Protein composition alters in vivo resorption of PEG-based hydrogels as monitored by contrast-enhanced MRI

Mimicking the extracellular world: from natural to fully synthetic matrices utilizing supramolecular biomaterials

The extracellular matrix (ECM) has evolved around complex covalent and non-covalent interactions to create impressive function-from cellular signaling to constant remodeling. A major challenge in the biomedical field is the de novo design and control of synthetic ECMs for applications ranging from tissue engineering to neuromodulation to bioelectronics. As we move towards recreating the ECM's complexity in hydrogels, the field has taken several approaches to recapitulate the main important features of the native ECM (i.e. mechanical, bioactive and dynamic properties). In this review, we first describe the wide variety of hydrogel systems that are currently used, ranging from fully natural to completely synthetic to hybrid versions, highlighting the advantages and limitations of each class. Then, we shift towards supramolecular hydrogels that show great potential for their use as ECM mimics due to their biomimetic hierarchical structure, inherent (controllable) dynamic properties and their modular design, allowing for precise control over their mechanical and biochemical properties. In order to make the next step in the complexity of synthetic ECM-mimetic hydrogels, we must leverage the supramolecular self-assembly seen in the native ECM; we therefore propose to use supramolecular monomers to create larger, hierarchical, co-assembled hydrogels with complex and synergistic mechanical, bioactive and dynamic features.

Development of a local controlled release system for therapeutic proteins in the treatment of skeletal muscle injuries and diseases

The present study aims to develop and characterize a controlled-release delivery system for protein therapeutics in skeletal muscle regeneration following an acute injury. The therapeutic protein, a membrane-GPI anchored protein called Cripto, was immobilized in an injectable hydrogel delivery vehicle for local administration and sustained release. The hydrogel was made of poly(ethylene glycol)-fibrinogen (PEG-Fibrinogen, PF), in the form of injectable microspheres. The PF microspheres exhibited a spherical morphology with an average diameter of approximately 100 micrometers, and the Cripto protein was uniformly entrapped within them. The release rate of Cripto from the PF microspheres was controlled by tuning the crosslinking density of the hydrogel, which was varied by changing the concentration of poly(ethylene glycol) diacrylate (PEG-DA) crosslinker. In vitro experiments confirmed a sustained-release profile of Cripto from the PF microspheres for up to 27 days. The released Cripto was biologically active and promoted the in vitro proliferation of mouse myoblasts. The therapeutic effect of PF-mediated delivery of Cripto in vivo was tested in a cardiotoxin (CTX)-induced muscle injury model in mice. The Cripto caused an increase in the in vivo expression of the myogenic markers Pax7, the differentiation makers eMHC and Desmin, higher numbers of centro-nucleated myofibers and greater areas of regenerated muscle tissue. Collectively, these results establish the PF microspheres as a potential delivery system for the localized, sustained release of therapeutic proteins toward the accelerated repair of damaged muscle tissue following acute injuries.

Digital Light Processing of 19F MRI-Traceable Gelatin-Based Biomaterial Inks towards Bone Tissue Regeneration

Gelatin-based photo-crosslinkable hydrogels are promising scaffold materials to serve regenerative medicine. They are widely applicable in additive manufacturing, which allows for the production of various scaffold microarchitectures in line with the anatomical requirements of the organ to be replaced or tissue defect to be treated. Upon their in vivo utilization, the main bottleneck is to monitor cell colonization along with their degradation (rate). In order to enable non-invasive visualization, labeling with MRI-active components like N-(2,2-difluoroethyl)acrylamide (DFEA) provides a promising approach. Herein, we report on the development of a gelatin-methacryloyl-aminoethyl-methacrylate-based biomaterial ink in combination with DFEA, applicable in digital light processing-based additive manufacturing towards bone tissue regeneration. The fabricated hydrogel constructs show excellent shape fidelity in line with the printing resolution, as DFEA acts as a small molecular crosslinker in the system. The constructs exhibit high stiffness (E = 36.9 ± 4.1 kPa, evaluated via oscillatory rheology), suitable to serve bone regeneration and excellent MRI visualization capacity. Moreover, in combination with adipose tissue-derived stem cells (ASCs), the 3D-printed constructs show biocompatibility, and upon 4 weeks of culture, the ASCs express the osteogenic differentiation marker Ca2+.

