Abstract
Injectable hydrogels are one of the most widely investigated and versatile technologies for drug delivery and tissue engineering applications. Hydrogels’ versatility arises from their tunable structure, which has been enabled by considerable advances in fields such as materials engineering, polymer science, and chemistry. Advances in these fields continue to lead to invention of new polymers, new approaches to crosslink polymers, new strategies to fabricate hydrogels, and new applications arising from hydrogels for improving healthcare. Various hydrogel technologies have received regulatory approval for healthcare applications ranging from cancer treatment to aesthetic corrections to spinal fusion. Beyond these applications, hydrogels are being studied in clinical settings for tissue regeneration, incontinence, and other applications. Here, we analyze the current clinical landscape of injectable hydrogel technologies, including hydrogels that have been clinically approved or are currently being investigated in clinical settings. We summarize our analysis to highlight key clinical areas that hydrogels have found sustained success in and further discuss challenges that may limit their future clinical translation.
Keywords: clinics, drug delivery, FDA, injectable materials, marketed products, regenerative, translational medicine
1. INTRODUCTION
Hydrogels are crosslinked hydrophilic polymer networks of high‐water content. Due to this high water content, hydrogels exhibit favorable biocompatibility and as such have been developed for and used in a variety of medical applications.1, 2, 3 Since hydrogel properties can be tuned through a variety of chemical approaches, their rational design and engineering has enabled new modalities for delivery of small molecules,4, 5, 6, 7 proteins,8, 9, 10, 11, 12 and cells13, 14 and as tissue engineering scaffolds for directing cell fate/lineage,15, 16 stem cell expansion, 17, 18, 19, 20 and tissue regeneration.21, 22, 23, 24, 25 Research efforts to develop hydrogels for biomedical applications represents one of the most studied areas at the interface of engineering and medicine. 26 The translation of hydrogels into the clinic, especially for advanced hydrogel systems, remains a challenge despite many products and current clinical investigations. This article summarizes the present state of hydrogel materials in clinical medicine. Of all the hydrogels currently in preclinical development, studied in clinical trials, and used in the clinic as approved products, injectable hydrogels represent a subset that are capable of providing the most versatile deployment as delivery/regeneration platforms. Injectable hydrogels can be directly applied to sites of interest, independent of, and in many cases conforming to, local geometric/physiological constraints. For detail on the various scientific aspects of hydrogels, please refer to the following recent reviews.26, 27, 28, 29, 30, 31 In this review, we provide a general overview of the clinical landscape of injectable hydrogels, highlighting both approved hydrogel systems and current clinical trials. We conclude with a discussion of current challenges to bolster translation of hydrogel materials.
2. CLINICAL LANDSCAPE OF HYDROGELS
As shown in Figure 1, the most abundant medical application of hydrogels is for soft contact lenses. As compared to glass lenses, hydrogels permit gas diffusion and retain water at the eye surface. 32 Clinical trials on new hydrogel lens products have focused on a variety of outcomes, including extending the time of continuous wear, adding pigments, and optimizing the lens geometry. To better delineate the various applications and types of hydrogels in the clinic, we separated them based on their structure in terms of hydrogel patches, coils, or bulk materials. Within each structural grouping, hydrogels that partition and deliver a therapeutic agent were separated from hydrogels that exert their therapeutic function without delivering a drug. For the purpose of our analysis, we defined a hydrogel patch as any material which is topically applied. Hydrogel patches are mostly used to provide an engineered barrier between compromised tissue and the external environment. For example, in the treatment of burn wounds, a hydrogel patch can achieve any combination of (a) preventing bacterial growth within the wound, (b) delivering therapeutics which accelerate healing, and (c) maintaining a moist tissue environment thereby reducing pain. Specific hydrogel patches are currently being explored for facilitating healing processes in diabetic ulcers, treating skin conditions such as eczema and psoriasis, and more. While accounting for only eight of the hydrogel clinical trials, hydrogel coils are useful as a replacement for platinum coils in the treatment of aneurism. For these applications, hydrogels of natural (i.e., gelatin) or synthetic (i.e., acrylamide, acrylic acid, glycolic acid) materials have been utilized, as well as composite structures (i.e., hydrogel‐coated metal coils). The third and most abundant class of hydrogels in the clinic, bulk hydrogels, is typically used for tissue augmentation or regeneration. Bulk hydrogels can act as a filler or replacement for soft tissue (e.g., to treat osteoarthritis, lipoatrophy), or provide mechanical support for compromised tissue (e.g., urinary incontinence, myocardial infarction). Current efforts include fabricating cell‐laden bulk hydrogels for tissue regeneration (e.g., kidney augmentation, myocardial regeneration). Similar to hydrogel patches, bulk hydrogels can themselves exert a therapeutic action or act as a depot for delivering a bioactive agent.
