TRPV4 activation enhances compressive properties and glycosaminoglycan deposition of equine neocartilage sheets.

Objective: To evaluate the effect of Transient Receptor Potential Vanilloid 4 (TRPV4) cation channel modulation on mesenchymal stromal cell (MSC)-derived neocartilage. Methods: RT-PCR was performed to evaluate mRNA levels of chondrogenic, hypertrophic and candidate mechanoresponsive genes in equine neocartilage sheets exposed to pulses of the TRPV4 agonist (GSK101) at different concentrations (N â€‹= â€‹10). Biochemical assays and mechanical tests (double indentation and unconfined compression) evaluated neocartilage properties (N â€‹= â€‹5). Results: GSK101 treatment (1 â€‹nM) increased ACAN levels after treatment for 1-h per day for 3 days. No increase was detected for hypertrophic markers RUNX2, MMP13, MMP1, ALP or COL10A1 at this concentration. This treatment regimen also increased sGAG content and enhanced compressive properties compared to untreated controls. GSK101 showed no effect on candidate mechanoresponsive genes at the time-point of analysis. Conclusions: Chemical activation of TRPV4 signalling can be used as a strategy to enhance matrix synthesis and maturation of MSC-derived engineered neocartilage and augment its load-bearing capacity.

Cell Cycle Synchronization of Primary and Cultured Articular Chondrocytes.

Cell cycle synchronization allows cells in a culture, originally at different stages of the cell cycle, to be brought to the same phase. It is normally performed by applying cell cycle arresting chemical agents to cells cultured in monolayer. While effective, isolated chondrocytes tend to dedifferentiate when cultured in monolayer and typically require 3D culturing methods to ensure phenotypic stability. Here, we describe both the conventional cell cycle synchronization method for cells in monolayer culture and an adapted method of synchronizing primary chondrocytes directly during the cell isolation process to limit potential dedifferentiation. Different methods including serum-starvation and treatment with thymidine, nocodazole, aphidicolin, and RO-3306 can synchronize the chondrocytes at different discrete phases. A cell purity of more than 90% in the S phase can be achieved with simultaneous cell isolation and synchronization using double thymidine treatment, generating a population of synchronized chondrocytes that show increased matrix synthesis when subsequently cultured in 3D.

Lithium chloride-induced primary cilia recovery enhances biosynthetic response of chondrocytes to mechanical stimulation.

Mechanical stimulation is commonly used in cartilage tissue engineering for enhancing tissue formation and improving the mechanical properties of resulting engineered tissues. However, expanded chondrocytes tend to dedifferentiate and lose expression of their primary cilia, which is necessary for chondrocyte mechanotransduction. As treatment with lithium chloride (LiCl) can restore passaged chondrocytes in monolayer, in this study, we investigated whether this approach would be effective in 3D culture and restore chondrocyte mechanosensitivity. Chondrocytes at different passages (P0 to P2) were treated with 0-50 mM LiCl for 24 h, with different pre-culture durations (0 to 4 days). The primary cilia incidence and length were measured in α-tubulin-stained images. Treated chondrocytes were cultured with or without dynamic compression to evaluate the effect of LiCl-induced primary cilia expression on matrix synthesis by mechanically stimulated chondrocytes. LiCl treatment of chondrocytes in 3D agarose culture increased primary cilia incidence and length, with significant increases in incidence and length using 50 mM LiCl compared to other concentrations (P < 0.05). This effect was further optimized by including a 4-day pre-culture prior to the 24-h 50 mM LiCl treatment. Importantly, LiCl-induced primary cilia expression increased chondrocyte mechanosensitivity. When stimulated with dynamic compression, LiCl-treated P1 chondrocytes increased collagen (1.4-fold, P < 0.1) and proteoglycan (1.5-fold, P < 0.05) synthesis compared to untreated, unstimulated cells. The LiCl treatment method described here can be used to restore primary cilia in passaged chondrocytes, transforming them into a mechanosensitive cell source for cartilage tissue engineering.

Effect of nutrient metabolism on cartilaginous tissue formation.

