Concise review: injectable biomaterials for the treatment of myocardial infarction and peripheral artery disease: translational challenges and progress

Injectable hyaluronic acid based microrods provide local micromechanical and biochemical cues to attenuate cardiac fibrosis after myocardial infarction

Repairing cardiac tissue after myocardial infarction (MI) is one of the most challenging goals in tissue engineering. Following ischemic injury, significant matrix remodeling and the formation of avascular scar tissue significantly impairs cell engraftment and survival in the damaged myocardium. This limits the efficacy of cell replacement therapies, demanding strategies that reduce pathological scarring to create a suitable microenvironment for healthy tissue regeneration. Here, we demonstrate the successful fabrication of discrete hyaluronic acid (HA)-based microrods to provide local biochemical and biomechanical signals to reprogram cells and attenuate cardiac fibrosis. HA microrods were produced in a range of physiological stiffness and shown to degrade in the presence of hyaluronidase. Additionally, we show that fibroblasts interact with these microrods in vitro, leading to significant changes in proliferation, collagen expression and other markers of a myofibroblast phenotype. When injected into the myocardium of an adult rat MI model, HA microrods prevented left ventricular wall thinning and improved cardiac function at 6 weeks post infarct.

Heart Repair Using Nanogel-Encapsulated Human Cardiac Stem Cells in Mice and Pigs with Myocardial Infarction

Stem cell transplantation is currently implemented clinically but is limited by low retention and engraftment of transplanted cells and the adverse effects of inflammation and immunoreaction when allogeneic or xenogeneic cells are used. Here, we demonstrate the safety and efficacy of encapsulating human cardiac stem cells (hCSCs) in thermosensitive poly(N-isopropylacrylamine-co-acrylic acid) or P(NIPAM-AA) nanogel in mouse and pig models of myocardial infarction (MI). Unlike xenogeneic hCSCs injected in saline, injection of nanogel-encapsulated hCSCs does not elicit systemic inflammation or local T cell infiltrations in immunocompetent mice. In mice and pigs with acute MI, injection of encapsulated hCSCs preserves cardiac function and reduces scar sizes, whereas injection of hCSCs in saline has an adverse effect on heart healing. In conclusion, thermosensitive nanogels can be used as a stem cell carrier: the porous and convoluted inner structure allows nutrient, oxygen, and secretion diffusion but can prevent the stem cells from being attacked by immune cells.

Exogenous miR-29B Delivery Through a Hyaluronan-Based Injectable System Yields Functional Maintenance of the Infarcted Myocardium

Myocardial infarction (MI) results in debilitating remodeling of the myocardial extracellular matrix (ECM). In this proof-of-principle study it was sought to modulate this aggressive remodeling by injecting a hyaluronic acid-based reservoir delivering exogenous microRNA-29B (miR-29B). This proof-of-principal study was executed whereby myocardial ischemia/reperfusion was performed on C57BL/6 mice for 45 min after which five 10 μL boluses of a hydrogel composed of thiolated hyaluronic acid cross-linked with poly (ethylene glycol) diacrylate, containing exogenous miR-29B as an active therapy, were injected into the border zone of the infarcted myocardium. Following surgery, the myocardial function of the animals was monitored up to 5 weeks. Delivering miR-29B locally using an injectable hyaluronan-based hydrogel resulted in the maintenance of myocardial function at 2 and 5 weeks following MI in this proof-of-principle study. In addition, while animals treated with the control of a nontargeting miR delivered using the hyaluronan-based hydrogel had a significant deterioration of myocardial function, those treated with miR-29B did not. Histological analysis revealed a significantly decreased presence of elastin and significantly less immature/newly deposited collagen fibers at the border zone of the infarct. Increased vascularity of the myocardial scar was also detected and Raman microspectroscopy discovered significantly altered ECM-specific biochemical signals at the border zone of the infarct. This preclinical proof-of-principle study demonstrates that an injectable hyaluronic acid hydrogel system could be capable of delivering miR-29B toward maintaining cardiac function following MI. In addition, Raman microspectroscopy revealed subtle, yet significant changes in ECM organization and maturity. These findings have great potential with regard to using injectable biomaterials as a local treatment for ischemic tissue and exogenous miRs to modulate tissue remodeling.