FIGURE 1.
Comprehensive analysis of hydrogels in clinical trials. Clinical trials which mentioned a hydrogel were identified using http://clinicaltrials.gov. Trials where the hydrogel was not used for a diagnostic or therapeutic purpose were removed. Trials which investigated the hydrogel cleaning solution, or another aspect of the hydrogel packaging, were also removed. In total, there were 425 clinical trials involving hydrogel materials. The primary clinical application of hydrogel materials was for soft contact lenses (202 unique clinical trials). With contact lenses excluded from further analysis, there were 223 clinical trials, spanning diverse medical applications. Trials which used the hydrogel as a tissue substitute, or a mechanical support to augment existing tissues (Regen Tissue) were analyzed separately from those which used hydrogels as a dressing or barrier to facilitate healing of an abrasion, burn, or ulcer (Regen wound). Of the 223 non‐lens hydrogel clinical trials, 8 used a hydrogel coil (cardiovascular application), 99 used a hydrogel patch, and 116 used a bulk hydrogel. Of the 116 bulk hydrogels, 31 were delivered via injection. Within the domain of injectable hydrogels, there are 28 approved clinical products and 31 devices in clinical trial (for full detail, see Tables 1 and 2). Within each hydrogel grouping (i.e., patch, bulk, injectable), we also stratified clinical trials by material origin (i.e., natural, synthetic, or unknown). Material origin was determined from either the clinical trial description or the device's U.S. patent. Solid lines denote categorization or clarification of a group, while dotted lines represent extraction of a particular subset
We also break down each hydrogel conformation (i.e., bulk, patch) by their material origin. Hydrogel biomaterials are either natural or synthetic in material origin. Natural hydrogels are typically derived from polypeptides (e.g., fibrin, collagen, gelatin) or polysaccharides (e.g., hyaluronic acid, alginate, cellulose, chitosan). 33 These natural polymers can gel through physical interactions between polymer chains, or in some cases by ionic crosslinking via multivalent ions. A major advantage of natural hydrogels, particularly as injectable or implantable materials, is that they are generally biocompatible and biodegradable. Synthetic hydrogels (e.g., poly(ethylene glycol), poly(acrylate) derivatives, poly(methacrylate) derivatives, poly(acrylamide), poly(lactic‐co‐glycolic acid), poly(vinyl alcohol), and poly(urethane)) afford high tunability during synthesis. This is because synthetic polymers’ molecular weights and crosslinking ratios can be controlled to tune hydrogel physical properties (i.e., swelling in water, viscosity, elasticity, porosity,). As such, material choice is one of the most important considerations for hydrogels, since material choice governs biocompatibility, porosity, degradability, and release of therapeutic agents. These material parameters may also be affected by the local physiological environment in which the hydrogel will be deployed.
Finally, we highlight injectable hydrogels, a highly investigated subset of hydrogels that can provide unique advantages due to their injection‐enabling properties. Injectable hydrogels can be delivered to various sites of the body with minimal discomfort while conforming to the site of application, thereby providing a versatile platform for applications in drug delivery and tissue engineering. Of all the clinical trials involving bulk hydrogels, injectable hydrogels make up 26%. For the purpose of this article, we focus on the clinical application and translation of injectable hydrogels.