Dynamic monitoring soft tissue healing via visualized Gd-crosslinked double network MRI microspheres

By integrating magnetic resonance-visible components with scaffold materials, hydrogel microspheres (HMs) become visible under magnetic resonance imaging(MRI), allowing for non-invasive, continuous, and dynamic monitoring of the distribution, degradation, and relationship of the HMs with local tissues. However, when these visualization components are physically blended into the HMs, it reduces their relaxation rate and specificity under MRI, weakening the efficacy of real-time dynamic monitoring. To achieve MRI-guided in vivo monitoring of HMs with tissue repair functionality, we utilized airflow control and photo-crosslinking methods to prepare alginate-gelatin-based dual-network hydrogel microspheres (G-AlgMA HMs) using gadolinium ions (Gd (III)), a paramagnetic MRI contrast agent, as the crosslinker. When the network of G-AlgMA HMs degrades, the cleavage of covalent bonds causes the release of Gd (III), continuously altering the arrangement and movement characteristics of surrounding water molecules. This change in local transverse and longitudinal relaxation times results in variations in MRI signal values, thus enabling MRI-guided in vivo monitoring of the HMs. Additionally, in vivo data show that the degradation and release of polypeptide (K2 (SL)6 K2 (KK)) from G-AlgMA HMs promote local vascular regeneration and soft tissue repair. Overall, G-AlgMA HMs enable non-invasive, dynamic in vivo monitoring of biomaterial degradation and tissue regeneration through MRI, which is significant for understanding material degradation mechanisms, evaluating biocompatibility, and optimizing material design.

Methacrylated Fibrinogen Hydrogels for 3D Cell Culture and Delivery

Methacrylation was performed on fibrinogen to design a new biomedical hydrogel for 3D cell culture or as a biodegradable delivery matrix for in vivo implantation. The methacrylation of denatured fibrinogen in solution was performed using methacrylic anhydride (MAA). The extent of fibrinogen methacrylation was quantified by proton NMR and controlled using stochiometric quantities of MAA during the reaction. The methacrylated fibrinogen (FibMA) hydrogels were formed by light-activated free-radical polymerization in the presence of macromolecular cross-linking polymers made from acrylated poly(ethylene glycol) (PEG). The biocompatibility and biodegradability of the FibMA hydrogels were characterized by in vitro assays and in vivo implantation experiments using quantitative magnetic resonance imaging (MRI) of the implant volume. The FibMA supported the growth and metabolic activity of human dermal fibroblasts in both 2D and 3D cultures. The methacrylation did not alter important biological attributes of the fibrinogen, including the ability to support cell adhesion and 3D cell culture, as well as to undergo proteolysis. Animal experiments confirmed the biodegradability of the FibMA for potential use as a scaffold in tissue engineering, as a bioink for 3D printing, or as a biodegradable matrix for in vivo sustained delivery of bioactive factors. STATEMENT OF SIGNIFICANCE: : This paper describes methacrylated fibrinogen (FibMA) and the formation of a biomedical hydrogel from FibMA for cell culture and other biomedical applications. Inspired from methacrylated gelatin (GelMA), the FibMA is made from blood-derived fibrinogen which is more suitable for clinical use. Sharing similar properties to other hydrogels made from methacrylated proteins, the FibMA has yet to be reported in the literature. In this manuscript, we provide the methodology to produce the FibMA hydrogels, we document the mechanical versatility of this new biomaterial, and we show the biocompatibility using 3D cell culture studies and in vivo implantations.

Research progress on albumin-based hydrogels: Properties, preparation methods, types and its application for antitumor-drug delivery and tissue engineering

Albumin is derived from blood plasma and is the most abundant protein in blood plasma, which has good mechanical properties, biocompatibility and degradability, so albumin is an ideal biomaterial for biomedical applications, and drug-carriers based on albumin can better reduce the cytotoxicity of drug. Currently, there are numerous reviews summarizing the research progress on drug-loaded albumin molecules or nanoparticles. In comparison, the study of albumin-based hydrogels is a relatively small area of research, and few articles have systematically summarized the research progress of albumin-based hydrogels, especially for drug delivery and tissue engineering. Thus, this review summarizes the functional features and preparation methods of albumin-based hydrogels, different types of albumin-based hydrogels and their applications in antitumor drugs, tissue regeneration engineering, etc. Also, potential directions for future research on albumin-based hydrogels are discussed.

