosslinks, and the mesh size of hybrid-hydrogel networks. These changes were proportional to the amount of dECM incorporated into the network. Dynamic stiffening increased the elastic modulus and decreased the swelling ratio, average molecular weight between crosslinks, and the mesh size of hybrid-hydrogels, as expected. Stiffening also activated human fibroblasts, as measured by increases in average cellular aspect ratio (1.59 ± 0.02 to 2.98 ± 0.20) and expression of α-smooth muscle actin (αSMA). Fibroblasts expressing αSMA increased from 25.8 to 49.1% upon dynamic stiffening, demonstrating that hybrid-hydrogels containing human dECM support investigation of dynamic mechanosensing. These results improve our understanding of the biomolecular networks formed within hybrid-hydrogels: this fully human phototunable hybrid-hydrogel system will enable researchers to control and decouple the biochemical changes that occur during fibrotic pathogenesis from the resulting increases in stiffness to study the dynamic cell-matrix interactions that perpetuate fibrotic diseases.

Laser-imprinting of micro-3D printed protein hydrogels enables real-time independent modification of substrate topography and elastic modulus

Independent control over the Young's modulus and topography of a hydrogel cell culture substrate is necessary to characterize how attributes of its adherent surface affect cellular responses. Arbitrary, real-time manipulation of these parameters at the micron scale would further provide cellular biologists and bioengineers with the tools to study and control numerous highly dynamic behaviors including cellular adhesion, motility, metastasis, and differentiation. Although physical, chemical, thermal, and light-based strategies have been developed to influence Young's modulus and topography of hydrogel substrates, independent control of these physical attributes has remained elusive, spatial resolution is often limited, and features commonly must be pre-patterned. We recently reported a strategy in which biomaterials having specified three-dimensional (3D) morphologies are micro-3D printed in a two-step process: laser-scanning bioprinting of a protein-based hydrogel, followed by biocompatible hydrogel re-scanning to create microscale imprinted features at user-defined times. In this approach, a pulsed near-infrared laser beam is focused within the printed hydrogel to promote matrix contraction through multiphoton crosslinking, where scanning the laser focus projects a user-defined topographical pattern on the surface without subjecting the hydrogel-solution interface to damaging light intensities. Here, we extend this strategy, demonstrating the ability to decouple dynamic topographical changes from changes in hydrogel Young's modulus at the substrate surface by increasing the isolation distance between the surface and re-scanning planes. Using atomic force microscopy, we show that robust topographic changes can be imposed without altering the Young's modulus measured at the substrate surface by scanning at a depth of greater than ~6 μm. Transmission electron microscopy of hydrogel thin sections reveals changes to hydrogel porosity and density distribution within scanned regions, and that such changes to the hydrogel matrix are highly localized to regions of laser exposure. These results represent valuable new capabilities for deconvolving the effects of substrate dynamic physical attributes on the behavior of adherent cells.

A switchable light-responsive azopolymer conjugating protein micropatterns with topography for mechanobiological studies

Stem cell shape and mechanical properties in vitro can be directed by geometrically defined micropatterned adhesion substrates. However, conventional methods are limited by the fixed micropattern design, which cannot recapitulate the dynamic changes of the natural cell microenvironment. Current methods to fabricate dynamic platforms usually rely on complex chemical strategies or require specialized apparatuses. Also, with these methods, the integration of dynamic signals acting on different length scales is not straightforward, whereas, in some applications, it might be beneficial to act on both a microscale level, that is, cell shape, and a nanoscale level, that is, cell adhesions. Here, we exploited a confocal laser-based technique on a light-responsive azopolymer displaying micropatterns of adhesive islands. The laser light promotes a directed mass migration and the formation of submicrometric topographic relieves. Also, by changing the surface chemistry, the surfacing topography affects cell spreading and shape. This method enabled us to monitor in a non-invasive manner the dynamic changes in focal adhesions, cytoskeleton structures, and nucleus conformation that followed the changes in the adhesive characteristic of the substrate. Focal adhesions reconfigured after the surfacing of the topography, and the actin filaments reoriented to coalign with the newly formed adhesive island. Changes in cell morphology also affected nucleus shape, chromatin conformation, and cell mechanics with different timescales. The reported strategy can be used to investigate mechanotransduction-related events dynamically by controlling cell adhesion at cell shape and focal adhesion levels. The integrated technique enables achieving a submicrometric resolution in a facile and cost-effective manner.