3. CLINICALLY APPROVED HYDROGEL PRODUCTS
Currently there are over 30 injectable hydrogel‐based products that have been approved by the FDA and/or EMA (Table 1 ). Perhaps the most well‐known examples are commercially successes such Medtronic's INFUSE® (approx. $750 million) and Endo's Vantas® (approx. $20 million). Here, we discuss these currently approved injectable hydrogel products.
TABLE 1.
Clinically approved injectable hydrogels, grouped by their material class and broad indication
| Name (company) | Hydrogel material/payload (gelation mechanism) | Injection type | Approved indication | Approval (year) |
|---|---|---|---|---|
| Cancer: synthetic | ||||
| SpaceOAR® Hydrogel (Augmenix, Inc.) | Polyethylene glycol (chemical reaction) | Percutaneous | For protecting vulnerable tissues during prostate cancer radiotherapy |
EMA (2010) FDA (2015) |
| Vantas® (Endo Pharmaceuticals) | Histrelin acetate, poly(2‐hydroxyethyl methacrylate), poly(2‐hydroxypropyl methacrylate) and gonadotropin releasing hormone (chemical reaction) | Subcutaneous | Palliative treatment of prostate cancer |
FDA (2004) EMA (2005) |
| Facial correction: synthetic | ||||
| Radiesse® (+) (Merz Pharmaceuticals) | Hydroxylapatite, carboxymethylcellulose with Lidocaine (physical interaction) | Dermis | Correction of wrinkles and folds, stimulation of natural collagen production | FDA (2015) |
| Radiesse® (Bioform Medical, Inc.) | Hydroxylapatite, carboxymethylcellulose (physical interaction) | Dermis | For correction of facial folds and wrinkles, signs of facial fat loss and volume loss |
EMA (2004) FDA (2006 for first indication) |
| Artefill® (Suneva Medical, Inc.) | Polymethylmethacrylate beads, collagen and lidocaine (physical interaction) | Dermis | Facial wrinkles and folds | FDA (2006) |
| Sculptra® (Sanofi Aventis U.S.) | Poly‐L‐lactic acid (physical interaction) | Dermis | For correction of signs of facial fat loss, shallow to deep contour deficiencies and facial wrinkles |
EMA (2000) FDA (2004 for first indication) |
| Facial correction: natural | ||||
| Belotero balance® (+) Lidocaine (Merz Pharmaceuticals) | Hyaluronic acid with lidocaine (chemical reaction) | Dermis | Moderate to severe facial wrinkles and folds | FDA (2019) |
| Revanesse® Versa+ | Hyaluronic acid with lidocaine (chemical reaction) | Dermis | Moderate to severe facial wrinkles and creases | FDA (2018) |
| Teosyal® RHA (Teoxane SA) | Hyaluronic acid (chemical reaction) | Dermis | Facial wrinkles and folds |
EMA (2015) FDA (2017) |
| Revanesse® Versa/Revanesse® Ultra (Prollenium Medical Technologies Inc.) | Hyaluronic acid (chemical reaction) | Dermis | Moderate to severe facial wrinkles and creases | FDA (2017) |
|
Restylane® Lyft, Restylane® Refyne, Restylane® Defyne (Galderma Laboratories, L.P.)