A major shortcoming in cartilage tissue engineering is the low biosynthetic response of chondrocytes. While different strategies have been investigated, a novel approach may be to control nutrient metabolism. Although known for their anaerobic metabolism, chondrocytes are more synthetically active under conditions that elicit mixed aerobic-anaerobic metabolism. Here, we postulate this metabolic switch induces HIF-1α signaling resulting in improved growth. Transition to different metabolic states can result in the pooling of metabolites, several of which can stabilize HIF-1α by interfering with PHD2. Chondrocytes cultured under increased media availability accelerated tissue deposition with the greatest effect occurring at 2 ml/106 cells. Under higher media availability, metabolism switched from anaerobic to mixed aerobic-anaerobic. Around this transition, maximal changes in PHD2 activity, HIF-1α expression, and HIF-1 target gene expression were observed. Loss-of-function studies using YC-1 confirmed the involvement of HIF-1. Lastly, targeted metabolomic studies revealed that intracellular lactate and succinate correlated with PHD2 activity. This study demonstrates that cartilaginous tissue formation can be regulated by nutrient metabolism and that this response is mediated through changes in HIF-1α signaling. By harnessing this newly identified metabolic switch, engineered cartilage implants may be developed without the need for sophisticated methods which could aid translation to the clinic.

Characterization of Mechanical and Dielectric Properties of Silicone Rubber.

Silicone rubber's silicone-oxygen backbones give unique material properties which are applicable in various biomedical devices. Due to the diversity of potential silicone rubber compositions, the material properties can vary widely. This paper characterizes the dielectric and mechanical properties of two different silicone rubbers, each with a different cure system, and in combination with silicone additives. A tactile mutator (Slacker™) and/or silicone thickener (Thi-vex™) were mixed with platinum-cured and condensation-cured silicone rubber in various concentrations. The dielectric constants, conductivities, and compressive and shear moduli were measured for each sample. Our study contributes novel information about the dielectric and mechanical properties of these two types of silicone rubber and how they change with the addition of two common silicone additives.

In vitro evaluation of novel titania-containing borate bioactive glass scaffolds.

Titanium-containing borate bioactive glass scaffolds (0, 5, 15, and 20 mol %, identified as BRT0, BRT1, BRT3, and BRT4) with a microstructure similar to that of human trabecular bone were prepared and evaluated in vitro for potential bone loss applications in revision total knee arthroplasty (rTKA). Methyl thiazolyl tetrazolium (MTT) cell viability assays of scaffold ion release extracts revealed that BRT0 scaffolds (0 mol % titanium) inhibited cell proliferation and activity at day 14. At day 30, all scaffold extracts decreased cell proliferation and activity significantly. However, live/dead cell assay results demonstrated that degradation products from all the scaffolds had no inhibitory effect on cell viability. Significant bactericidal efficacies of BRT3 extracts against Escherishia coli (Gram-negative) and BRT1 extracts against Staphylococcus aureus and Staphylococcus epidermidis (both Gram-positive bacteria) were demonstrated. Finally, evaluation of the cell/bioactive glass surface interactions showed well-spread cells on the surface of the BRT3 glass discs and BRT1 and BRT3 scaffolds, when compared to BRT0 and BRT4 scaffolds. The results indicate that by changing the Ti4+ :B3+ ratio, the ion release and consequently cell proliferation could be improved. in vitro results in this study demonstrate that BRT3 scaffolds could be a promising candidate for addressing bone loss in rTKAs; however, in vivo studies would be required to evaluate the effect of a dynamic environment on the cell and tissue response to the fabricated scaffolds.

Cell Cycle Synchronization of Primary Articular Chondrocytes Enhances Chondrogenesis.

OBJECTIVE: Although tissue engineering is a promising option for articular cartilage repair, it has been challenging to generate functional cartilaginous tissue. While the synthetic response of chondrocytes can be influenced by various means, most approaches treat chondrocytes as a homogeneous population that would respond similarly. However, isolated cells heterogeneously progress through the cell cycle, which can affect macromolecular biosynthesis. As it is possible to synchronize cells within discrete cell cycle phases, the purpose of this study was to investigate the effects of cell cycle synchronization on the chondrogenic potential of primary articular chondrocytes. DESIGN: Different methods of cell synchronization (serum starvation, thymidine, nocodazole, aphidicolin, and RO-3306) were tested for their ability to synchronize primary articular chondrocytes during the process of cell isolation. Cells (unsynchronized and synchronized) were then encapsulated in alginate gels, cultured for 4 weeks, and analyzed for their structural and biochemical properties. RESULTS: The double-thymidine method yielded the highest level of cell purity, with cells synchronized in S phase. While the cells started to lose synchronization after 24 hours, tissue constructs developed from initially S phase synchronized cells had significantly higher glycosaminoglycan and collagen II amounts than those developed using unsynchronized cells. CONCLUSIONS: Initial synchronization led to long-term changes in cartilaginous tissue formation. This effect was postulated to be due to the rapid auto-induction of TGF-ßs by actively dividing S phase cells, thereby stimulating chondrogenesis. Cell synchronization methods may also be applied in conjunction with redifferentiation methods to improve the chondrogenic potential of dedifferentiated or diseased chondrocytes.