The effects of surface topography modification on hydrogel properties

Hydrogel has been an attractive biomaterial for tissue engineering, drug delivery, wound healing, and contact lens materials, due to its outstanding properties, including high water content, transparency, biocompatibility, tissue mechanical matching, and low toxicity. As hydrogel commonly possesses high surface hydrophilicity, chemical modifications have been applied to achieve the optimal surface properties to improve the performance of hydrogels for specific applications. Ideally, the effects of surface modifications would be stable, and the modification would not affect the inherent hydrogel properties. In recent years, a new type of surface modification has been discovered to be able to alter hydrogel properties by physically patterning the hydrogel surfaces with topographies. Such physical patterning methods can also affect hydrogel surface chemical properties, such as protein adsorption, microbial adhesion, and cell response. This review will first summarize the works on developing hydrogel surface patterning methods. The influence of surface topography on interfacial energy and the subsequent effects on protein adsorption, microbial, and cell interactions with patterned hydrogel, with specific examples in biomedical applications, will be discussed. Finally, current problems and future challenges on topographical modification of hydrogels will also be discussed.

Stimuli-Responsive Biomaterials: Scaffolds for Stem Cell Control

Stem cell fate is closely intertwined with microenvironmental and endogenous cues within the body. Recapitulating this dynamic environment ex vivo can be achieved through engineered biomaterials which can respond to exogenous stimulation (including light, electrical stimulation, ultrasound, and magnetic fields) to deliver temporal and spatial cues to stem cells. These stimuli-responsive biomaterials can be integrated into scaffolds to investigate stem cell response in vitro and in vivo, and offer many pathways of cellular manipulation: biochemical cues, scaffold property changes, drug release, mechanical stress, and electrical signaling. The aim of this review is to assess and discuss the current state of exogenous stimuli-responsive biomaterials, and their application in multipotent stem cell control. Future perspectives in utilizing these biomaterials for personalized tissue engineering and directing organoid models are also discussed.

Approaches for the synthesis of o-nitrobenzyl and coumarin linkers for use in photocleavable biomaterials and bioconjugates and their biomedical applications

Photocleavable biomaterials and bioconjugates are particularly interesting because light sources are easy to obtain and the responsiveness of materials is convenient to control. In recent years, various photocleavable biomaterials and bioconjugates have been synthesized for the control of payload release, regulation of biomolecule activity, 3D cell culture, and investigation of molecular mechanisms. Photocleavable linkers are crucial components of photocleavable biomaterials, which significantly influence the photoresponsive capabilities of materials. Photosensitive molecules, such as o-nitrobenzyls and coumarins, have been extensively developed as photocleavable linkers. In the present review, we provide comprehensive knowledge regarding the synthetic strategies of o-nitrobenzyl and coumarin derived linkers with various functional groups and their applications for the construction of photocleavable biomaterials and bioconjugates. Finally, the biomedical applications of o-nitrobenzyl and coumarin-based photocleavable biomaterials and bioconjugates will be summarized and discussed.

Clickable decellularized extracellular matrix as a new tool for building hybrid-hydrogels to model chronic fibrotic diseases in vitro

Fibrotic disorders account for over one third of mortalities worldwide. Despite great efforts to study the cellular and molecular processes underlying fibrosis, there are currently few effective therapies. Dual-stage polymerization reactions are an innovative tool for recreating heterogeneous increases in extracellular matrix (ECM) modulus, a hallmark of fibrotic diseases in vivo. Here, we present a clickable decellularized ECM (dECM) crosslinker incorporated into a dynamically responsive poly(ethylene glycol)-α-methacrylate (PEGαMA) hybrid-hydrogel to recreate ECM remodeling in vitro. An off-stoichiometry thiol-ene Michael addition between PEGαMA (8-arm, 10 kg mol-1) and the clickable dECM resulted in hydrogels with an elastic modulus of E = 3.6 ± 0.24 kPa, approximating healthy lung tissue (1-5 kPa). Next, residual αMA groups were reacted via a photo-initiated homopolymerization to increase modulus values to fibrotic levels (E = 13.4 ± 0.82 kPa) in situ. Hydrogels with increased elastic moduli, mimicking fibrotic ECM, induced a significant increase in the expression of myofibroblast transgenes. The proportion of primary fibroblasts from dual-reporter mouse lungs expressing collagen 1a1 and alpha-smooth muscle actin increased by approximately 60% when cultured on stiff and dynamically stiffened hybrid-hydrogels compared to soft. Likewise, fibroblasts expressed significantly increased levels of the collagen 1a1 transgene on stiff regions of spatially patterned hybrid-hydrogels compared to the soft areas. Collectively, these results indicate that hybrid-hydrogels are a new tool that can be implemented to spatiotemporally induce a phenotypic transition in primary murine fibroblasts in vitro.