Chondral and Osteochondral Femoral Cartilage Lesions Treated with GelrinC: Significant Improvement of Radiological Outcome Over Time and Zonal Variation of the Repair Tissue Based on T2 Mapping at 24 Months

OBJECTIVE

To prospectively assess the efficacy of GelrinC in the treatment of chondral and osteochondral femoral cartilage lesions using morphological (Magnetic Resonance Observation of Cartilage Repair Tissue [MOCART]) and quantitative (T₂-mapping) magnetic resonance imaging (MRI).

DESIGN

This study was designed as a prospective single-arm, open label, multicenter study. Morphological magnetic resonance imaging (MRI) for MOCART assessment and T₂ mapping was performed 1 week and 6, 12, 18, and 24 months after GelrinC implantation. Evaluation of T₂ mapping was based on the assessment of global T₂ indices (T₂ of the repair tissue [RT] divided by T₂ of healthy reference cartilage) and zonal variation.

RESULTS

Fifty-six (20 female) patients were prospectively enrolled. The mean MOCART score significantly increased from baseline to the 24-month follow-up with 88.8 (95% CI, 85.8-91.9; P < 0.001) for all lesions combined as well as 86.8 (95% CI, 83.0-90.6) for chondral lesions and 94.1 (95% CI, 68.55-100) for osteochondral lesions. Furthermore, based on T₂ mapping, significant zonal variation of the RT was observed at 24 months (P = 0.039), which did not differ significantly from healthy reference cartilage (P = 0.6).

CONCLUSION

Increasing MOCART scores were observed throughout the follow-up period, indicative of maturation of the cartilage repair. Significant zonal variation of the RT at 24 months might indicate the transformation into hyaline cartilage-like RT. Slightly differing morphological outcome between chondral and osteochondral lesions, but similar global and zonal T₂ indices at 24 months, support the potential of GelrinC as a treatment option for both lesion types.

3D bioprinting of a trachea-mimetic cellular construct of a clinically relevant size

Despite notable advances in extrusion-based 3D bioprinting, it remains a challenge to create a clinically-sized cellular construct using extrusion-based 3D printing due to long printing times adversely affecting cell viability and functionality. Here, we present an advanced extrusion-based 3D bioprinting strategy composed of a two-step printing process to facilitate creation of a trachea-mimetic cellular construct of clinically relevant size. A porous bellows framework is first printed using typical extrusion-based 3D printing. Selective printing of cellular components, such as cartilage rings and epithelium lining, is then performed on the outer grooves and inner surface of the bellows framework by a rotational printing process. With this strategy, 3D bioprinting of a trachea-mimetic cellular construct of clinically relevant size is achieved in significantly less total printing time compared to a typical extrusion-based 3D bioprinting strategy which requires printing of an additional sacrificial material. Tracheal cartilage formation was successfully demonstrated in a nude mouse model through a subcutaneous implantation study of trachea-mimetic cellular constructs wrapped with a sinusoidal-patterned tubular mesh preventing rapid resorption of cartilage rings in vivo. This two-step 3D bioprinting for a trachea-mimetic cellular construct of clinically relevant size can provide a fundamental step towards clinical translation of 3D bioprinting based tracheal reconstruction.

Proanthocyanidin-crosslinked collagen/konjac glucomannan hydrogel with improved mechanical properties and MRI trackable biodegradation for potential tissue engineering scaffolds

Collagen (Col) has been intensively exploited as a biomaterial for its excellent biocompatibility, biodegradation and bioactivity. However, the poor mechanical properties and rapid biodegradation of reconstituted collagen hydrogels have always been the bottlenecks for their further development especially for vascular tissue engineering. Herein, based on the self-assembly characteristics of collagen, a ternary hydrogel scaffold, comprising rigid collagen molecules, flexible konjac glucomannan (KGM) chains and biocompatible crosslinkers of proanthocyanidin (PA), has been designed to achieve a synergistic interaction for essentially optimizing the mechanical properties of the so-obtained Col/KGM/PA hydrogel, which possesses not only substantially improved strength but also good elasticity. PA endows these scaffolds with controllable biodegradation and anti-calcification and antioxidant activities. TEM discovered the co-existence of two types of fibrils with distinctly different arrangement patterns, explaining the contribution of KGM macromolecules to elasticity generation. The in vivo variations of Col/KGM/PA implants are visualized in real-time by magnetic resonance imaging (MRI). Moreover, a quantitative technique of MRI T₂-mapping combined with histology is designed to visualize the in vivo biodegradation mechanism of layer-by-layer erosion for these hydrogels. Simultaneously, three different relationships between the respective processes of in vivo degradation and in vivo dehydration of these controlled hydrogel implants were clearly revealed by this technique. Such a designed Col/KGM/PA composite hydrogel realizes the essential integration of good biocompatibility, controllable biodegradation and improved mechanical properties for developing a desired scaffold material for tissue engineering applications.