Tantalum-containing mesoporous bioactive glass powder for hemostasis.

This study evaluates the hemostatic properties of tantalum-containing mesoporous bioactive glasses (Ta-MBGs) through a suite of in-vitro methods: hemolysis percentage, zeta potential, blood coagulation assays (Activated Partial Thromboplastin Time - APTT and Prothrombin Time - PT) and cytotoxicity tests. Five compositions of Ta-MBG, with x mol% Ta2O5 added to the glass series (80-x)SiO2-15CaO-5P2O5-xTa2O5 where x=0 (0Ta), x=0.5 (0.5Ta), x=1 (1Ta), x=5 (5Ta), and x=10 (10Ta) mol%, were synthesised. The hemostatic potential of all the Ta-MBGs was confirmed by their negative zeta potential (-23 to -31 mV), which enhances the intrinsic pathway of blood coagulation. The hemolysis percentages of all Ta-MBGs except 10Ta showed statistically significant reductions compared to the same experiments carried out both in the absence of a sample ('no treatment' group) and in the presence of 10Ta. These observations validate the consideration of Ta-MBGs as hemostatic agents as they do not cause significant lysis of red blood cells. Cytotoxicity analysis revealed that Ta-MBGs had no effect on bovine fibroblast viability. Furthermore, a reduction in both APTT (a test to evaluate the intrinsic pathway of coagulation) and PT (a test to evaluate the extrinsic pathway) signified enhancement of hemostasis: 5Ta caused a significant reduction in APTT compared to 'no treatment', 1Ta and 10Ta and a significant reduction in PT compared to 0Ta. Therefore, we conclude that 5mol% of Ta optimised the hemostatic properties of these mesoporous bioactive glasses.

Optimization of culture media to enhance the growth of tissue engineered cartilage.

Tissue engineering is a promising option for cartilage repair. However, several hurdles still need to be overcome to develop functional tissue constructs suitable for implantation. One of the most common challenges is the general low capacity of chondrocytes to synthesize cartilage-specific extracellular matrix (ECM). While different approaches have been explored to improve the biosynthetic response of chondrocytes, several studies have demonstrated that the nutritional environment (e.g., glucose concentration and media volume) can have a profound effect on ECM synthesis. Thus, the purpose of this study was to optimize the formulation of cell culture media to upregulate the accumulation of cartilaginous ECM constituents (i.e., proteoglycans and collagen) by chondrocytes in 3D culture. Using response surface methodology, four different media factors (basal media, media volume, glucose, and glutamine) were first screened to determine optimal media formulations. Constructs were then cultured under candidate optimal media formulations for 4 weeks and analyzed for their biochemical and structural properties. Interestingly, the maximal accumulation of proteoglycans and collagen appeared to be elicited by different media formulations. Most notably, proteoglycan accumulation was favored by high volume, low glucose-containing DMEM/F12 (1:1) media whereas collagen accumulation was favored by high volume, high glucose-containing F12 media. While high glutamine-containing media elicited increased DNA content, glutamine concentration had no apparent effect on ECM accumulation. Therefore, optimizing the nutritional environment during chondrocyte culture appears to be a promising, straight-forward approach to improve cartilaginous tissue formation. Future work will investigate the combined effects of the nutritional environment and external stimuli.

Engineering of scaffold-free tri-layered auricular tissues for external ear reconstruction.