Interplay between degradability and integrin signaling on mesenchymal stem cell function within poly(ethylene glycol) based microporous annealed particle hydrogels

Microporous annealed particle (MAP) hydrogels are promising materials for delivering therapeutic cells. It has previously been shown that spreading and mechanosensing activation of human mesenchymal stem cells (hMSCs) incorporated in these materials can be modulated by tuning the modulus of the microgel particle building blocks. However, the effects of degradability and functionalization with different integrin-binding peptides on cellular responses has not been explored. In this work, RGDS functionalized and enzymatically degradable poly(ethylene glycol) (PEG) microgels were annealed into MAP hydrogels via thiol-ene click chemistry and photopolymerization. During cell-mediated degradation, the microgel surfaces were remodeled to wrinkles or ridges, but the scaffold integrity was maintained. Moreover, cell spreading, proliferation, and secretion of extracellular matrix proteins were significantly enhanced in faster matrix metalloproteinase degrading (KCGPQGIWGQCK) MAP hydrogels compared to non-degradable controls after 8 days of culture. We subsequently evaluated paracrine activity by hMSCs seeded in the MAP hydrogels functionalized with either RGDS or c(RRETAWA), which is specific for α5β1 integrins, and evaluated the interplay between degradability and integrin-mediated signaling. Importantly, c(RRETAWA) functionalization upregulated secretion of bone morphogenetic protein-2 overall and on a per cell basis, but this effect was critically dependent on microgel degradability. In contrast, RGDS functionalization led to higher overall vascular endothelial growth factor secretion in degradable scaffolds due to the high cell number. These results demonstrate that integrin-binding peptides can modulate hMSC behavior in PEG-based MAP hydrogels, but the results strongly depend on the susceptibility of the microgel building blocks to cell-mediated matrix remodeling. This relationship should be considered in future studies aiming to further develop these materials for stem cell delivery and tissue engineering applications. STATEMENT OF SIGNIFICANCE: Microporous annealed particle (MAP) hydrogels are attracting increasing interest for tissue repair and regeneration and have shown superior results compared to conventional hydrogels in multiple applications. Here, we studied the impact of MAP hydrogel degradability and functionalization with different integrin-binding peptides on human mesenchymal stem cells (hMSCs) that were incorporated during particle annealing. Degradability was found to improve cell growth, spreading, and extracellular matrix production regardless of the integrin-binding peptide. Moreover, in degradable MAP hydrogels the integrin-binding peptide c(RRETAWA) was found to increase osteogenic protein expression by hMSCs compared to RGDS-functionalized MAP hydrogels. These results have important implications for the development of a MAP hydrogel-based hMSC delivery system for bone tissue engineering.

Tissue-informed engineering strategies for modeling human pulmonary diseases

Chronic pulmonary diseases, including idiopathic pulmonary fibrosis (IPF), pulmonary hypertension (PH), and chronic obstructive pulmonary disease (COPD), account for staggering morbidity and mortality worldwide but have limited clinical management options available. Although great progress has been made to elucidate the cellular and molecular pathways underlying these diseases, there remains a significant disparity between basic research endeavors and clinical outcomes. This discrepancy is due in part to the failure of many current disease models to recapitulate the dynamic changes that occur during pathogenesis in vivo. As a result, pulmonary medicine has recently experienced a rapid expansion in the application of engineering principles to characterize changes in human tissues in vivo and model the resulting pathogenic alterations in vitro. We envision that engineering strategies using precision biomaterials and advanced biomanufacturing will revolutionize current approaches to disease modeling and accelerate the development and validation of personalized therapies. This review highlights how advances in lung tissue characterization reveal dynamic changes in the structure, mechanics, and composition of the extracellular matrix in chronic pulmonary diseases and how this information paves the way for tissue-informed engineering of more organotypic models of human pathology. Current translational challenges are discussed as well as opportunities to overcome these barriers with precision biomaterial design and advanced biomanufacturing techniques that embody the principles of personalized medicine to facilitate the rapid development of novel therapeutics for this devastating group of chronic diseases.