Fibrinogen-Based Hydrogel Modulus and Ligand Density Effects on Cell Morphogenesis in Two-Dimensional and Three-Dimensional Cell Cultures

There is a need to further explore the convergence of mechanobiology and dimensionality with systematic investigations of cellular response to matrix mechanics in 2D and 3D cultures. Here, a semisynthetic hydrogel capable of supporting both 2D and 3D cell culture is applied to investigate cell response to matrix modulus and ligand density. The culture materials are fabricated from adducts of polyethylene glycol (PEG) or PluronicF127 and fibrinogen fragments, formed into hydrogels by free-radical polymerization, and characterized by shear rheology. Control over the modulus of the materials is accomplished by changing the concentration of synthetic PEG-diacrylate crosslinker (0.5% w/v), and by altering the molecular length of the PEG (10 and 20 kDa). Control over ligand density is accomplished by changing fibrinogen concentrations from 3 to 12 mg mL^-1 . In 2D culture, cell motility parameters, including cell speed and persistence time are significantly increased with increasing modulus. In both 2D and 3D culture, cells express vinculin and there is evidence of focal adhesion formation in the high stiffness materials. The modulus- and ligand-dependent morphogenesis response from the cells in 2D culture is contradictory to the same measured response in 3D culture. In 2D culture, anchorage-dependent cells become more elongated and significantly increase their size with increasing ligand density and matrix modulus. In 3D culture, the same anchorage-dependent cells become less spindled and significantly reduce their size in response to increasing ligand density and matrix modulus. These differences arise from dimensionality constraints, most notably the encapsulation of cells in a non-porous hydrogel matrix. These insights underscore the importance of mechanical properties in regulating cell morphogenesis in a 3D culture milieu. The versatility of the hydrogel culture environment further highlights the significance of a modular approach when developing materials that aim to optimize the cell culture environment.

MRI Visualization of Injectable Ureidopyrimidinone Hydrogelators by Supramolecular Contrast Agent Labeling

Information about the in vivo location, shape, degradation, or erosion rate of injected in situ gelating hydrogels can be obtained with magnetic resonance imaging (MRI). Herein, an injectable supramolecular ureidopyrimidinone-based hydrogel (UPy-PEG) is functionalized with a modified Gadolinium(III)-DOTA complex (UPy-Gd) for contrast enhanced MRI. The contrast agent is designed to supramolecularly interact with the hydrogel network to enable high-quality imaging of this hydrogel. The applicability of the approach is demonstrated with successful visualization of the Gd-labeled UPy-PEG hydrogel after targeted intramyocardial catheter injection in a pig heart.

Assessment of in vivo degradation profiles of hyaluronic acid hydrogels using temporal evolution of chemical exchange saturation transfer (CEST) MRI

Hyaluronic acid (HA) hydrogels have found a wide range of applications in biomedicine: regenerative medicine to drug delivery applications. In vivo quantitative assessment of these hydrogels using magnetic resonance imaging (MRI) provides an effective, accurate, safe, and non-invasive translational approach to assess the biodegradability of HA hydrogels. Chemical exchange saturation transfer (CEST) is an MRI contrast enhancement technique that overcomes the concentration limitation of other techniques like magnetic resonance spectroscopy (MRS) by detecting metabolites at up to two orders of magnitude or higher. In this study, HA hydrogels were synthesized based on different crosslinking agents and assessed using CEST MRI to investigate the in vivo degradation profiles of these gels in a mouse subcutaneous injection model over a three-month period. Nature of crosslinking agents was found to influence their degradation profiles. Since CEST MRI provides a unique chemical signature to visualize HA hydrogels, our studies proved that this technique could be used as a guide in the hydrogel optimization process for drug delivery and regenerative medicine applications.

