Self-Healing Injectable Hydrogels for Tissue Regeneration

Abstract

Biomaterials with the ability to self-heal and recover their structural integrity offer many advantages for applications in biomedicine. The past decade has witnessed the rapid emergence of a new class of self-healing biomaterials commonly termed injectable, or printable in the context of 3D printing. These self-healing injectable biomaterials, mostly hydrogels and other soft condensed matter based on reversible chemistry, are able to temporarily fluidize under shear stress and subsequently recover their original mechanical properties. Self-healing injectable hydrogels offer distinct advantages compared to traditional biomaterials. Most notably, they can be administered in a locally targeted and minimally invasive manner through a narrow syringe without the need for invasive surgery. Their moldability allows for a patient-specific intervention and shows great prospects for personalized medicine. Injected hydrogels can facilitate tissue regeneration in multiple ways owing to their viscoelastic and diffusive nature, ranging from simple mechanical support, spatiotemporally controlled delivery of cells or therapeutics, to local recruitment and modulation of host cells to promote tissue regeneration. Consequently, self-healing injectable hydrogels have been at the forefront of many cutting-edge tissue regeneration strategies. This study provides a critical review of the current state of self-healing injectable hydrogels for tissue regeneration. As key challenges toward further maturation of this exciting research field, we identify (i) the trade-off between the self-healing and injectability of hydrogels vs their physical stability, (ii) the lack of consensus on rheological characterization and quantitative benchmarks for self-healing injectable hydrogels, particularly regarding the capillary flow in syringes, and (iii) practical limitations regarding translation toward therapeutically effective formulations for regeneration of specific tissues. Hence, here we (i) review chemical and physical design strategies for self-healing injectable hydrogels, (ii) provide a practical guide for their rheological analysis, and (iii) showcase their applicability for regeneration of various tissues and 3D printing of complex tissues and organoids.

Figure 1

Schematic behavior of a self-healing injectable hydrogel with (i) gel-like properties at rest, (ii) fluidization under shear due to reversible chemistry and/or alignment in the flow field, and (iii) self-healing of the original structure and mechanical properties after flow.

Figure 2

Overview of common noncovalent chemical interactions used for the design of self-healing injectable hydrogels. Examples for electrostatic, metal coordination, hydrophobic, and hydrogen bonding are based on refs (91), (92), (32), and (93), respectively. Adapted with permission from ref (4). Copyright 2018 Wiley.

Figure 3

Overview of common strategies for the design of self-healing injectable hydrogels based on dynamic covalent interactions. Examples, from left to right, are based on refs (146), (147) and (148), respectively. Adapted with permission from ref (4). Copyright 2018 Wiley.

Figure 4

Overview of common strategies for the design of self-healing injectable hydrogels based on multiple interactions (dual cross-linked) or multiple materials (double network). Examples, from left to right, are based on refs (165) and (166), respectively. Adapted with permission from ref (4). Copyright 2018 Wiley.

Figure 5

Overview of common physical forms of self-healing injectable hydrogels. Examples, from left to right, are based on refs (32), (136), (61), (110), and (192), respectively. Adapted with permission from ref (4). Copyright 2018 Wiley.

Figure 6

Overview of rheological protocols for the quantification of apparent yield stress, capillary extrusion, and self-healing capacity of self-healing injectable hydrogels. (A) Flow curves down to low shear rates with Herschel–Bulkley model fit. (B) Stress ramp to detect onset of material flow. (C) Oscillatory stress sweeps with apparent yield stress extraction at crossover of G′ tangents of the linear and nonlinear regime. (D) Flow curves of the same hydrogel obtained by oscillatory, capillary, and steady shear rheology. (E) (left) Schematic of hydrogel plug flow in a syringe needle. (right) Experimental data on hydrogel velocity and shear rate profile as a function of capillary radius compared to cell suspension. (F) Self-healing test using alternating low-high strain cycles. (G) Self-healing determined by oscillatory time sweep following preshear at a shear rate of 1000 1/s for varying periods or upon deposition of the hydrogel through a 26-gauge needle. (H) Example of stimulus-induced hydrogel strengthening to enhance mechanical hydrogel properties after extrusion. Note: Graphs show different materials. (A,C) Replotted with permission from ref (265). Copyright 2016 Elsevier. (B,D) Replotted with permission from ref (227). Copyright 2019 American Chemical Society. (E) Replotted with permission from refs (44) and (234). Copyright 2012 and 2018 American Chemical Society. (F) Replotted with permission from ref (31). Copyright 2017 American Chemical Society. (G) Replotted with permission from ref (261). Copyright 2010 Royal Society of Chemistry, (H) Replotted with permission from ref (270). Copyright 2020 Wiley.