Optoregulated Biointerfaces to Trigger Cellular Responses

Optoregulated biointerfaces offer the possibility to manipulate the interactions between cell membrane receptors and the extracellular space. This Invited Feature Article summarizes recent efforts by our group and others during the past decade to develop light-responsive biointerfaces to stimulate cells and elicit cellular responses using photocleavable protecting groups (PPG) as our working tool. This article begins by providing a brief introduction to available PPGs, with a special focus on the widely used o-nitrobenzyl family, followed by an overview of molecular design principles for the control of bioactivity in the context of cell-material interactions and the characterization methods to use in following the photoreaction at surfaces. We present various light-guided cellular processes using PPGs, including cell adhesion, release, migration, proliferation, and differentiation, both in vitro and in vivo. Finally, this Invited Feature Article closes with our perspective on the current status and future challenges of this topic.

Spatio-Temporal Control of Cell Adhesion: Toward Programmable Platforms to Manipulate Cell Functions and Fate

Biophysical and biochemical signals of material surfaces potently regulate cell functions and fate. In particular, micro- and nano-scale patterns of adhesion signals can finely elicit and affect a plethora of signaling pathways ultimately affecting gene expression, in a process known as mechanotransduction. Our fundamental understanding of cell-material signals interaction and reaction is based on static culturing platforms, i.e., substrates exhibiting signals whose configuration is time-invariant. However, cells in-vivo are exposed to arrays of biophysical and biochemical signals that change in time and space and the way cells integrate these might eventually dictate their behavior. Advancements in fabrication technologies and materials engineering, have recently enabled the development of culturing platforms able to display patterns of biochemical and biophysical signals whose features change in time and space in response to external stimuli and according to selected programmes. These dynamic devices proved to be particularly helpful in shedding light on how cells adapt to a dynamic microenvironment or integrate spatio-temporal variations of signals. In this work, we present the most relevant findings in the context of dynamic platforms for controlling cell functions and fate in vitro. We place emphasis on the technological aspects concerning the fabrication of platforms displaying micro- and nano-scale dynamic signals and on the physical-chemical stimuli necessary to actuate the spatio-temporal changes of the signal patterns. In particular, we illustrate strategies to encode material surfaces with dynamic ligands and patterns thereof, topographic relieves and mechanical properties. Additionally, we present the most effective, yet cytocompatible methods to actuate the spatio-temporal changes of the signals. We focus on cell reaction and response to dynamic changes of signal presentation. Finally, potential applications of this new generation of culturing systems for in vitro and in vivo applications, including regenerative medicine and cell conditioning are presented.

In Situ Imprinting of Topographic Landscapes at the Cell-Substrate Interface

In their native environments, adherent cells encounter dynamic topographical cues involved in promoting differentiation, orientation, and migration. Ideally, such processes would be amenable to study in cell culture using tools capable of imposing dynamic, arbitrary, and reversible topographic features without perturbing environmental conditions or causing chemical and/or structural disruptions to the substrate surface. To address this need, we report here development of an in vitro strategy for challenging cells with dynamic topographical experiences in which protein-based hydrogel substrate surfaces are modified in real time by positioning a pulsed, near-infrared laser focus within the hydrogel, promoting chemical cross-linking which results in local contraction of the protein matrix. Scanning the laser focus through arbitrary patterns directed by a dynamic reflective mask creates an internal contraction pattern that is projected onto the hydrogel surface as features such as rings, pegs, and grooves. By subjecting substrates to a sequence of scan patterns, we show that topographic features can be created, then eliminated or even reversed. Because laser-induced shrinkage can be confined to 3D voxels isolated from the cell-substrate interface, hydrogel modifications are made without damaging cells or disrupting the chemical or structural integrity of the surface.