Carbon nanotube-incorporated collagen hydrogels improve cell alignment and the performance of cardiac constructs

Carbon nanotubes (CNTs) provide an essential 2-D microenvironment for cardiomyocyte growth and function. However, it remains to be elucidated whether CNT nanostructures can promote cell–cell integrity and facilitate the formation of functional tissues in 3-D hydrogels. Here, single-walled CNTs were incorporated into collagen hydrogels to fabricate (CNT/Col) hydrogels, which improved mechanical and electrical properties. The incorporation of CNTs (up to 1 wt%) exhibited no toxicity to cardiomyocytes and enhanced cell adhesion and elongation. Through the use of immunohistochemical staining, transmission electron microscopy, and intracellular calcium-transient measurement, the incorporation of CNTs was found to improve cell alignment and assembly remarkably, which led to the formation of engineered cardiac tissues with stronger contraction potential. Importantly, cardiac tissues based on CNT/Col hydrogels were noted to have better functionality. Collectively, the incorporation of CNTs into the Col hydrogels improved cell alignment and the performance of cardiac constructs. Our study suggests that CNT/Col hydrogels offer a promising tissue scaffold for cardiac constructs, and might serve as injectable biomaterials to deliver cell or drug molecules for cardiac regeneration following myocardial infarction in the near future.

Designing Acellular Injectable Biomaterial Therapeutics for Treating Myocardial Infarction and Peripheral Artery Disease

As the number of global deaths attributed to cardiovascular disease continues to rise, viable treatments for cardiovascular events such as myocardial infarction (MI) or conditions like peripheral artery disease (PAD) are critical. Recent studies investigating injectable biomaterials have shown promise in promoting tissue regeneration and functional improvement, and in some cases, incorporating other therapeutics further augments the beneficial effects of these biomaterials. In this review, we aim to emphasize the advantages of acellular injectable biomaterial-based therapies, specifically material-alone approaches or delivery of acellular biologics, in regards to manufacturability and the capacity of these biomaterials to regenerate or repair diseased tissue. We will focus on design parameters and mechanisms that maximize therapeutic efficacy, particularly, improved functional perfusion and neovascularization regarding PAD and improved cardiac function and reduced negative left ventricular (LV) remodeling post-MI. We will then discuss the rationale and challenges of designing new injectable biomaterial-based therapies for the clinic.

Delivery of growth factor-based therapeutics in vascular diseases: Challenges and strategies

Either cardiovascular or peripheral vascular diseases have become the major cause of morbidity and mortality worldwide. Recently, growth factors therapeutics, whatever administrated in form of exogenous growth factors or their relevant genes have been discovered to be an effective strategy for the prevention and therapy of vascular diseases, because of their promoting angiogenesis. Besides, as an alternative, stem cell-based therapy has been also developed in view of their paracrine-mediated effect or ability of differentiation toward angiogenesis-related cells under assistance of growth factors. Despite of being specific and potent, no matter growth factors or stem cells-based therapy, their full clinical transformation is limited from bench to bedside. In this review, the potential choices of therapeutic modes based on types of different growth factors or stem cells were firstly summarized for vascular diseases. The confronted various challenges such as lack of non-invasive delivery method, the physiochemical challenge, the short half-life time, and poor cell survival, were carefully analyzed for these therapeutic modes. Various strategies to overcome these limitations are put forward from the perspective of drug delivery. The expertised design of a suitable delivery form will undoubtedly provide valuable insight into their clinical application in the regenerative medicine.

Biohybrid cardiac ECM-based hydrogels improve long term cardiac function post myocardial infarction

Injectable scaffolds for cardiac tissue regeneration are a promising therapeutic approach for progressive heart failure following myocardial infarction (MI). Their major advantage lies in their delivery modality that is considered minimally invasive due to their direct injection into the myocardium. Biomaterials comprising such scaffolds should mimic the cardiac tissue in terms of composition, structure, mechanical support, and most importantly, bioactivity. Nonetheless, natural biomaterial-based gels may suffer from limited mechanical strength, which often fail to provide the long-term support required by the heart for contraction and relaxation. Here we present newly-developed injectable scaffolds, which are based on solubilized decellularized porcine cardiac extracellular matrix (pcECM) cross-linked with genipin alone or engineered with different amounts of chitosan to better control the gel's mechanical properties while still leveraging the ECM biological activity. We demonstrate that these new biohybrid materials are naturally remodeled by mesenchymal stem cells, while supporting high viabilities and affecting cell morphology and organization. They exhibit neither in vitro nor in vivo immunogenicity. Most importantly, their application in treating acute and long term chronic MI in rat models clearly demonstrates the significant therapeutic potential of these gels in the long-term (12weeks post MI). The pcECM-based gels enable not only preservation, but also improvement in cardiac function eight weeks post treatment, as measured using echocardiography as well as hemodynamics. Infiltration of progenitor cells into the gels highlights the possible biological remodeling properties of the ECM-based platform.