OBJECTIVES: Current strategies for external ear reconstruction can lead to donor site morbidity and/or surgical complications. Tissue-engineered auricular tissues may provide readily available reconstructive materials that resemble native auricular tissue, which is composed of a cartilaginous region sandwiched between two perichondrial layers. We previously developed scaffold-free bi-layered auricular tissues, consisting of a perichondrial layer and a cartilaginous layer, by cultivating chondrocytes and perichondrial cells in a continuous flow bioreactor. Here, we aimed to improve construct properties and develop strategies to engineer tri-layered auricular constructs that better mimic native auricular tissue. STUDY DESIGN: Experimental study. METHODS: Different concentrations of insulin-like growth factor (IGF)-1 and insulin were supplemented during bioreactor culture to determine conditions for engineering bi-layered constructs. We also investigated two methods of engineering tri-layered constructs. Method 1 used Ficoll separation to isolate perichondrial cells, followed by the seeding of isolated perichondrial cells onto the opposing side of the bi-layered constructs. Method 2 involved the growth of the bi-layered constructs in osteogenic culture medium. RESULTS: The combination of 10 nM IGF-1 and 100 nM insulin led to increased collagen content in the engineered bi-layered constructs. For developing tri-layered constructs, method 2 yielded thicker constructs with better mechanical and biochemical properties compared to method 1. In addition, the presence of the perichondrial layers protected the engineered constructs from tissue calcification. CONCLUSION: Auricular tissues with a biomimetic microstructure can be created by growing chondrocytes and perichondrial cells in a continuous flow bioreactor, followed by cultivation in osteogenic medium. LEVEL OF EVIDENCE: NA Laryngoscope, 129:E272-E283, 2019.

Stochastic Resonance with Dynamic Compression Improves the Growth of Adult Chondrocytes in Agarose Gel Constructs.

Dynamic mechanical stimulation has been an effective method to improve the growth of tissue engineering cartilage constructs derived from immature cells. However, when more mature cell populations are used, results are often variable due to the differing responses of these cells to external stimuli. This can be especially detrimental in the case of mechanical loading. In previous studies, multi-modal mechanical stimulation in the form of stochastic resonance was shown to be effective at improving the growth of young bovine chondrocytes. Thus, the aim of this study was to investigate the short-term and long-term effects of stochastic resonance on two groups of bovine chondrocytes, adult (> 30 month) and juvenile (~ 18 months). While the juvenile cells outperformed the adult cells in terms of their anabolic response to loading, combined mechanical loading for both age groups resulted in greater matrix synthesis compared to compressive loading alone. In the adult cells, potential pathological tissue formation was evident with the presence of cell clustering. However, the presence of broad-band mechanical vibrations (alone or with compressive loading) appeared to mitigate this response and allow these cells to attain a growth response similar to the juvenile, unstimulated cells. Therefore, the use of stochastic resonance appears to show promise as a method to improve the formation and properties of tissue engineered cartilage constructs, irrespective of cell age.

Comparisons of Auricular Cartilage Tissues from Different Species.

OBJECTIVES: Tissue engineering of auricular cartilage has great potential in providing readily available materials for reconstructive surgeries. As the field of tissue engineering moves forward to developing human tissues, there needs to be an interspecies comparison of the native auricular cartilage in order to determine a suitable animal model to assess the performance of engineered auricular cartilage in vivo. METHODS: Here, we performed interspecies comparisons of auricular cartilage by comparing tissue microstructure, protein localization, biochemical composition, and mechanical properties of auricular cartilage tissues from rat, rabbit, pig, cow, and human. RESULTS: Human, pig, and cow auricular cartilage have smaller lacunae compared to rat and rabbit cartilage ( P < .05). Despite differences in tissue microstructure, human auricular cartilage has similar biochemical composition to both rat and rabbit. Auricular cartilage from pig and cow, alternatively, display significantly higher glycosaminoglycan and collagen contents compared to human, rat, and rabbit ( P < .05). The mechanical properties of human auricular cartilage were comparable to that of all 4 animal species. CONCLUSIONS: This is the first study that compares the microstructural, biochemical, and mechanical properties of auricular cartilage from different species. This study showed that different experimental animal models of human auricular cartilage may be suitable in different cases.

Scaffold-free cartilage tissue engineering with a small population of human nasoseptal chondrocytes.

OBJECTIVE: Cartilage tissue engineering is a promising approach to provide suitable materials for nasal reconstruction; however, it typically requires large numbers of cells. We have previously shown that a small number of chondrocytes cultivated within a continuous flow bioreactor can elicit substantial tissue growth, but translation to human chondrocytes is not trivial. Here, we aimed to demonstrate the application of the bioreactor to generate large-sized tissues from a small population of primary human nasoseptal chondrocytes. STUDY DESIGN: Experimental study. METHODS: Chondrocytes were cultured in the bioreactor using different medium compositions, with varying amounts of serum and with or without growth factors. Resulting engineered tissues were analyzed for physical properties, biochemical composition, tissue microstructure, and protein localization. RESULTS: Bioreactor-cultivated constructs grown with serum and growth factors (basic fibroblast growth factor and transforming growth factor beta 2) had greater thickness, as well as DNA and glycosaminoglycan (GAG) contents, compared to low serum and no growth factor controls. These constructs also showed the most intense proteoglycan and collagen II staining. CONCLUSION: The combination of bioreactor conditions, serum, and growth factors allowed the generation of large, thick scaffold-free human cartilaginous tissues that resembled the native nasoseptal cartilage. There also may be implications for patient selection in future clinical applications of these engineered tissues because their GAG content decreased with donor age. LEVEL OF EVIDENCE: NA. Laryngoscope, 127:E91-E99, 2017.

