Gas-modulating microcapsules for spatiotemporal control of hypoxia.

Oxygen is a vital molecule involved in regulating development, homeostasis, and disease. The oxygen levels in tissue vary from 1 to 14% with deviations from homeostasis impacting regulation of various physiological processes. In this work, we developed an approach to encapsulate enzymes at high loading capacity, which precisely controls the oxygen content in cell culture. Here, a single microcapsule is able to locally perturb the oxygen balance, and varying the concentration and distribution of matrix-embedded microcapsules provides spatiotemporal control. We demonstrate attenuation of hypoxia signaling in populations of stem cells, cancer cells, endothelial cells, cancer spheroids, and intestinal organoids. Varying capsule placement, media formulation, and timing of replenishment yields tunable oxygen gradients, with concurrent spatial growth and morphogenesis in a single well. Capsule containing hydrogel films applied to chick chorioallantoic membranes encourages neovascularization, providing scope for topical treatments or hydrogel wound dressings. This platform can be used in a variety of formats, including deposition in hydrogels, as granular solids for 3D bioprinting, and as injectable biomaterials. Overall, this platform's simplicity and flexibility will prove useful for fundamental studies of oxygen-mediated processes in virtually any in vitro or in vivo format, with scope for inclusion in biomedical materials for treating injury or disease.

Pluripotent stem cell-derived mesenchymal stromal cells improve cardiac function and vascularity after myocardial infarction.

BACKGROUND AIMS: Mesenchymal stromal cells (MSCs) have been shown to improve cardiac function after injury and are the subject of ongoing clinical trials. In this study, the authors tested the cardiac regenerative potential of an induced pluripotent stem cell-derived MSC (iPSC-MSC) population (Cymerus MSCs) in a rat model of myocardial ischemia-reperfusion (I/R). Furthermore, the authors compared this efficacy with bone marrow-derived MSCs (BM-MSCs), which are the predominant cell type in clinical trials. METHODS: Four days after myocardial I/R injury, rats were randomly assigned to (i) a Cymerus MSC group (n = 15), (ii) a BM-MSC group (n = 15) or (iii) a vehicle control group (n = 14). For cell-treated animals, a total of 5 × 106 cells were injected at three sites within the infarcted left ventricular (LV) wall. RESULTS: One month after cell transplantation, Cymerus MSCs improved LV function (assessed by echocardiography) compared with vehicle and BM-MSCs. Interestingly, Cymerus MSCs enhanced angiogenesis without sustained engraftment or significant impact on infarct scar size. Suggesting safety, Cymerus MSCs had no effect on inducible tachycardia or the ventricular scar heterogeneity that provides a substrate for cardiac re-entrant circuits. CONCLUSIONS: The authors here demonstrate that intra-myocardial administration of iPSC-MSCs (Cymerus MSCs) provide better therapeutic effects compared with conventional BM-MSCs in a rodent model of myocardial I/R. Because of its manufacturing scalability, iPSC-MSC therapy offers an exciting opportunity for an "off-the-shelf" stem cell therapy for cardiac repair.

Enzyme Responsive Inverse Opal Hydrogels.

Structured color in nature is controlled by nano- and micro-structured interfaces giving rise to a photonic bandgap. This study presents a biomimetic optical material based on polymeric inverse opals that respond to enzyme activity. Polymer colloids provide a template in which acryloyl-functionalized poly(ethylene glycol) is integrated; dissolution of the colloids leads to a hydrogel inverse opal that can be lithographically patterned using transfer printing. Incorporating enzyme substrates within the voids provides a material that responds to the presence of proteases through a shift in the optical properties.

Anti-inflammatory ω-3 endocannabinoid epoxides.