STATEMENT OF SIGNIFICANCE

This work describes the development of new injectable scaffolds for cardiac tissue regeneration that are based on solubilized porcine cardiac extracellular matrix (ECM), combined with natural biomaterials: genipin, and chitosan. The design of such scaffolds aims at leveraging the natural bioactivity and unique structure of cardiac ECM, while overcoming its limited mechanical strength, which may fail to provide the long-term support required for heart contraction and relaxation. Here, we present a biocompatible gel-platform with custom-tailored mechanical properties that significantly improve cardiac function when injected into rat hearts following acute and chronic myocardial infarction. We clearly demonstrate the substantial therapeutic potential of these scaffolds, which not only preserved heart functions but also alleviated MI damage, even after the formation of a mature scar tissue.

Weak Bond-Based Injectable and Stimuli Responsive Hydrogels for Biomedical Applications

Here we define hydrogels crosslinked by weak bonds as physical hydrogels. They possess unique features including reversible bonding, shear thinning and stimuli-responsiveness. Unlike covalently crosslinked hydrogels, physical hydrogels do not require triggers to initiate chemical reactions for in situ gelation. The drug can be fully loaded in a pre-formed hydrogel for delivery with minimal cargo leakage during injection. These benefits make physical hydrogels useful as delivery vehicles for applications in biomedical engineering. This review focuses on recent advances of physical hydrogels crosslinked by weak bonds: hydrogen bonds, ionic interactions, host-guest chemistry, hydrophobic interactions, coordination bonds and π-π stacking interactions. Understanding the principles and the state of the art of gels with these dynamic bonds may give rise to breakthroughs in many biomedical research areas including drug delivery and tissue engineering.

Vascular induction and cell infiltration into peptide-modified bioactive silk fibroin hydrogels

A vascular-inducing peptide was produced recombinantly to stimulate the replacement of silk fibroin hydrogels by vascularized tissuesin vivo.

Striated muscle function, regeneration, and repair

As the only striated muscle tissues in the body, skeletal and cardiac muscle share numerous structural and functional characteristics, while exhibiting vastly different size and regenerative potential. Healthy skeletal muscle harbors a robust regenerative response that becomes inadequate after large muscle loss or in degenerative pathologies and aging. In contrast, the mammalian heart loses its regenerative capacity shortly after birth, leaving it susceptible to permanent damage by acute injury or chronic disease. In this review, we compare and contrast the physiology and regenerative potential of native skeletal and cardiac muscles, mechanisms underlying striated muscle dysfunction, and bioengineering strategies to treat muscle disorders. We focus on different sources for cellular therapy, biomaterials to augment the endogenous regenerative response, and progress in engineering and application of mature striated muscle tissues in vitro and in vivo. Finally, we discuss the challenges and perspectives in translating muscle bioengineering strategies to clinical practice.

Design of Injectable Materials to Improve Stem Cell Transplantation

Stem cell-based therapies are steadily gaining traction for regenerative medicine approaches to treating disease and injury throughout the body. While a significant body of work has shown success in preclinical studies, results often fail to translate in clinical settings. One potential cause is the massive transplanted cell death that occurs post injection, preventing functional integration with host tissue. Therefore, current research is focusing on developing injectable hydrogel materials to protect cells during delivery and to stimulate endogenous regeneration through interactions of transplanted cells and host tissue. This review explores the design of targeted injectable hydrogel systems for improving the therapeutic potential of stem cells across a variety of tissue engineering applications with a focus on hydrogel materials that have progressed to the stage of preclinical testing.

Evidence for Mechanisms Underlying the Functional Benefits of a Myocardial Matrix Hydrogel for Post-MI Treatment

BACKGROUND

There is increasing need for better therapies to prevent the development of heart failure after myocardial infarction (MI). An injectable hydrogel derived from decellularized porcine ventricular myocardium has been shown to halt the post-infarction progression of negative left ventricular remodeling and decline in cardiac function in both small and large animal models.

OBJECTIVES

This study sought to elucidate the tissue-level mechanisms underlying the therapeutic benefits of myocardial matrix injection.