About Chemical Strategies to Fabricate Cell-Instructive Biointerfaces with Static and Dynamic Complexity

Properly functioning cell-instructive biointerfaces are critical for healthy integration of biomedical devices in the body and serve as decisive tools for the advancement of our understanding of fundamental cell biological phenomena. Studies are reviewed that use covalent chemistries to fabricate cell-instructive biointerfaces. These types of biointerfaces typically result in a static presentation of predefined cell-instructive cues. Chemically defined, but dynamic cell-instructive biointerfaces introduce spatiotemporal control over cell-instructive cues and present another type of biointerface, which promises a more biomimetic way to guide cell behavior. Therefore, strategies that offer control over the lateral sorting of ligands, the availability and molecular structure of bioactive ligands, and strategies that offer the ability to induce physical, chemical and mechanical changes in situ are reviewed. Specific attention is paid to state-of-the-art studies on dynamic, cell-instructive 3D materials. Future work is expected to further deepen our understanding of molecular and cellular biological processes investigating cell-type specific responses and the translational steps toward targeted in vivo applications.

Spatiotemporal hydrogel biomaterials for regenerative medicine

Hydrogels mimic many of the physical properties of soft tissue and are widely used biomaterials for tissue engineering and regenerative medicine. Synthetic hydrogels have been developed to recapitulate many of the healthy and diseased states of native tissues and can be used as a cell scaffold to study the effect of matricellular interactions in vitro. However, these matrices often fail to capture the dynamic and heterogenous nature of the in vivo environment, which varies spatially and during events such as development and disease. To address this deficiency, a variety of manufacturing and processing techniques are being adapted to the biomaterials setting. Among these, photochemistry is particularly well suited because these reactions can be performed in precise three-dimensional space and at specific moments in time. This spatiotemporal control over chemical reactions can also be performed over a range of cell- and tissue-relevant length scales with reactions that proceed efficiently and harmlessly at ambient conditions. This review will focus on the use of photochemical reactions to create dynamic hydrogel environments, and how these dynamic environments are being used to investigate and direct cell behavior.

Light-Responsive Hierarchically Structured Liquid Crystal Polymer Networks for Harnessing Cell Adhesion and Migration

Extracellular microenvironment is highly dynamic where spatiotemporal regulation of cell-instructive cues such as matrix topography tightly regulates cellular behavior. Recapitulating dynamic changes in stimuli-responsive materials has become an important strategy in regenerative medicine to generate biomaterials which closely mimic the natural microenvironment. Here, light responsive liquid crystal polymer networks are used for their adaptive and programmable nature to form hybrid surfaces presenting micrometer scale topographical cues and changes in nanoscale roughness at the same time to direct cell migration. This study shows that the cell speed and migration patterns are strongly dependent on the height of the (light-responsive) micrometer scale topographies and differences in surface nanoroughness. Furthermore, switching cell migration patterns upon in situ temporal changes in surface nanoroughness, points out the ability to dynamically control cell behavior on these surfaces. Finally, the possibility is shown to form photoswitchable topographies, appealing for future studies where topographies can be rendered reversible on demand.

Dynamically Tunable Cell Culture Platforms for Tissue Engineering and Mechanobiology

Human tissues are sophisticated ensembles of many distinct cell types embedded in the complex, but well-defined, structures of the extracellular matrix (ECM). Dynamic biochemical, physicochemical, and mechano-structural changes in the ECM define and regulate tissue-specific cell behaviors. To recapitulate this complex environment in vitro, dynamic polymer-based biomaterials have emerged as powerful tools to probe and direct active changes in cell function. The rapid evolution of polymerization chemistries, structural modulation, and processing technologies, as well as the incorporation of stimuli-responsiveness, now permit synthetic microenvironments to capture much of the dynamic complexity of native tissue. These platforms are comprised not only of natural polymers chemically and molecularly similar to ECM, but those fully synthetic in origin. Here, we review recent in vitro efforts to mimic the dynamic microenvironment comprising native tissue ECM from the viewpoint of material design. We also discuss how these dynamic polymer-based biomaterials are being used in fundamental cell mechanobiology studies, as well as towards efforts in tissue engineering and regenerative medicine.