Restylane® Silk (Valeant Pharmaceuticals North America LLC/Medicis)
Restylane® Injectable Gel (Medicis Aesthetics Holdings, Inc.) |
Hyaluronic acid with Lidocaine (chemical reaction) | Subcutaneous, dermis, lips | For correction of volume deficit, facial folds and wrinkles, midface contour deficiencies, and perioral rhytids |
EMA (2010) FDA (2012 for first indication) |
| Belotero balance® (Merz Pharmaceuticals) | Hyaluronic acid (chemical reaction) | Dermis | Moderate to severe facial wrinkles and folds |
EMA (2004) FDA (2011) |
| Juvéderm® XC (Allergan, Inc.) | Hyaluronic acid with lidocaine (chemical reaction) | Facial tissue | Correction of facial wrinkles and folds | FDA (2010) |
| Evolence® Collagen Filler (Colbar Lifescience l) | Collagen (chemical reaction) | Dermis | Moderate to deep facial wrinkles and folds |
EMA (2004) FDA (2008) |
| Elevess® (Anika Therapeutics) | Hyaluronic acid with lidocaine (chemical reaction) | Dermis | Moderate to severe facial wrinkles and folds |
FDA (2006) EMA (2007) |
| Juvéderm®/Voluma XC/Ultra XC/Volbella XC/ Vollure XC (Allergan, Inc) | Hyaluronic acid (chemical reaction) | Facial tissue, cheek, lips | For correction of facial wrinkles and folds, volume loss, and lip augmentation. |
EMA (2000) FDA (2006 for first indication) |
| Hylaform® (Hylan B gel), Captique Injectable Gel, Prevelle Silk (Genzyme Biosurgery) | Modified hyaluronic acid derived from a bird (avian) source (chemical reaction) | Dermis | Correction of moderate to severe facial wrinkles and folds |
EMA (1995) FDA (2004) |
| Collagen Implant, CosmoDerm® 1 human‐based collagen, CosmoDerm® 2 human‐based collagen CosmoPlast® human‐based collagen (Inamed Corporation/Allergan, Inc.) | Human collagen (CosmoDerm: physical interaction, CosmoPlast: chemical reaction) | Superficial papillary dermis | For correction of soft tissue contour deficiencies, such as wrinkles and acne scars | FDA & EMA (2003) |
| Fibrel® (Serono Laboratories) | Collagen (physical interaction) | Dermis | For correction of depressed cutaneous scars | FDA (1988) |
| Zyplast(R)® and Zyderm(R)® (Inamed Corporation/Allergan, Inc.) | Bovine collagen (chemical reaction) | Dermis | For correction of contour deficiencies | FDA and EMA (1981) |
| Spinal fusion: natural | ||||
| EUFLEXXA® (Ferring Pharmaceuticals Inc.) | Hyaluronic acid (physical interaction) | Intra‐articular | Knee osteoarthritis |
FDA (2004) EMA (2005) |
| INFUSE® bone graft (Medtronic Sofamor Danek USA, Inc.) | Collagen and recombinant human bone morphogenetic protein‐2 (physical interaction) | Spinal injection | Spinal fusion, and spine, oral‐maxillofacial and orthopedic trauma surgeries | FDA (2002 for first indication) |
| Osteogenic protein 1(OP‐1®) implant, OP‐1® Putty (Stryker Biotech) | Collagen, carboxymethylcellulose, and recombinant OP‐1 (physical interaction) | Spinal injection | Posterolateral lumbar spinal fusion | FDA (2001) |
| Other: synthetic | ||||
| TraceIT® Hydrogel Tissue Marker (Augmenix, Inc.) | Polyethylene glycol (chemical reaction) | Percutaneous | Improved soft tissue alignment for image guided therapy | FDA (2013) |
| Supprelin LA® (Indevus Pharmaceuticals, Inc.) | Histrelin acetate, Poly(2‐hydroxyethyl methacrylate) (chemical reaction) | Subcutaneous | Central precocious puberty |
EMA (2005) FDA (2007) |
| Bulkamid® hydrogel (Searchlight Pharma) | Polyacrylamide (chemical reaction) | Transurtheral | Female stress urinary incontinence |
EMA (2003) FDA (2006) |
| Coaptite® (BioForm Medical, Inc.) | Calcium hydroxylapatite, sodium carboxymethylcellulose, glycerin (physical interaction) | Submucosal | Female stress urinary incontinence |
EMA (2001) FDA (2005) |
| Other: natural | ||||
| Algisyl‐LVR® Hydrogel Implant (LoneStar Heart, Inc.) | Alginate (physical interaction) | Percutaneous | Advanced heart failure | EMA (2014) |
3.1. Facial correction/aesthetic products