METHODS

Myocardial matrix or saline was injected into infarcted myocardium 1 week after ischemia-reperfusion in Sprague-Dawley rats. Cardiac function was evaluated by magnetic resonance imaging and hemodynamic measurements at 5 weeks after injection. Whole transcriptome microarrays were performed on RNA isolated from the infarct at 3 days and 1 week after injection. Quantitative polymerase chain reaction and histologic quantification confirmed expression of key genes and their activation in altered pathways.

RESULTS

Principal component analysis of the transcriptomes showed that samples collected from myocardial matrix-injected infarcts are distinct and cluster separately from saline-injected control subjects. Pathway analysis indicated that these differences are due to changes in several tissue processes that may contribute to improved cardiac healing after MI. Matrix-injected infarcted myocardium exhibits an altered inflammatory response, reduced cardiomyocyte apoptosis, enhanced infarct neovascularization, diminished cardiac hypertrophy and fibrosis, altered metabolic enzyme expression, increased cardiac transcription factor expression, and progenitor cell recruitment, along with improvements in global cardiac function and hemodynamics.

CONCLUSIONS

These results indicate that the myocardial matrix alters several key pathways after MI creating a pro-regenerative environment, further demonstrating its promise as a potential post-MI therapy.

Biosynthetic Polymers as Functional Materials

The synthesis of functional polymers encoded with biomolecules has been an extensive area of research for decades. As such, a diverse toolbox of polymerization techniques and bioconjugation methods has been developed. The greatest impact of this work has been in biomedicine and biotechnology, where fully synthetic and naturally derived biomolecules are used cooperatively. Despite significant improvements in biocompatible and functionally diverse polymers, our success in the field is constrained by recognized limitations in polymer architecture control, structural dynamics, and biostabilization. This Perspective discusses the current status of functional biosynthetic polymers and highlights innovative strategies reported within the past five years that have made great strides in overcoming the aforementioned barriers.

Effects of Matrix Metalloproteinases on the Performance of Platelet Fibrin Gel Spiked With Cardiac Stem Cells in Heart Repair

Stem cells and biomaterials have been studied for therapeutic cardiac repair. Previous studies have shown the beneficial effects of platelet fibrin gel and cardiac stem cells when cotransplanted into rodent hearts with myocardial infarction (MI). We hypothesized that matrix metalloproteinases (MMPs) play an important role in such protection. Thus, the present study is designed to elucidate the effects of MMP inhibition on the therapeutic benefits of intramyocardial injection of platelet fibrin gel spiked with cardiac stem cells (cell-gel) in a rat model of acute MI. In vitro, broad-spectrum MMP inhibitor GM6001 undermines cell spreading and cardiomyocyte contraction. In a syngeneic rat model of myocardial infarction, MMP inhibition blunted the recruitment of endogenous cardiovascular cells into the injected biomaterials, therefore hindering de novo angiogenesis and cardiomyogenesis. Echocardiography and histology 3 weeks after treatment revealed that metalloproteinase inhibition diminished the functional and structural benefits of cell-gel in treating MI. Reduction of host angiogenesis, cardiomyocyte cycling, and MMP-2 activities was evident in animals treated with GM6001. Our findings suggest that MMPs play a critical role in the therapeutic benefits of platelet fibrin gel spiked with cardiac stem cells for treating MI.

SIGNIFICANCE

In this study, the effects of matrix metalloproteinase inhibition on the performance of platelet gel spiked with cardiac stem cells (cell-gel) for heart regeneration are explored. The results demonstrate that matrix metalloproteinases are required for cell-gel to exert its benefits in cardiac repair. Inhibition of matrix metalloproteinases reduces cell engraftment, host angiogenesis, and recruitment of endogenous cardiovascular cells in rats with heart attack.

Decellularized myocardial matrix hydrogels: In basic research and preclinical studies

A variety of decellularized materials have been developed that have demonstrated potential for treating cardiovascular diseases and improving our understanding of cardiac development. Of these biomaterials, decellularized myocardial matrix hydrogels have shown great promise for creating cellular microenvironments representative of the native cardiac tissue and treating the heart after a myocardial infarction. Decellularized myocardial matrix hydrogels derived from porcine cardiac tissue form a nanofibrous hydrogel once thermally induced at physiological temperatures. Use of isolated cardiac extracellular matrix in 2D and 3D in vitro platforms has demonstrated the capability to provide tissue specific cues for cardiac cell growth and differentiation. Testing of the myocardial matrix hydrogel as a therapy after myocardial infarction in both small and large animal models has demonstrated improved left ventricular function, increased cardiac muscle, and cellular recruitment into the treated infarct. Based on these results, steps are currently being taken to translate these hydrogels into a clinically used injectable biomaterial therapy. In this review, we will focus on the basic science and preclinical studies that have accelerated the development of decellularized myocardial matrix hydrogels into an emerging novel therapy for treating the heart after a myocardial infarction.