Engineered Models of Confined Cell Migration

Cells in the body are physically confined by neighboring cells, tissues, and the extracellular matrix. Although physical confinement modulates intracellular signaling and the underlying mechanisms of cell migration, it is difficult to study in vivo. Furthermore, traditional two-dimensional cell migration assays do not recapitulate the complex topographies found in the body. Therefore, a number of experimental in vitro models that confine and impose forces on cells in well-defined microenvironments have been engineered. We describe the design and use of microfluidic microchannel devices, grooved substrates, micropatterned lines, vertical confinement devices, patterned hydrogels, and micropipette aspiration assays for studying cell responses to confinement. Use of these devices has enabled the delineation of changes in cytoskeletal reorganization, cell-substrate adhesions, intracellular signaling, nuclear shape, and gene expression that result from physical confinement. These assays and the physiologically relevant signaling pathways that have been elucidated are beginning to have a translational and clinical impact.

Evaluation of a bilayered, micropatterned hydrogel dressing for full-thickness wound healing

Nearly 12 million wounds are treated in emergency departments throughout the United States every year. The limitations of current treatments for complex, full-thickness wounds are the driving force for the development of new wound treatment devices that result in faster healing of both dermal and epidermal tissue. Here, a bilayered, biodegradable hydrogel dressing that uses microarchitecture to guide two key steps in the proliferative phase of wound healing, re-epithelialization, and revascularization, was evaluated in vitro in a cell migration assay and in vivo in a bipedicle ischemic rat wound model. Results indicate that the Sharklet™-micropatterned apical layer of the dressing increased artificial wound coverage by up to 64%, P = 0.024 in vitro. In vivo evaluation demonstrated that the bilayered dressing construction enhanced overall healing outcomes significantly compared to untreated wounds and that these outcomes were not significantly different from a leading clinically available wound dressing. Collectively, these results demonstrate high potential for this new dressing to effectively accelerate wound healing.

Convergence of Highly Resolved and Rapid Screening Platforms with Dynamically Engineered, Cell Phenotype-Prescriptive Biomaterials

Biophysical and biochemical cues from the cellular microenvironment initiate intracellular signaling through cellular membrane receptors and trigger specific cell developmental programs. Extracellular substrates and matrix scaffolds engineered to mimic cell's native physiological environment must incorporate the multifactorial parameters (composition, micro and nanoscale organization and topography) of the extracellular matrix as well as the dynamic nature of the matrix. The design of such engineered biomaterials is challenged by the inherent complexity and dynamic nature of the cell-extracellular matrix reciprocity, while the validation of robust microenvironments requires a deeper, higher content phenotypic resolution of cell-matrix interactions alongside a rapid screening capability. To this end, high-throughput platforms are integral to facilitating the screening and optimization of complex engineered microenvironments for directing desired cell developmental pathway. This review highlights the recent advances in biomaterial platforms that present dynamic cues and enable high throughput screening of cell's response to a combination of micro-environmental factors. We also address some newer techniques involving high content image informatics to elucidate emergent cellular behaviors with a focus on stem cell regenerative endpoints.

Dynamically tunable polymer microwells for directing mesenchymal stem cell differentiation into osteogenesis

Dynamically tunable geometric microwells have great capacity to regulate the cytoskeletal structure and differentiation of mesenchymal stem cells along adipogenesis and osteogenesis pathways.

Porous and strong three-dimensional carbon nanotube coated ceramic scaffolds for tissue engineering

A method to coat high-quality uniform coatings of carbon nanotubes throughout 3D porous structures is developed. Testing of their physical and biological properties demonstrate their potential for application in tissue engineering.

Hybrid inverse opals for regulating cell adhesion and orientation

Cell adhesion and alignment are two important considerations in tissue engineering applications as they can regulate the subsequent cell proliferation activity and differentiation program. Although many effects have been applied to regulate the adhesion or alignment of cells by using physical and chemical methods, it is still a challenge to regulate these cell behaviors simultaneously. Here, we present novel substrates with tunable nanoscale patterned structures for regulating the adhesion and alignment of cells. The substrates with different degrees of pattern orientation were achieved by customizing the amount of stretching applied to polymer inverse opal films. Cells cultured on these substrates showed an adjustable morphology and alignment. Moreover, soft hydrogels, which have poor plasticity and are difficult to cast into patterned structures, were applied to infiltrate the inverse opal structure. We demonstrated that the adhesion ratio of cells could be regulated by these hybrid substrates, as well as adjusting the cell morphology and alignment. These features of functional inverse opal substrates make them suitable for important applications in tissue engineering.