The improved biocompatibility especially of hyaluronic acid (HA)‐based hydrogels have found a wide application as temporary dermal fillers in soft tissue augmentation. Juvéderm®, one of the leading HA‐based products currently in the market is widely used for the correction of age‐related volume loss, moderate to severe facial wrinkles and folds and lip augmentation. These HA containing hydrogels, once injected beneath the skin surface, integrate into the dermal layer and attract and bind to water molecules, thus providing volume and fullness to the skin lasting for about 6–12 months. Some of these dermal fillers including Juvéderm XC®, Elevess®, Prevelle Silk®, Revanesse® Versa +, Restylane® contain lidocaine as an additional component to minimize the pain or discomfort associated with injection. Collagen is also commonly used and has the shortest lasting effect (3–4 months) among the injectable filler materials. Calcium hydroxylapatite and Poly‐L‐lactic acid are among the long‐lasting synthetic absorbable filler materials with effects lasting up to 1–2 years. Artefill®, the only nonabsorbable material (polymethylmethacrylate beads) product approved by the FDA to date, is a permanent synthetic injectable filler used for the treatment of nasolabial folds. The increased numbers of approved naturally derived hydrogels in comparison to synthetic ones as aesthetic products could be attributed to improved biocompatibility.
3.2. Cancer products
The hydrogel's ability to encapsulate and release small molecules and biologics have been used for synthetic depot systems with improved, sustained local therapeutic delivery. Endo's Vantas ® has been approved by the FDA as a subcutaneous hormonal therapy to prevent testosterone‐dependent prostate cancer cells to grow following testosterone release. Vantas® employs a cylindrically shaped diffusion‐controlled reservoir system to sustain the release of gonadotropin releasing hormone (GnRH) or luteinizing hormone releasing hormone (LH‐RH) over time. The implant contains Histrelin acetate, a nonapeptide analogue of GnRH with added potency compared to leuprolide acetate‐poly(lactic‐co‐glycolic acid) (PLGA)‐based nonhydrogel products (Lupron depot® and Eligard®). Collectively, these hydrogel depots provide considerable benefits in patient acceptance and convenience through providing long‐lasting benefits with a single injection. Augmenix's SpaceOAR® hydrogel is the latest of all to be approved for protecting patients undergoing radiation therapy for prostate cancer. Surprisingly, none of the natural hydrogels have been approved for use in cancer products, possibly due to the advantages in using synthetic gels as sustained release platforms.
3.3. Spinal fusion products
As scaffolding materials, hydrogels have garnered remarkable attention due to their ability to provide mechanical support to the existing natural tissues. Sustaining the functional transition of the scaffold during the healing process remains as one of the critical challenges in the hydrogel‐based product design in regenerative medicine. Stryker's, OP‐1® implant was one of the first products approved by the FDA (2001) to promote bone growth. However, later in 2004, OP‐1® surgical putty containing the OP‐1 protein and collagen was designated as a “humanitarian use device” for the treatment of rare conditions due to failing to demonstrate the efficacy of the device. Medtronic's autograft replacement therapy, Infuse® bone graft received FDA approval in 2002 due to its improved osteoinductivity. The product containing recombinant human bone morphogenetic protein‐2 (rhBMP‐2) together with the absorbable collagen sponge carrier (ACS) initiates the bone induction process once injected into the spine and eventually leads to bone remodeling and new blood vessels formation. Ferring's EUFLEXXA® was approved in 2004 for treating knee pain caused by osteoarthritis. Currently there are no synthetic hydrogel products that have been approved for use in regenerative market presumably owing to their low biocompatibility.