Clinical studies suggest that diets rich in ω-3 polyunsaturated fatty acids (PUFAs) provide beneficial anti-inflammatory effects, in part through their conversion to bioactive metabolites. Here we report on the endogenous production of a previously unknown class of ω-3 PUFA-derived lipid metabolites that originate from the crosstalk between endocannabinoid and cytochrome P450 (CYP) epoxygenase metabolic pathways. The ω-3 endocannabinoid epoxides are derived from docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) to form epoxyeicosatetraenoic acid-ethanolamide (EEQ-EA) and epoxydocosapentaenoic acid-ethanolamide (EDP-EA), respectively. Both EEQ-EAs and EDP-EAs are endogenously present in rat brain and peripheral organs as determined via targeted lipidomics methods. These metabolites were directly produced by direct epoxygenation of the ω-3 endocannabinoids, docosahexanoyl ethanolamide (DHEA) and eicosapentaenoyl ethanolamide (EPEA) by activated BV-2 microglial cells, and by human CYP2J2. Neuroinflammation studies revealed that the terminal epoxides 17,18-EEQ-EA and 19,20-EDP-EA dose-dependently abated proinflammatory IL-6 cytokines while increasing anti-inflammatory IL-10 cytokines, in part through cannabinoid receptor-2 activation. Furthermore the ω-3 endocannabinoid epoxides 17,18-EEQ-EA and 19,20-EDP-EA exerted antiangiogenic effects in human microvascular endothelial cells (HMVEC) and vasodilatory actions on bovine coronary arteries and reciprocally regulated platelet aggregation in washed human platelets. Taken together, the ω-3 endocannabinoid epoxides' physiological effects are mediated through both endocannabinoid and epoxyeicosanoid signaling pathways. In summary, the ω-3 endocannabinoid epoxides are found at concentrations comparable to those of other endocannabinoids and are expected to play critical roles during inflammation in vivo; thus their identification may aid in the development of therapeutics for neuroinflammatory and cerebrovascular diseases.

Interfacial geometry dictates cancer cell tumorigenicity.

Within the heterogeneous architecture of tumour tissue there exists an elusive population of stem-like cells that are implicated in both recurrence and metastasis. Here, by using engineered extracellular matrices, we show that geometric features at the perimeter of tumour tissue will prime a population of cells with a stem-cell-like phenotype. These cells show characteristics of cancer stem cells in vitro, as well as enhanced tumorigenicity in murine models of primary tumour growth and pulmonary metastases. We also show that interfacial geometry modulates cell shape, adhesion through integrin α5ß1, MAPK and STAT activity, and initiation of pluripotency signalling. Our results for several human cancer cell lines suggest that interfacial geometry triggers a general mechanism for the regulation of cancer-cell state. Similar to how a growing tumour can co-opt normal soluble signalling pathways, our findings demonstrate how cancer can also exploit geometry to orchestrate oncogenesis.

The importance of matrix in cardiomyogenesis: Defined substrates for maturation and chamber specificity.

Human embryonic stem cell-derived cardiomyocytes (hESC-CM) are a promising source of cardiac cells for disease modelling and regenerative medicine. However, current protocols invariably lead to mixed population of cardiac cell types and often generate cells that resemble embryonic phenotypes. Here we developed a combinatorial approach to assess the importance of extracellular matrix proteins (ECMP) in directing the differentiation of cardiomyocytes from human embryonic stem cells (hESC). We did this by focusing on combinations of ECMP commonly found in the developing heart with a broad goal of identifying combinations that promote maturation and influence chamber specific differentiation. We formulated 63 unique ECMP combinations fabricated from collagen 1, collagen 3, collagen 4, fibronectin, laminin, and vitronectin, presented alone and in combinations, leading to the identification of specific ECMP combinations that promote hESC proliferation, pluripotency, and germ layer specification. When hESC were subjected to a differentiation protocol on the ECMP combinations, it revealed precise protein combinations that enhance differentiation as determined by the expression of cardiac progenitor markers kinase insert domain receptor (KDR) and mesoderm posterior transcription factor 1 (MESP1). High expression of cardiac troponin (cTnT) and the relative expression of myosin light chain isoforms (MLC2a and MLC2v) led to the identification of three surfaces that promote a mature cardiomyocyte phenotype. Action potential morphology was used to assess chamber specificity, which led to the identification of matrices that promote chamber-specific cardiomyocytes. This study provides a matrix-based approach to improve control over cardiomyocyte phenotypes during differentiation, with the scope for translation to cardiac laboratory models and for the generation of functional chamber specific cardiomyocytes for regenerative therapies.