Orthogonally engineering matrix topography and rigidity to regulate multicellular morphology

Programmable polymer substrates, which mimic the variable extracellular matrices in living systems, are used to regulate multicellular morphology, via orthogonally modulating the matrix topography and elasticity. The multicellular morphology is dependent on the competition between cell-matrix adhesion and cell-cell adhesion. Decreasing the cell-matrix adhesion provokes cytoskeleton reorganization, inhibits lamellipodial crawling, and thus enhances the leakiness of multicellular morphology.

Multi-lineage differentiation of human mesenchymal stromal cells on the biophysical microenvironment of cell-derived matrix

We obtained fibroblast- (FDM) and preosteoblast- (PDM) derived matrices in vitro from their respective cells. Our hypothesis was that these naturally occurring cell-derived matrices (CDMs) would provide a better microenvironment for the multi-lineage differentiation of human mesenchymal stromal cells (hMSCs) than those based on traditional single-protein-based platforms. Cells cultured for 5-6 days were decellularized with detergents and enzymes. The resulting matrices showed a fibrillar surface texture. Under osteogenic conditions, human bone-marrow-derived stromal cells (HS-5) exhibited higher amounts of both mineralized nodule formation and alkaline phosphatase (ALP) expression than those cultured on plastic or gelatin. Osteogenic markers (Col I, osteopontin, and cbfa1) and ALP activity from cells cultured on PDM were notably upregulated at 4 weeks. The use of FDM significantly improved the cellular expression of chondrogenic markers (Sox 9 and Col II), while downregulating that of Col I at 4 weeks. Both CDMs were more effective in inducing cellular synthesis of glycosaminoglycan content than control substrates. We also investigated the effect of matrix surface texture on hMSC (PT-2501) differentiation; soluble matrix (S-matrix)-coated substrates exhibited a localized fibronectin (FN) alignment, whereas natural matrix (N-matrix)-coated substrates preserved the naturally formed FN fibrillar alignment. hMSCs cultured for 4 weeks on N-matrices under osteogenic or chondrogenic conditions deposited a greater amount of calcium and proteoglycan than those cultured on S-matrices as assessed by von Kossa and Safranin O staining. In contrast to the expression levels of lineage-specific markers for cells cultured on gelatin, FN, or S-matrices, those cultured on N-matrices yielded highly upregulated levels. This study demonstrates not only the capacity of CDM for being an effective inductive template for the multi-lineage differentiation of hMSCs, but also the critical biophysical role that the matrix fibrillar texture itself plays on the induction of stem cell differentiation.

Clickable, photodegradable hydrogels to dynamically modulate valvular interstitial cell phenotype

Biophysical cues are widely recognized to influence cell phenotype. While this evidence was established using static substrates, there is growing interest in creating stimulus-responsive biomaterials that better recapitulate the dynamic extracellular matrix. Here, a clickable, photodegradable hydrogel substrate that allows the user to precisely control substrate elasticity and topography in situ is presented. The hydrogels are synthesized by reacting an 8-arm poly(ethylene glycol) alkyne with an azide-functionalized photodegradable crosslinker. The utility of this platform by exploiting its photoresponsive properties to modulate the phenotype of porcine aortic valvular interstitial cells (VICs) is demonstrated. First, VIC phenotype is monitored, in response to initial substratum modulus and static topographic cues. Higher modulus (E ≈ 15 kPa) substrates induce higher levels of activation (≈70% myofibroblasts) versus soft (E ≈ 3 kPa) substrates (≈20% myofibroblasts). Microtopographies that induce VIC alignment and elongation on low modulus substrates also stimulate activation. Finally, VIC phenotype is monitored in response to sequential in situ manipulations. The results illustrate that VIC activation on stiff surfaces (≈70% myofibroblasts) can be partially reversed by reducing surface modulus (≈30% myofibroblats) and subsequently re-activated by anisotropic topographies (≈60% myofibroblasts). Such dynamic substrates afford unique opportunities to decipher the complex role of matrix cues on the plasticity of VIC activation.