The assembly state and charge of amphiphilic β-sheet peptides affect blood clotting

Hydrogels composed of designed β-sheet amphiphilic peptides have been exploited in several biomedical applications. Here the peptide's charge shows influence on blood compatibility with antithrombotic or no effects on clotting blood given by the anionic and zwitterionic peptides.

Degradable acetalated dextran microparticles for tunable release of an engineered hepatocyte growth factor fragment

Injectable biomaterials are promising as new therapies to treat myocardial infarction (MI). One useful property of biomaterials is the ability to protect and sustain release of therapeutic payloads. In order to create a platform for optimizing the release rate of cardioprotective molecules we utilized the tunable degradation of acetalated dextran (AcDex). We created microparticles with three distinct degradation profiles and showed that the consequent protein release profiles could be modulated within the infarcted heart. This enabled us to determine how delivery rate impacted the efficacy of a model therapeutic, an engineered hepatocyte growth factor fragment (HGF-f). Our results showed that the cardioprotective efficacy of HGF-f was optimal when delivered over three days post-intramyocardial injection, yielding the largest arterioles, fewest apoptotic cardiomyocytes bordering the infarct and the smallest infarcts compared to empty particle treatment four weeks after injection. This work demonstrates the potential of using AcDex particles as a delivery platform to optimize the time frame for delivering therapeutic proteins to the heart.

Integration of mesenchymal stem cells with nanobiomaterials for the repair of myocardial infarction

The integration of nanobiomaterials with stem cells represents a promising strategy for the treatment of myocardial infarction. While stem cells and nanobiomaterials each demonstrated partial success in cardiac repair individually, the therapeutic efficacy of the clinical settings for each of these has been low. Hence, a combination of nanobiomaterials with stem cells is vigorously studied to create synergistic effects for treating myocardial infarction. To date, various types of nanomaterials have been incorporated with stem cells to control cell fate, modulate the therapeutic behavior of stem cells, and make them more suitable for cardiac repair. Here, we review the current stem cell therapies for cardiac repair and describe the combinatorial approaches of using nanobiomaterials and stem cells to improve therapeutic efficacy for the treatment of myocardial infarction.

Myocardial infarction: stem cell transplantation for cardiac regeneration

It is estimated that by 2030, almost 23.6 million people will perish from cardiovascular disease, according to the WHO. The review discusses advances in stem cell therapy for myocardial infarction, including cell sources, methods of differentiation, expansion selection and their route of delivery. Skeletal muscle cells, hematopoietic cells and mesenchymal stem cells (MSCs) and embryonic stem cells (ESCs)-derived cardiomyocytes have advanced to the clinical stage, while induced pluripotent cells (iPSCs) are yet to be considered clinically. Delivery of cells to the sites of injury and their subsequent retention is a major issue. The development of supportive scaffold matrices to facilitate stem cell retention and differentiation are analyzed. The review outlines clinical translation of conjugate stem cell-based cellular therapeutics post-myocardial infarction.

Enzyme-Responsive Nanoparticles for Targeted Accumulation and Prolonged Retention in Heart Tissue after Myocardial Infarction

A method for targeting to and retaining intravenously injected nanoparticles at the site of acute myocardial infarction in a rat model is described. Enzyme-responsive peptide-polymer amphiphiles are assembled as spherical micellar nanoparticles, and undergo a morphological transition from spherical-shaped, discrete materials to network-like assemblies when acted upon by matrix metalloproteinases (MMP-2 and MMP-9), which are up-regulated in heart tissue post-myocardial infarction.

The feasibility and safety of Algisyl-LVR™ as a method of left ventricular augmentation in patients with dilated cardiomyopathy: initial first in man clinical results

BACKGROUND

A tissue engineering approach to augment the left ventricular wall has been suggested as a means to treat patients with advanced heart failure. This study evaluated the safety and feasibility of Algisyl-LVR™ as a method of left ventricular augmentation in patients with dilated cardiomyopathy undergoing open-heart surgery.