Advancing Synthetic Hydrogels through Nature-Inspired Materials Chemistry.

Synthetic extracellular matrix (ECM) mimics that can recapitulate the complex biochemical and mechanical nature of native tissues are needed for advanced models of development and disease. Biomedical research has heavily relied on the use of animal-derived biomaterials, which is now impeding their translational potential and convoluting the biological insights gleaned from in vitro tissue models. Natural hydrogels have long served as a convenient and effective cell culture tool, but advances in materials chemistry and fabrication techniques now present promising new avenues for creating xenogenic-free ECM substitutes appropriate for organotypic models and microphysiological systems. However, significant challenges remain in creating synthetic matrices that can approximate the structural sophistication, biochemical complexity, and dynamic functionality of native tissues. This review summarizes key properties of the native ECM, and discusses recent approaches used to systematically decouple and tune these properties in synthetic matrices. The importance of dynamic ECM mechanics, such as viscoelasticity and matrix plasticity, is also discussed, particularly within the context of organoid and engineered tissue matrices. Emerging design strategies to mimic these dynamic mechanical properties are reviewed, such as multi-network hydrogels, supramolecular chemistry, and hydrogels assembled from biological monomers.

Mechanical and biological behavior of double network hydrogels reinforced with alginate versus gellan gum.

Alginate and gellan gum have both been used by researchers as reinforcing networks to create tough and biocompatible polyethylene glycol (PEG) based double network (DN) hydrogels; however, the relative advantages and disadvantages of each approach are not understood. This study directly compares the mechanical and biological properties of polyethylene glycol di-methacrylate (PEGDMA) hybrid DN hydrogels reinforced with either gellan gum or sodium alginate using PEGDMA concentrations from 10 to 20 wt% and reinforcing network concentrations of 1 and 2 wt%. The findings demonstrate that gellan gum reinforcement is more effective at increasing the strength, stiffness, and toughness of PEGDMA DN hydrogels. In contrast, alginate reinforcement yields DN hydrogels with greater stretchability compared to gellan gum reinforced PEGDMA. Furthermore, separate measurements of toughness via unnotched work of rupture testing and notched fracture toughness testing showed a strong correlation of these two properties for a single reinforcing network type, but not across the two types of reinforcing networks. This suggests that additional notched fracture toughness experiments are important for understanding the full mechanical response when comparing different tough DN hydrogel systems. Regarding the biological response, after conjugation of matrix protein to the surface of both materials robust cell attachment and spreading was supported with higher yes associated protein (YAP) nuclear expression observed in populations adhering to the stiffer gellan gum-PEGDMA material. This study provides valuable insights regarding how to design double network hydrogels for specific property requirements, e.g., for use in biomedical devices, as scaffolding for tissue engineering, or in soft robotic applications.

Mesenchymal stem cell-secretome laden photopolymerizable hydrogels for wound healing.

Mesenchymal stem cell-derived secretome represents an emerging acellular therapeutic which possess significant opportunity for clinical applications due to its anti-inflammatory, immunomodulatory, and wound healing properties. However, maintaining therapeutic efficacy and ensuring stability of cell-based products is challenging, requiring a robust delivery method. Therefore, we designed a hydrogel-based scaffold loaded with CK Cell Technologies' proprietary Mesenchymal stem cell-secretome for controlled release treatment of acute and chronic wounds. We incorporated both conditioned media (CM) and extracellular vesicles (EVs) into gelatin methacryloyl (GelMA) hydrogels and demonstrated how we can tune the diffusive release of the EVs from them. To demonstrate viability of the approach, we developed a wound healing scratch assay where we see in situ release of CM and EVs promote enhanced migration of human dermal fibroblasts (hDFs). We see the colocalization of these EVs in the fibroblasts using fluorescent microscopy. Finally, as a surrogate for in vivo neovascularization, we conducted an in vitro tube formation assay for the MSC-secretome using matrigel-embedded human microvascular endothelial cells. By adding CM and EVs, we observe an increase in tubulogenesis. Collectively, our data demonstrates by tuning the GelMA properties, we can influence the controlled release of the MSC-secretome for a wound dressing and bandage application for chronic and acute wounds.