Complex dynamic substrate control: dual-tone hydrogel photoresists allow double-dissociation of topography and modulus

Complex substrate control is demonstrated with a dual-tone hydrogel photoresist. By exposing a photodegradable hydrogel to UV light through a photomask, both swollen and eroded micropatterns with a decreased modulus can be created on the surface under different exposure conditions. This provides an important tool for investigating the synergistic effects of spatially heterogeneous mechanical and topological cues on cell behavior.

Cell-material interactions revealed via material techniques of surface patterning

Cell-material interactions constitute a key fundamental topic in biomaterials study. Various cell cues and matrix cues as well as soluble factors regulate cell behaviors on materials. These factors are coupled with each other as usual, and thus it is very difficult to unambiguously elucidate the role of each regulator. The recently developed material techniques of surface patterning afford unique ways to reveal the underlying science. This paper reviews the pertinent material techniques to fabricate patterns of microscale and nanoscale resolutions, and corresponding cell studies. Some issues are emphasized, such as cell localization on patterned surfaces of chemical contrast, and effects of cell shape, cell size, cell-cell contact, and seeding density on differentiation of stem cells. Material cues to regulate cell adhesion, cell differentiation and other cell events are further summed up. Effects of some physical properties, such as surface topography and matrix stiffness, on cell behaviors are also discussed; nanoscaled features of substrate surfaces to regulate cell fate are summarized as well. The pertinent work sheds new insight into the cell-material interactions, and is stimulating for biomaterial design in regenerative medicine, tissue engineering, and high-throughput detection, diagnosis, and drug screening.

Tunable and dynamic soft materials for three-dimensional cell culture

The human body is complex and hierarchically structured, composed of cells residing within the extracellular matrix (ECM) of tissues that are assembled into organs, all working together to complete a given function. One goal of current biomaterials research is to capture some of this complexity outside of the body for understanding the underlying biology of development, repair, and disease and to devise new strategies for regenerative medicine or disease treatment. Polymeric materials have arisen as powerful tools to mimic the native ECM, giving experimenters a way to capture key aspects of the native cellular environment outside of the body. In particular, dynamic materials allow changes in the properties of these ECM mimics during an experiment, affording an additional degree of control for the experimenter. In this tutorial review, the basic cellular processes of cell migration, proliferation, and differentiation will be overviewed to motivate design considerations for polymeric ECM mimics, and examples will be given of how classes of dynamic materials are being used to study each cellular process.

A Diels-Alder modulated approach to control and sustain the release of dexamethasone and induce osteogenic differentiation of human mesenchymal stem cells

We report a new approach to controlled drug release based upon exploiting the dynamic equilibrium that exists between Diels-Alder reactants and products, demonstrating the release of a furan containing dexamethasone peptide (dex-KGPQG-furan) from a maleimide containing hydrogel. Using a reaction-diffusion model, the release kinetics were tuned to achieve sustained concentrations conducive to osteogenic differentiation of human mesenchymal stem cells (hMSCs). Efficacy was first demonstrated in a 2D culture model, in which dexamethasone release induced significant increases in alkaline phosphatase (ALP) activity and mineral deposition in hMSCs compared to a dexamethasone-free treatment. The results were similar to that observed with a soluble dexamethasone treatment. More dramatic differences were observed in 3D culture, where co-encapsulation of a dexamethasone releasing hydrogel depot within an hMSC-laden extracellular matrix mimetic poly(ethylene glycol) hydrogel resulted in a local and robust osteogenic differentiation. ALP activity reached levels that were up to six times higher than the dexamethasone free treatment. Interestingly, at 5 and 10 day time points, the ALP activity exceeded the dexamethasone positive control, suggesting a potential benefit of sustained release in 3D culture. After 21 days, substantial mineralization comparable to the positive control was also observed in the hydrogels. Collectively, these results demonstrate Diels-Alder modulated release as an effective and versatile new platform for controlled drug delivery that may prove especially beneficial for sustaining the release of low molecular weight molecules in hydrogel systems.