METHODS AND RESULTS

Eleven male patients (aged 44 to 74years) with advanced heart failure (NYHA class 3 or 4), a left ventricular ejection fraction (LVEF) of <40% and requiring conventional heart surgery received Algisyl-LVR delivered into the LV myocardial free wall. Serial echocardiography, assessment of NYHA class, Kansas City Cardiomyopathy Questionnaire (KCCQ) and 24-hour Holter monitoring were obtained at baseline, days 3 and 8 (for echocardiography and Holter monitoring), and at 3, 6, 12, 18 and 24months. A total of 9 (81.8%) patients completed 24months of follow-up. Two patients withdrew consent after day 8 and at the 3month visit. Operative mortality was 0% (n=10 with 30day follow-up). There were no adverse events attributed to Algisyl-LVR. Mean LVEF improved from 27.1 (±7.3) % at screening to a mean LVEF of 34.8 (±18.6) % 24months post-discharge. Improvements of NYHA class were corroborated with improvements in KCCQ summary scores. Holter monitor data showed a significant decrease in the episodes of nonsustained ventricular tachycardia following administration of Algisyl-LVR.

CONCLUSIONS

Administration of Algisyl-LVR to patients with advanced HF at the time of cardiac surgery is feasible and safe; warranting continued development of Algisyl-LVR as a potential therapy in patients with advanced HF.

A prospective comparison of alginate-hydrogel with standard medical therapy to determine impact on functional capacity and clinical outcomes in patients with advanced heart failure (AUGMENT-HF trial)

Aims

AUGMENT-HF was an international, multi-centre, prospective, randomized, controlled trial to evaluate the benefits and safety of a novel method of left ventricular (LV) modification with alginate-hydrogel.

Methods

Alginate-hydrogel is an inert permanent implant that is directly injected into LV heart muscle and serves as a prosthetic scaffold to modify the shape and size of the dilated LV. Patients with advanced chronic heart failure (HF) were randomized (1 : 1) to alginate-hydrogel (n = 40) in combination with standard medical therapy or standard medical therapy alone (Control, n = 38). The primary endpoint of AUGMENT-HF was the change in peak VO₂ from baseline to 6 months. Secondary endpoints included changes in 6-min walk test (6MWT) distance and New York Heart Association (NYHA) functional class, as well as assessments of procedural safety.

Results

Enrolled patients were 63 ± 10 years old, 74% in NYHA functional class III, had a LV ejection fraction of 26 ± 5% and a mean peak VO₂ of 12.2 ± 1.8 mL/kg/min. Thirty-five patients were successfully treated with alginate-hydrogel injections through a limited left thoracotomy approach without device-related complications; the 30-day surgical mortality was 8.6% (3 deaths). Alginate-hydrogel treatment was associated with improved peak VO₂ at 6 months—treatment effect vs. Control: +1.24 mL/kg/min (95% confidence interval 0.26–2.23, P = 0.014). Also 6MWT distance and NYHA functional class improved in alginate-hydrogel-treated patients vs. Control (both P < 0.001).

Conclusion

Alginate-hydrogel in addition to standard medical therapy for patients with advanced chronic HF was more effective than standard medical therapy alone for improving exercise capacity and symptoms. The results of AUGMENT-HF provide proof of concept for a pivotal trial.

Trial Registration Number

NCT01311791.

Developing injectable nanomaterials to repair the heart

Injectable nanomaterials have been designed for the treatment of myocardial infarction, particularly during the acute stages of inflammation and injury. Among these strategies, injectable nanofibrous hydrogel networks or nanoparticle complexes may be delivered alone or with a therapeutic to improve heart function. Intramyocardial delivery of these materials localizes treatments to the site of injury. As an alternative, nanoparticles may be delivered intravenously, which provides the ultimate minimally invasive approach. These systems take advantage of the leaky vasculature after myocardial infarction, and may be designed to specifically target the injured region. The translational applicability of both intramyocardial and intravenous applications may provide safe and effective solutions upon optimizing the timing of the treatments and biodistribution.