Hydrogel composition and laser micropatterning to regulate sciatic nerve regeneration

Treatment of peripheral nerve injuries has evolved over the past several decades to include the use of sophisticated new materials endowed with trophic and topographical cues that are essential for in vivo nerve fibre regeneration. In this research, we explored the use of an advanced design strategy for peripheral nerve repair, using biological and semi-synthetic hydrogels that enable controlled environmental stimuli to regenerate neurons and glial cells in a rat sciatic nerve resection model. The provisional nerve growth conduits were composed of either natural fibrin or adducts of synthetic polyethylene glycol and fibrinogen or gelatin. A photo-patterning technique was further applied to these 3D hydrogel biomaterials, in the form of laser-ablated microchannels, to provide contact guidance for unidirectional growth following sciatic nerve injury. We tested the regeneration capacity of subcritical nerve gap injuries in rats treated with photo-patterned materials and compared these with injuries treated with unpatterned hydrogels, either stiff or compliant. Among the factors tested were shear modulus, biological composition, and micropatterning of the materials. The microchannel guidance patterns, combined with appropriately matched degradation and stiffness properties of the material, proved most essential for the uniform tissue propagation during the nerve regeneration process.

Multimodal Hydrogel-Based Platform To Deliver and Monitor Cardiac Progenitor/Stem Cell Engraftment

Retention and survival of transplanted cells are major limitations to the efficacy of regenerative medicine, with short-term paracrine signals being the principal mechanism underlying current cell therapies for heart repair. Consequently, even improvements in short-term durability may have a potential impact on cardiac cell grafting. We have developed a multimodal hydrogel-based platform comprised of a poly(ethylene glycol) network cross-linked with bioactive peptides functionalized with Gd(III) in order to monitor the localization and retention of the hydrogel in vivo by magnetic resonance imaging. In this study, we have tailored the material for cardiac applications through the inclusion of a heparin-binding peptide (HBP) sequence in the cross-linker design and formulated the gel to display mechanical properties resembling those of cardiac tissue. Luciferase-expressing cardiac stem cells (CSC-Luc2) encapsulated within these gels maintained their metabolic activity for up to 14 days in vitro. Encapsulation in the HBP hydrogels improved CSC-Luc2 retention in the mouse myocardium and hind limbs at 3 days by 6.5- and 12- fold, respectively. Thus, this novel heparin-binding based, Gd(III)-tagged hydrogel and CSC-Luc2 platform system demonstrates a tailored, in vivo detectable theranostic cell delivery system that can be implemented to monitor and assess the transplanted material and cell retention.

Diamagnetic chemical exchange saturation transfer (diaCEST) affords magnetic resonance imaging of extracellular matrix hydrogel implantation in a rat model of stroke

Extracellular matrix (ECM) is widely used as an inductive biological scaffold to repair soft tissue after injury by promoting functional site-appropriate remodeling of the implanted material. However, there is a lack of non-invasive analysis methods to monitor the remodeling characteristics of the ECM material after implantation and its biodegradation over time. We describe the use of diamagnetic chemical exchange saturation transfer (CEST) magnetic resonance imaging to monitor the distribution of an ECM hydrogel after intracerebral implantation into a stroke cavity. In vitro imaging indicated a robust concentration-dependent detection of the ECM precursor and hydrogel at 1.8 and 3.6 ppm, which broadly corresponded to chondroitin sulfate and fibronectin. This detection was robust to changes in pH and improved at 37 °C. In vivo implantation of ECM hydrogel into the stroke cavity in a rat model corresponded macroscopically to the distribution of biomaterial as indicated by histology, but mismatches were also evident. Indeed, CEST imaging detected an endogenous "increased deposition". To account for this endogenous activity, pre-implantation images were subtracted from post-implantation images to yield a selective visualization of hydrogel distribution in the stroke cavity and its evolution over 7 days. The CEST detection of ECM returned to baseline within 3 days due to a decrease in fibronectin and chondroitin sulfate in the hydrogel. The distribution of ECM hydrogel within the stroke cavity is hence feasible in vivo, but further advances are required to warrant a selective long-term monitoring in the context of biodegradation.