ROS-mediated anticancer effects of EGFR-targeted nanoceria.

The therapeutic effectiveness of anticancer drugs, including nanomedicines, can be enhanced with active receptor-targeting strategies. Epidermal growth factor receptor (EGFR) is an important cancer biomarker, constitutively expressed in sarcoma patients of different histological types. The present work reports materials and in vitro biomedical analyses of silanized (passive delivery) and/or EGF-functionalized (active delivery) ceria nanorods exhibiting highly defective catalytically active surfaces. The EGFR-targeting efficiency of nanoceria was confirmed by receptor-binding studies. Increased cytotoxicity and reactive oxygen species (ROS) production were observed for EGF-functionalized nanoceria owing to enhanced cellular uptake by HT-1080 fibrosarcoma cells. The uptake was confirmed by TEM and confocal microscopy. Silanized nanoceria demonstrated negligible/minimal cytotoxicity toward healthy MRC-5 cells at 24 and 48 h, whereas this was significant at 72 h owing to a nanoceria accumulation effect. In contrast, considerable cytotoxicity toward the cancer cells was exhibited at all three times points. The ROS generation and associated cytotoxicity were moderated by the equilibrium between catalysis by ceria, generation of cell debris, and blockage of active sites. EGFR-targeting is shown to enhance the uptake levels of nanoceria by cancer cells, subsequently enhancing the overall anticancer activity and therapeutic performance of ceria.

Geometric cues for directing the differentiation of mesenchymal stem cells.

Significant efforts have been directed to understanding the factors that influence the lineage commitment of stem cells. This paper demonstrates that cell shape, independent of soluble factors, has a strong influence on the differentiation of human mesenchymal stem cells (MSCs) from bone marrow. When exposed to competing soluble differentiation signals, cells cultured in rectangles with increasing aspect ratio and in shapes with pentagonal symmetry but with different subcellular curvature-and with each occupying the same area-display different adipogenesis and osteogenesis profiles. The results reveal that geometric features that increase actomyosin contractility promote osteogenesis and are consistent with in vivo characteristics of the microenvironment of the differentiated cells. Cytoskeletal-disrupting pharmacological agents modulate shape-based trends in lineage commitment verifying the critical role of focal adhesion and myosin-generated contractility during differentiation. Microarray analysis and pathway inhibition studies suggest that contractile cells promote osteogenesis by enhancing c-Jun N-terminal kinase (JNK) and extracellular related kinase (ERK1/2) activation in conjunction with elevated wingless-type (Wnt) signaling. Taken together, this work points to the role that geometric shape cues can play in orchestrating the mechanochemical signals and paracrine/autocrine factors that can direct MSCs to appropriate fates.

Biomechanical, biophysical and biochemical modulators of cytoskeletal remodelling and emergent stem cell lineage commitment.

Across complex, multi-time and -length scale biological systems, redundancy confers robustness and resilience, enabling adaptation and increasing survival under dynamic environmental conditions; this review addresses ubiquitous effects of cytoskeletal remodelling, triggered by biomechanical, biophysical and biochemical cues, on stem cell mechanoadaptation and emergent lineage commitment. The cytoskeleton provides an adaptive structural scaffold to the cell, regulating the emergence of stem cell structure-function relationships during tissue neogenesis, both in prenatal development as well as postnatal healing. Identification and mapping of the mechanical cues conducive to cytoskeletal remodelling and cell adaptation may help to establish environmental contexts that can be used prospectively as translational design specifications to target tissue neogenesis for regenerative medicine. In this review, we summarize findings on cytoskeletal remodelling in the context of tissue neogenesis during early development and postnatal healing, and its relevance in guiding lineage commitment for targeted tissue regeneration. We highlight how cytoskeleton-targeting chemical agents modulate stem cell differentiation and govern responses to mechanical cues in stem cells' emerging form and function. We further review methods for spatiotemporal visualization and measurement of cytoskeletal remodelling, as well as its effects on the mechanical properties of cells, as a function of adaptation. Research in these areas may facilitate translation of stem cells' own healing potential and improve the design of materials, therapies, and devices for regenerative medicine.