Figure 7

Schematic overview of tissue regeneration strategies employing self-healing injectable hydrogels.

Figure 8

Overview of applications of self-healing injectable hydrogels for regeneration of different tissues. (A) Cardiac hydrogel injection to provide mechanical support and prevent left ventricular remodeling after myocardial infarction compared to injection of phosphate buffered saline (PBS, scale bar = 0.5 mm). Reproduced with permission from ref (31). Copyright 2017 American Chemical Society. Reproduced with permission from ref (352). Copyright 2009 Elsevier. (B) (left) Hydrogel injection into stroke cavity with simultaneous drainage of extracellular fluid (ECF) to maintain intracerebral pressure. (right) Histological image showing the injected hydrogel in the stroke cavity (orange, scale bar = 1 mm). Reproduced with permission from ref (36). Copyright 2015 Elsevier. (C) Images of excised fibrosarcomas 20 days after intratumoral injection of doxorubicin in a hydrogel matrix, free doxorubicin, or saline. Reproduced with permission from ref (379). Copyright 2015 American Chemical Society. (D) Infrared image of an intratumorally administered injectable hydrogel containing excitable nanoparticles for photothermal therapy of hypoxia-resistant breast tumors. Reproduced with permission from ref (321). Copyright 2021 Elsevier. (E) Histological image showing the recruitment of dendritic cells (purple) by cytokine-loaded hydrogels from subcutaneous tissue. Reproduced with permission from ref (380). Copyright 2019 American Chemical Society. (F) Closure of Pseudomonas aeruginosa infected wounds dressed with gauze or antibacterial hydrogels. Reproduced with permission from ref (381). Copyright 2019 Elsevier. (G) Histological image of calvarial bone with hematoxylin and eosin staining three months postinjury showing mature collagen-rich bone for fast relaxing hydrogels and sparse disorganized collagen without mature bone for slow relaxing hydrogels. Scale bar corresponds to 180 μm. Reproduced with permission from ref (382). Copyright 2017 Wiley. (H) Injection of a hydrogel as vitreous substitute during vitrectomy. Reproduced with permission from ref (383). Copyright 2021 Elsevier.

Figure 9

Schematic overview of applications of self-healing injectable hydrogels as 3D extrusion printing inks and/or support matrices in freeform 3D printing.

Figure 10

Overview of self-healing injectable hydrogels to obtain complex tissue constructs and organ-like structures. (A) Dorsal root ganglions (red) in isotropic fibrin hydrogels (left) and with anisotropic magnetically aligned microgels (right, green) promoting directed neurite growth. Reproduced with permission from ref (402). Copyright 2017 American Chemical Society. (B) Multimaterial printing of a human tendon muscle replacement with gradient mechanical properties and rigid UV-curable contact points (E-MAX 904, blue) and elastic alginate/polyacrylamide composite (red). Reproduced with permission from ref (629). Copyright 2017 Elsevier. (C) Freeform 3D printing of a sacrificial ink in a UV-curable hydrogel matrix allows omnidirectional printing of complex hollow vasculature structures (scale bar = 10 mm). Reproduced with permission from ref (606) . Copyright 2019 Wiley. (D) Deposition of cell spheroids rich in cardiomyocytes (red, “healthy”) or rich in fibroblasts (green, “scarred”) in a hydrogel matrix to obtain complex microtissues for the study of tissue defects and drug screening (scale bar = 100 μm, zoom in = 50 μm). Reproduced with permission from ref (654). Copyright 2021 Springer Nature. (E) Personalized multicellular 3D printed heart with hollow structures and vasculature. From left: The CAD model, the printed heart in the support hydrogel (scale bar = 5 mm), and 3D confocal image showing cardiomyocytes (pink) and endothelial cells (orange, scale bar = 1 mm). Reproduced with permission from ref (639). Copyright 2019 Wiley. (F) 3D printed triculture liver model with physiologically relevant hexagonal architecture enhances morphological organization, liver-specific gene expression, and metabolic activity of hepatic progenitor cells (green, red = supporting endodermal and mesodermal cells, scale bar = 500 μm). Reproduced with permission from ref (655). Copyright 2016 National Academy of Sciences.