Fabrication and characterization of injectable hydrogels derived from decellularized skeletal and cardiac muscle

Biomaterials, which can contain appropriate biomechanical and/or biochemical cues, are increasingly being investigated as potential scaffolds for tissue regeneration and/or repair for treating myocardial infarction, heart failure, and peripheral artery disease. Specifically, injectable hydrogels are touted for their minimally invasive delivery, ability to self-assemble in situ, and capacity to encourage host tissue regeneration. Here we present detailed methods for fabricating and characterizing decellularized injectable cardiac and skeletal muscle extracellular matrix (ECM) hydrogels. The ECM derived hydrogels have low cellular and DNA content, retain sulfated glycosaminoglycans and other extracellular matrix proteins such as collagen, gel at physiologic temperature and pH, and assume a nanofibrous architecture. These injectable hydrogels are amenable to minimally invasive, tissue specific biomaterial therapies for treating myocardial infarction and peripheral artery disease.

Clinical applications of naturally derived biopolymer-based scaffolds for regenerative medicine

Naturally derived polymeric biomaterials, such as collagens, silks, elastins, alginates, and fibrins are utilized in tissue engineering due to their biocompatibility, bioactivity, and tunable mechanical and degradation kinetics. The use of these natural biopolymers in biomedical applications is advantageous because they do not release cytotoxic degradation products, are often processed using environmentally-friendly aqueous-based methods, and their degradation rates within biological systems can be manipulated by modifying the starting formulation or processing conditions. For these reasons, many recent in vivo investigations and FDA-approval of new biomaterials for clinical use have utilized natural biopolymers as matrices for cell delivery and as scaffolds for cell-free support of native tissues. This review highlights biopolymer-based scaffolds used in clinical applications for the regeneration and repair of native tissues, with a focus on bone, skeletal muscle, peripheral nerve, cardiac muscle, and cornea substitutes.

Injectable PEGylated fibrinogen cell-laden microparticles made with a continuous solvent- and oil-free preparation method

A new methodology is reported for the continuous, solvent- and oil-free production of photopolymerizable microparticles containing encapsulated human dermal fibroblasts. A precursor solution of cells in photoreactive poly(ethylene glycol) (PEG)-fibrinogen (PF) polymer was transported through a transparent injector exposed to light irradiation before being atomized in a jet-in-air nozzle. Shear rheometry data revealed the crosslinking kinetics of the PF/cell solution, which was then used to determine the amount of irradiation required to partially polymerize the mixture just prior to atomization. The partially polymerized drops of PF/cells fell into a gelation bath for further crosslinking until fully polymerized hydrogel microparticles were formed. As the drops of solution exited the air-in-jet nozzle, their viscosity was designed to be sufficiently high so as to prevent rapid mixing and/or dilution in the gelation bath, but without undergoing complete gelation in the nozzle. Several parameters of this system were varied to control the size and polydispersity of the microparticles, including the cell density, the flow rate and the air pressure in the nozzle. The system was capable of producing cell-laden microparticles with an average diameter of between 88.1 to 347.1 μm, and a dispersity of between 1.1 and 2.4, depending on the parameters chosen. Varying the precursor flow rate and/or cell density was beneficial in controlling the size and polydispersity of the microparticles; all microparticles exhibited very high cell viability, which was not affected by these parameters. In conclusion, this dropwise photopolymerization methodology for preparing cell-laden microparticles is an attractive alternative to existing techniques that use harsh solvents/oils and offer limited control over particle size and polydispersity.

Microfluidic Isolation of CD34-Positive Skin Cells Enables Regeneration of Hair and Sebaceous Glands In Vivo

Skin stem cells resident in the bulge area of hair follicles and at the basal layer of the epidermis are multipotent and able to self-renew when transplanted into full-thickness defects in nude mice. Based on cell surface markers such as CD34 and the α6-integrin, skin stem cells can be extracted from tissue-derived cell suspensions for engraftment using the gold standard cell separation technique of fluorescence-activated cell sorting (FACS). This paper describes an alternative separation method using microfluidic devices coated with degradable antibody-functionalized hydrogels. The microfluidic method allows direct injection of tissue digestate (no preprocessing tagging of cells is needed), is fast (45 minutes from injected sample to purified cells), and scalable. This method is used in this study to isolate CD34-positive (CD34+) cells from murine skin tissue digestate, and the functional capability of these cells is demonstrated by transplantation into nude mice using protocols developed by other groups for FACS-sorted cells. Specifically, the transplantation of microfluidic isolated CD34+ cells along with dermal and epidermal cells was observed to generate significant levels of hair follicles and sebaceous glands consistent with those observed previously with FACS-sorted cells.