Hydrogel Microtumor Arrays to Evaluate Nanotherapeutics.

Nanoparticle drug formulations have many advantages for cancer therapy due to benefits in targeting selectivity, lack of systemic toxicity, and increased drug concentration in the tumor microenvironment after delivery. However, the promise of nanomedicine is limited by preclinical models that fail to accurately assess new drugs before entering human trials. In this work a new approach to testing nanomedicine using a microtumor array formed through hydrogel micropatterning is demonstrated. This technique allows partitioning of heterogeneous cell states within a geometric pattern-where boundary regions of curvature prime the stem cell-like fraction-allowing to simultaneously probe drug uptake and efficacy in different cancer cell fractions with high reproducibility. Using melanoma cells of different metastatic potential, a relationship between stem fraction and nanoparticle uptake is discovered. Deformation cytometry reveals that the stem cell-like population exhibits a more mechanically deformable cell membrane. Since the stem fraction in a tumor is implicated in drug resistance, recurrence, and metastasis, the findings suggest that nanoparticle drug formulations are well suited for targeting this dangerous cell population in cancer therapy.

In situ formation of osteochondral interfaces through "bone-ink" printing in tailored microgel suspensions.

Osteochondral tissue has a complex hierarchical structure spanning subchondral bone to articular cartilage. Biomaterials approaches to mimic and repair these interfaces have had limited success, largely due to challenges in fabricating composite hard-soft interfaces with living cells. Biofabrication approaches have emerged as attractive methods to form osteochondral analogues through additive assembly of hard and soft components. We have developed a unique printing platform that is able to integrate soft and hard materials concurrently through freeform printing of mineralized constructs within tunable microgel suspensions containing living cells. A library of microgels based on gelatin were prepared, where the stiffness of the microgels and a liquid "filler" phase can be tuned for bioprinting while simultaneously directing differentiation. Tuning microgel stiffness and filler content differentially directs chondrogenesis and osteogenesis within the same construct, demonstrating how this technique can be used to fabricate osteochondral interfaces in a single step. Printing of a rapidly setting calcium phosphate cement, so called "bone-ink" within a cell laden suspension bath further guides differentiation, where the cells adjacent to the nucleated hydroxyapatite phase undergo osteogenesis with cells in the surrounding medium undergoing chondrogenesis. In this way, bone analogues with hierarchical structure can be formed within cell-laden gradient soft matrices to yield multiphasic osteochondral constructs. This technique provides a versatile one-pot biofabrication approach without harsh post-processing which will aid efforts in bone disease modelling and tissue engineering. STATEMENT OF SIGNIFICANCE: This paper demonstrates the first example of a biofabrication approach to rapidly form osteochondral constructs in a single step under physiological conditions. Key to this advance is a tunable suspension of extracellular matrix microgels that are packed together with stem cells, providing a unique and modular scaffolding for guiding the simultaneous formation of bone and cartilage tissue. The physical properties of the suspension allow direct writing of a ceramic "bone-ink", resulting in an ordered structure of microscale hydrogels, living cells, and bone mimics in a single step. This platform reveals a simple approach to making complex skeletal tissue for disease modelling, with the possibility of repairing and replacing bone-cartilage interfaces in the clinic using a patient's own cells.

Electrostatic Assembly of Multiarm PEG-Based Hydrogels as Extracellular Matrix Mimics: Cell Response in the Presence and Absence of RGD Cell Adhesive Ligands.