3.4. Other indications
Other than aesthetics, cancer and spinal fusion products, hydrogels have been approved in many other indications. LoneStar's Algisyl‐LVR® is the only hydrogel product derived from alginate that has been approved for advanced heart failure. While both Bulkamid® and Coaptite® have obtained approvals for the treatment of urinary incontinence in females, application of polyacrylamide‐based Bulkamid® for urinary disorders has witnessed a surge in the clinical trials. Additionally, TraceIT® hydrogel tissue marker was approved by the FDA for image‐guided soft tissue alignment in 2013 and has established this PEG‐based hydrogel system as one of the most explored synthetic hydrogel systems for imaging and spacing in cancer treatments in the clinical trials.
4. CURRENT HYDROGEL CLINICAL TRIALS
Considerable efforts in the clinic are focused on evaluating new injectable hydrogel‐based systems as therapeutic, diagnostic, and aesthetic agents. In this section, we will briefly review (a) the current landscape of hydrogel‐based systems currently being investigated in the clinic (Table 2 ) and (b) the key technologies attempting to integrate advanced functions and improved biocompatibility into the hydrogel structures.
TABLE 2.
Examples of current clinical trials for injectable hydrogels
| Name (sponsor company/university) | Hydrogel material/payload (gelation mechanism) | Injection type | Indications | http://clinicaltrials.gov identifier (phase) |
|---|---|---|---|---|
| Tissue regeneration: synthetic | ||||
| Argiform (Research Centre BIOFORM) | Polyacrylamide/silver ions (chemical reaction) | Intra‐articular | Knee osteoarthritis | NCT03897686 (NA) |
| Aquamid (Henning Bliddal) | Polyacrylamide (chemical reaction) | Intra‐articular | Knee osteoarthritis | NCT03060421 (NA) |
| PAAG‐OA (Contura) | Polyacrylamide (chemical reaction) | Intra‐articular | Knee osteoarthritis | NCT04045431 (NA) |
| Aquamid (A2 Reumatologi Og Idrætsmedicin) | Polyacrylamide (chemical reaction) | Intra‐articular | Knee osteoarthritis | NCT03067090 (NA) |
| GelStix® Nucleus augmentation device (Dr med. Paolo Maino Viceprimario Anestesiologia) | Polyacrylonitrile (chemical reaction) | Intra‐discal | Degenerative disc disease | NCT02763956 (NA) |
| Tissue regeneration: natural | ||||
| Hymovis Viscoelastic Hydrogel (Fidia Farmaceutici s.p.a.) | High molecular weight hyaluronan (physical interaction) | Intra‐articular | Osteoarthritis | NCT01372475 (Ph III) |
| HYADD® 4 Hydrogel (Fidia Farmaceutici s.p.a.) | Non‐crosslinked hyaluronic acid alkylamide (physical interaction) | Intra‐articular | Knee osteoarthritis | NCT02187549 (NA) |
| Promedon | Hydroxyethyl cellulose (physical interaction) | Knee | Osteoarthritis | NCT04061733 (NA) |
| Algisyl‐LVR® device (LoneStar Heart, Inc.) | Alginate (physical interaction) | Intra‐myocardial | Heart failure and dilated cardiomyopathy | NCT01311791 (Ph II/III) |
| Algisyl device (LoneStar Heart, Inc.) | Alginate (physical interaction) | Intra‐myocardial | Moderate to severe heart failure | NCT03082508 (NA) |
| Neo‐kidney augment (inRegen) | Gelatin with selected renal cells (chemical reaction) | Kidney | Type 2 diabetes and chronic kidney disease | NCT02525263 (Ph II) |
| Renal autologous cell therapy (inRegen) | Gelatin with renal autologous cells (chemical reaction) | Renal cortex | Chronic kidney disease from congenital anomalies of kidney and urinary tract | NCT04115345 (Ph I) |
| The Second Affiliated Hospital of Chongqing Medical University | Unknown/human amniotic epithelial cells (mechanism unknown) | Uterine cavity | Asherman's syndrome | NCT03223454 (Ph I) |