Biomaterials in myocardial tissue engineering.

Cardiovascular disease is the leading cause of death in the developed world, and as such there is a pressing need for treatment options. Cardiac tissue engineering emerged from the need to develop alternative sources and methods of replacing tissue damaged by cardiovascular diseases, as the ultimate treatment option for many who suffer from end-stage heart failure is a heart transplant. In this review we focus on biomaterial approaches to augmenting injured or impaired myocardium, with specific emphasis on: the design criteria for these biomaterials; the types of scaffolds - composed of natural or synthetic biomaterials or decellularized extracellular matrix - that have been used to develop cardiac patches and tissue models; methods to vascularize scaffolds and engineered tissue; and finally, injectable biomaterials (hydrogels) designed for endogenous repair, exogenous repair or as bulking agents to maintain ventricular geometry post-infarct. The challenges facing the field and obstacles that must be overcome to develop truly clinically viable cardiac therapies are also discussed.

Hydrogels with integrin-binding angiopoietin-1-derived peptide, QHREDGS, for treatment of acute myocardial infarction.

BACKGROUND: Hydrogels are being actively investigated for direct delivery of cells or bioactive molecules to the heart after myocardial infarction (MI) to prevent cardiac functional loss. We postulate that immobilization of the prosurvival angiopoietin-1-derived peptide, QHREDGS, to a chitosan-collagen hydrogel could produce a clinically translatable thermoresponsive hydrogel to attenuate post-MI cardiac remodeling. METHODS AND RESULTS: In a rat MI model, QHREDGS-conjugated hydrogel (QHG213H), control gel, or PBS was injected into the peri-infarct/MI zone. By in vivo tracking and chitosan staining, the hydrogel was demonstrated to remain in situ for 2 weeks and was cleared in ≈3 weeks. By echocardiography and pressure-volume analysis, the QHG213H hydrogel significantly improved cardiac function compared with the controls. Scar thickness and scar area fraction were also significantly improved with QHG213H gel injection compared with the controls. There were significantly more cardiomyocytes, determined by cardiac troponin-T staining, in the MI zone of the QHG213H hydrogel group; and hydrogel injection did not induce a significant inflammatory response as assessed by polymerase chain reaction and an inflammatory cytokine assay. The interaction of cardiomyocytes and cardiac fibroblasts with QHREDGS was found to be mediated by ß1-integrins. CONCLUSIONS: We demonstrated for the first time that the QHG213H peptide-modified hydrogel can be injected in the beating heart where it remains localized for a clinically effective period. Moreover, the QHG213H hydrogel induced significant cardiac functional and morphological improvements after MI relative to the controls.

Perfusable branching microvessel bed for vascularization of engineered tissues.

Vascularization is critical for the survival of engineered tissues in vitro and in vivo. In vivo, angiogenesis involves endothelial cell proliferation and sprouting followed by connection of extended cellular processes and subsequent lumen propagation through vacuole fusion. We mimicked this process in engineering an organized capillary network anchored by an artery and a vein. The network was generated by inducing directed capillary sprouting from vascular explants on micropatterned substrates containing thymosin ß4-hydrogel. The capillary outgrowths connected between the parent explants by day 21, a process that was accelerated to 14 d by application of soluble VEGF and hepatocyte growth factor. Confocal microscopy and transmission electron microscopy indicated the presence of tubules with lumens formed by endothelial cells expressing CD31, VE-cadherin, and von Willebrand factor. Cardiac tissues engineered around the resulting vasculature exhibited improved functional properties, cell striations, and cell-cell junctions compared with tissues without prevascularization. This approach uniquely allows easy removal of the vasculature from the microfabricated substrate and easy seeding of the tissue specific cell types in the parenchymal space.

Controlled delivery of thymosin ß4 for tissue engineering and cardiac regenerative medicine.