Synthetic hydrogels have been used widely as extracellular matrix (ECM) mimics due to the ability to control and mimic physical and biochemical cues observed in natural ECM proteins such as collagen, laminin, and fibronectin. Most synthetic hydrogels are formed via covalent bonding resulting in slow gelation which is incompatible with drop-on-demand 3D bioprinting of cells and injectable hydrogels for therapeutic delivery. Herein, we developed an electrostatically crosslinked PEG-based hydrogel system for creating high-throughput 3D in vitro models using synthetic hydrogels to mimic the ECM cancer environment. A 3-arm PEG-based polymer backbone was first modified with either permanent cationic charged moieties (2-(methacryloyloxy)ethyl trimethylammonium) or permanent anionic charged moieties (3-sulfopropyl methacrylate potassium salt). The resulting charged polymers can be conjugated further with various amounts of cell adhesive RGD motifs (0, 25, 75, and 98%) to study the influences of RGD motifs on breast cancer (MCF-7) spheroid formation. Formation, stability, and mechanical properties of hydrogels were tested with, and without, RGD to evaluate the cellular response to material parameters in a 3D environment. The hydrogels can be degraded in the presence of salts at room temperature by breaking the interaction of oppositely charged polymer chains. MCF-7 cells could be released with high viability through brief exposure to NaCl solution. Flow cytometry characterization demonstrated that embedded MCF-7 cells proliferate better in a softer (60 Pa) 3D hydrogel environment compared to those that are stiffer (1160 Pa). As the stiffness increases, the RGD motif plays a role in promoting cell proliferation in the stiffer hydrogel. Flow cytometry characterization demonstrated that embedded MCF-7 cells proliferate better in a softer (60 Pa) 3D hydrogel environment compared to those that are stiffer (1160 Pa). As the stiffness increases, the RGD motif plays a role in promoting cell proliferation in the stiffer hydrogel. Additionally, cell viability was not impacted by the tested hydrogel stiffness range between 60 to 1160 Pa. Taken together, this PEG-based tuneable hydrogel system shows great promise as a 3D ECM mimic of cancer extracellular environments with controllable biophysical and biochemical properties. The ease of gelation and dissolution through salt concentration provides a way to quickly harvest cells for further analysis at any given time of interest without compromising cell viability.

Hierarchical assembly of tryptophan zipper peptides into stress-relaxing bioactive hydrogels.

Soft materials in nature are formed through reversible supramolecular assembly of biological polymers into dynamic hierarchical networks. Rational design has led to self-assembling peptides with structural similarities to natural materials. However, recreating the dynamic functional properties inherent to natural systems remains challenging. Here we report the discovery of a short peptide based on the tryptophan zipper (trpzip) motif, that shows multiscale hierarchical ordering that leads to emergent dynamic properties. Trpzip hydrogels are antimicrobial and self-healing, with tunable viscoelasticity and unique yield-stress properties that allow immediate harvest of embedded cells through a flick of the wrist. This characteristic makes Trpzip hydrogels amenable to syringe extrusion, which we demonstrate with examples of cell delivery and bioprinting. Trpzip hydrogels display innate bioactivity, allowing propagation of human intestinal organoids with apical-basal polarization. Considering these extensive attributes, we anticipate the Trpzip motif will prove a versatile building block for supramolecular assembly of soft materials for biotechnology and medicine.

Magnetic Nanofibrous Hydrogels for Dynamic Control of Stem Cell Differentiation.

The extracellular matrix in tissue consists of complex heterogeneous soft materials with hierarchical structure and dynamic mechanical properties dictating cell and tissue level function. In many natural matrices, there are nanofibrous structures that serve to guide cell activity and dictate the form and function of tissue. Synthetic hydrogels with integrated nanofibers can mimic the structural properties of native tissue; however, model systems with dynamic mechanical properties remain elusive. Here we demonstrate modular nanofibrous hydrogels that can be reversibly stiffened in response to applied magnetic fields. Iron oxide nanoparticles were incorporated into gelatin nanofibers through electrospinning, followed by chemical stabilization and fragmentation. These magnetoactive nanofibers can be mixed with virtually any hydrogel material and reversibly stiffen the matrix at a low fiber content (≤3%). In contrast to previous work, where a large quantity of magnetic material disallowed cell encapsulation, the low nanofiber content allows matrix stiffening with cells in 3D. Using adipose derived stem cells, we show how nanofibrous matrices are beneficial for both osteogenesis and adipogenesis, where stiffening the hydrogel with applied magnetic fields enhances osteogenesis while discouraging adipogenesis. Skeletal myoblast progenitors were used as a model of tissue morphogenesis with matrix stiffening augmenting myogenesis and multinucleated myotube formation. The ability to reversibly stiffen fibrous hydrogels through magnetic stimulation provides a useful tool for studying nanotopography and dynamic mechanics in cell culture, with a scope for stimuli responsive materials for tissue engineering.