Hydrogel as a bioactive material to regulate stem cell fate

The encapsulation of stem cells in a hydrogel substrate provides a promising future in biomedical applications. However, communications between hydrogels and stem cells is complicated; various factors such as porosity, different polymer types, stiffness, compatibility and degradation will lead to stem cell survival or death. Hydrogels mimic the three-dimensional extracellular matrix to provide a friendly environment for stem cells. On the other hand, stem cells can sense the surroundings to make the next progression, stretching out, proliferating or just to remain. As such, understanding the correlation between stem cells and hydrogels is crucial. In this Review, we first discuss the varying types of the hydrogels and stem cells, which are most commonly used in the biomedical fields and further investigate how hydrogels interact with stem cells from the perspective of their biomedical application, while providing insights into the design and development of hydrogels for drug delivery, tissue engineering and regenerative medicine purpose. In addition, we compare the results such as stiffness, degradation time and pore size as well as peptide types of hydrogels from respected journals. We also discussed most recently magnificent materials and their effects to regulate stem cell fate.

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Hydrogels as Extracellular Matrix (ECM) mimics stem cells proliferation and differentiation.

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Discuss how hydrogels interact with stem cells from the perspective of their biomedical applications.

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Recent magnificent materials and their effects to regulate stem cells fate.

A Novel Strategy for Softening Gelatin-Bioactive-Glass Hybrids

The brittle structure of polymer-bioactive-glass hybrids is a hurdle for their biomedical applications. To address this issue here, we developed a novel method to cease the overcondensation of bioactive-glass by polymer cross-linking. Here, an organosilane-functionalized gelatin methacrylate (GelMA) is covalently bonded to a bioactive-glass during the sol-gel process, and the condensation of silica networks is controlled by photo-cross-linking of GelMA. The physicochemical properties and mechanical strength of these hybrids are tunable by the incorporation of secondary cross-linking agents. These hydrogels display elastic properties with ultimate compression strain above 0.2 mm·mm(-1) and tunable compressive modulus in the range of 42-530 kPa. In addition, these hydrogels are bioactive because they promoted the alkaline phosphatase activity of bone progenitor cells. They are also well-tolerated in the mice subcutaneous model. Therefore, our method is efficient for the prevention of overcondensation and allows preparation of soft bioactive hydrogels from organic-inorganic matrices, suitable for soft and hard tissue regeneration.

Functional biomedical hydrogels for in vivo imaging

In vivo imaging of biomedical hydrogels enables real-time and non-invasive visualization of the status of structure and function of hydrogels.

An integrated nanoplatform for theranostics via multifunctional core–shell ferrite nanocubes

A novel integrated nanoplatform facilitates excellent targeted MR imaging guided synergism of magnetothermal and chemotherapy based on magnetic core–shell ferrite nanocubes (MNCs).

Visualization of in situ hydrogels by MRI in vivo

Chitosan and PEG-based self-healable in situ hydrogel developed as a long-term MRI reporter.

PET imaging of a collagen matrix reveals its effective injection and targeted retention in a mouse model of myocardial infarction

Injectable biomaterials have shown promise for cardiac regeneration therapy. However, little is known regarding their retention and distribution upon application in vivo. Matrix imaging would be useful for evaluating these important properties. Herein, hexadecyl-4-[(18)F]fluorobenzoate ((18)F-HFB) and Qdot labeling was used to evaluate collagen matrix delivery in a mouse model of myocardial infarction (MI). At 1 wk post-MI, mice received myocardial injections of (18)F-HFB- or Qdot-labeled matrix to assess its early retention and distribution (at 10 min and 2h) by positron emission tomography (PET), or fluorescence imaging, respectively. PET imaging showed that the bolus of matrix at 10 min redistributed evenly within the ischemic territory by 2h. Ex vivo biodistribution revealed myocardial matrix retention of ∼ 65%, which correlated with PET results, but may be an underestimate since (18)F-HFB matrix labeling efficiency was ∼ 82%. For covalently linked Qdots, labeling efficiency was ∼ 96%. Ex vivo Qdot quantification showed that ∼ 84% of the injected matrix was retained in the myocardium. Serial non-invasive PET imaging and validation by fluorescence imaging confirmed the effectiveness of the collagen matrix to be retained and redistributed within the infarcted myocardium. This study identifies matrix-targeted imaging as a promising modality for assessing the biodistribution of injectable biomaterials for application in the heart.