Thymosin ß4 (Tß4) is a peptide with multiple biological functions. Here, we focus on the role of Tß4 in vascularization, and review our studies of the controlled delivery of Tß4 through its incorporation in biomaterials. Tß4 promotes vascularization through VEGF induction and AcSDKP-induced migration and differentiation of endothelial cells. We developed a collagen-chitosan hydrogel for the controlled release of Tß4 over 28 days. In vitro, the Tß4-encapsulated hydrogel increased migration of endothelial cells and tube formation from epicardial explants that were cultivated on top of the hydrogel, compared to Tß4-free hydrogel and soluble Tß4 in the culture medium. In vivo, subcutaneously injected Tß4-containing collagen-chitosan hydrogel in rats led to enhanced vascularization compared to Tß4-free hydrogel and collagen hydrogel with Tß4. Furthermore, the injection of the Tß4-encapsulated hydrogel in the infarct region improved angiogenesis, reduced tissue loss, and retained left ventricular wall thickness after myocardial infarction in rats.

Biofabrication enables efficient interrogation and optimization of sequential culture of endothelial cells, fibroblasts and cardiomyocytes for formation of vascular cords in cardiac tissue engineering.

We previously reported that preculture of fibroblasts (FBs) and endothelial cells (ECs) prior to cardiomyocytes (CMs) improved the structural and functional properties of engineered cardiac tissue compared to culture of CMs alone or co-culture of all three cell types. However, these approaches did not result in formation of capillary-like cords, which are precursors to vascularization in vivo. Here we hypothesized that seeding the ECs first on Matrigel and then FBs 24 h later to stabilize the endothelial network (sequential preculture) would enhance cord formation in engineered cardiac organoids. Three sequential preculture groups were tested by seeding ECs (D4T line) at 8%, 15% and 31% of the total cell number on Matrigel-coated microchannels and incubating for 24 h. Cardiac FBs were then seeded (32%, 25% and 9% of the total cell number, respectively) and incubated an additional 24 h. Finally, neonatal rat CMs (60% of the total cell number) were added and the organoids were cultivated for seven days. Within 24 h, the 8% EC group formed elongated cords which eventually developed into beating cylindrical organoids, while the 15% and 31% EC groups proliferated into flat EC monolayers with poor viability. Excitation threshold (ET) in the 8% EC group (3.4 ± 1.2 V cm(-1)) was comparable to that of the CM group (3.3 ± 1.4 V cm(-1)). The ET worsened with increasing EC seeding density (15% EC: 4.4 ± 1.5 V cm(-1); 31% EC: 4.9 ± 1.5 V cm(-1)). Thus, sequential preculture promoted vascular cord formation and enhanced architecture and function of engineered heart tissues.

Hydrogel substrate stiffness and topography interact to induce contact guidance in cardiac fibroblasts.

Previous studies demonstrated the importance of substrate stiffness and topography on the phenotype of many different cell types including fibroblasts. Yet the interaction of these two physical parameters remains insufficiently characterized, in particular for cardiac fibroblasts. Most studies focusing on contact guidance use rigid patterned substrates. It is not known how the ability of cardiac fibroblasts to follow grooves and ridges changes as the substrate stiffness is decreased to match the range of stiffness found in native heart tissues. This report demonstrates a significant interactive effect of substrate stiffness and topography on cardiac fibroblast elongation and orientation using polyacrylamide substrates of different stiffness and topography.

Controlled release of thymosin ß4 from injected collagen-chitosan hydrogels promotes angiogenesis and prevents tissue loss after myocardial infarction.

AIMS: Acute myocardial infarction (MI) leads to fibrosis and severe left ventricular wall thinning. Enhancing vascularization within the infarct reduces cell death and maintains a thick left ventricular wall, which is essential for proper cardiac function. Here, we evaluated the controlled delivery of thymosin ß4 (Tß4), which supports cardiomyocyte survival by inducing vascularization and upregulating Akt activity, in the treatment of MI. MATERIALS & METHODS: We injected collagen-chitosan hydrogel with controlled release of Tß4 into the infarct after performing left anterior descending artery ligation in rats. RESULTS: Tß4-encapsulated hydrogel (thymosin) significantly reduced tissue loss post-MI (13 ± 4%), compared with 58 ± 3% and 30 ± 8% tissue loss for no treatment (MI only) and Tß4-free hydrogel (control). Significantly more Factor VIII-positive blood vessels with diameter >50 µm were in the thymosin group compared with both MI only and control (p < 0.0001), showing Tß4-induced vascularization. Wall thickness was positively correlated with the mature blood vessel density (r = 0.9319; p < 0.0001). CONCLUSION: Controlled release of Tß4 within the infarct enhances angiogenesis and presence of cardiomyocytes that are necessary for cardiac repair.