Evaluation of the Immune Response to Chitosan-graft-poly(caprolactone) Biopolymer Scaffolds.

Biomimetic scaffolds recreating key elements of the architecture and biological activity of the extracellular matrix have enormous potential for soft tissue engineering applications. Combining appropriate mechanical properties with select biological cues presents a challenge for bioengineering, as natural materials are most bioactive but can lack mechanical integrity, while synthetic polymers have strength but are often biologically inert. Blends of synthetic and natural materials, aiming to combine the benefits of each, have shown promise but inherently require a compromise, diluting down favorable properties in each polymer to accommodate the other. Here, we electrospun a material comprising chitosan, a natural polysaccharide, and polycaprolactone (PCL), one of the most widely studied synthetic polymers used in materials engineering. In contrast to a classical blend, here PCL was chemically grafted onto the chitosan backbone to create chitosan-graft-polycaprolactone (CS-g-PCL) and then combined further with unmodified PCL to generate scaffolds with discreet chitosan functionalization. These small amounts of chitosan led to significant changes in scaffold architecture and surface chemistry, reducing the fiber diameter, pore size, and hydrophobicity. Interestingly, all CS-g-PCL-containing blends were stronger than control PCL, though with reduced elongation. In in vitro assessments, increasing the CS-g-PCL content led to significant improvements in in vitro blood compatibility compared to PCL alone while increasing fibroblast attachment and proliferation. In a mouse subcutaneous implantation model, a higher CS-g-PCL content improved the immune response to the implants. Macrophages in tissues surrounding CS-g-PCL scaffolds decreased proportionately to the chitosan content by up to 65%, with a corresponding decrease in pro-inflammatory cytokines. These results suggest that CS-g-PCL is a promising hybrid material comprising natural and synthetic polymers with tailorable mechanical and biological properties, justifying further development and in vivo evaluation.

Directing stem cell fate by controlling the affinity and density of ligand-receptor interactions at the biomaterials interface.

Sticky situation: the differentiation of mesenchymal stem cells can be influenced by the affinity and density of an immobilized ligand for the integrin receptors. Cells adherent to monolayers that present the high-affinity, cyclic-RGD peptide (left) show increased expression of osteogenic markers, while cells on monolayers presenting the lower-affinity, linear-RGD peptide (right) express early markers of myogenesis at a high density and neurogenesis at a low density of the ligand.

Cell-Laden Gradient Microgel Suspensions for Spatial Control of Differentiation During Biofabrication.

During tissue development, stem and progenitor cells form functional tissue with high cellular diversity and intricate micro- and macro-architecture. Current approaches have attempted to replicate this process with materials cues or through spontaneous cell self-organization. However, cell-directed and materials-directed organization are required simultaneously to achieve biomimetic structure and function. Here, it is shown how integrating live adipose derived stem cells with gradient microgel suspensions steers divergent differentiation outcomes. Microgel matrices composed of small particles are found to promote adipogenic differentiation, while larger particles fostered increased cell spreading and osteogenic differentiation. Tuning the matrix formulation demonstrates that early cell adhesion and spreading dictate differentiation outcome. Combining small and large microgels into gradients spatially directs proliferation and differentiation over time. After 21 days of culture, osteogenic conditions foster significant mineralization within the individual microgels, thereby providing cell-directed changes in composition and mechanics within the gradient porous scaffold. Freeform printing of high-density cell suspensions is performed across these gradients to demonstrate the potential for hierarchical tissue biofabrication. Interstitial porosity influences cell expansion from the print and microgel size guides spatial differentiation, thereby providing scope to fabricate tissue gradients at multiple scales through integrated and printed cell populations.