| Naofumi Takehara | Gelatin with basic fibroblast growth factor (mechanism unknown) | Intra‐myocardial | Ischemic cardiomyopathy | NCT00981006 (Ph I) |
| VentriGel (Ventrix, Inc.) | Native myocardial extracellular matrix (physical interaction) | Trans‐endocardially | Myocardial infarction | NCT02305602 (Ph I) |
| Cancer applications: synthetic | ||||
| Absorbable Radiopaque Tissue Marker (Sidney Kimmel Comprehensive Cancer Center at Johns Hopkins) | Polyethylene glycol/TraceIT® (chemical reaction) | Between pancreas and duodenum | Imaging of pancreatic adenocarcinoma | NCT03307564 |
| Memorial Sloan Kettering Cancer Center | Polyethylene glycol (chemical reaction) | Visceral pleura | Lung biopsy | NCT02224924 (Ph III) |
| Absorbable Radiopaque Tissue Marker (Washington University School of Medicine) | Polyethylene glycol/TraceIT® (chemical reaction) | Resection bed | Imaging of oropharyngeal cancer | NCT03713021 (Ph I) |
| Absorbable Radiopaque Hydrogel Spacer (Thomas Zilli, University Hospital, Geneva) | Polyethylene glycol/TraceIT® (chemical reaction) | Between the target (prostate/vagina) and the organ (rectum) | Spacing in radiation therapy for rectal cancer | NCT03258541 (NA) |
| Augmenix, Inc. | Polyethylene glycol/SpaceOAR® (chemical reaction) | Between the rectum and prostate | Spacing in radiation therapy for prostate cancer | NCT01538628 (Ph III) |
| Royal North Shore Hospital | Polyethylene glycol/SpaceOAR® (chemical reaction) | Between the rectum and prostate | Spacing in radiation therapy for prostate cancer | NCT02212548 (NA) |
| University of Washington | Polyethylene glycol/TraceIT® (chemical reaction) | Around circumference of the tumor bed | Imaging of bladder carcinoma | NCT03125226 |
| Cancer applications: natural | ||||
| Gut Guarding Gel (National Cheng‐Kung University Hospital) | Sodium alginate/calcium lactate (physical interaction) | Submucosal | Gastroenterological tumor and polyps | NCT03321396 (NA) |
| Incontinence: synthetic | ||||
| Bulkamid (Karolinska Institutet) | Polyacrylamide (chemical reaction) | Transurethral | Midurethral sling surgery | NCT02776423 |
| Bulkamid (Cantonal Hospital, Frauenfeld) | Polyacrylamide/botulinum toxin A (chemical reaction) | Intra‐vesical | Mixed urinary incontinence | NCT02815046 (NA) |
| Bulkamid (Contura) | Polyacrylamide (chemical reaction) | Transurethral | Stress urinary incontinence | NCT00629083 (NA) |
| Bulkamid (Helsinki University Central Hospital) | Polyacrylamide (chemical reaction) | Transurethral | Stress urinary incontinence | NCT02538991 (NA) |
| Bulkamid (Karolinska Institutet) | Polyacrylamide (chemical reaction) | Submucosal | Anal incontinence | NCT02550899 (Ph IV) |
| Other: synthetic | ||||
| Ocular Therapeutix, Inc. | Polyethylene glycol/OTX‐TKI (chemical reaction) | Intra‐vitreal | Neovascular age‐related macular degeneration | NCT03630315 (Ph I) |
| EUTROPHILL hydrogel (Assistance Publique ‐ Hôpitaux de Paris) | Polyacrylamide (chemical reaction) | Subcutaneous | HIV‐related facial lipoatrophy | NCT01077765 (Ph III) |
| Frequency Therapeutics | Poloxamer/FX‐322 (physical interaction) | Intra‐tympanic | Sensorineural hearing loss | NCT04120116 (Ph II) |
4.1. Hydrogels in regenerative medicine
Since the first FDA approval of Infuse® bone graft over 18 years ago, surprisingly there are few injectable hydrogel products in the market for wound care, tissue engineering and regeneration. However, with the advancement in new material chemistries, polymer physics, fabrication capabilities, and understanding of tissue engineering, currently hydrogel scaffolds investigated in regenerative medicine comprises the largest number of clinical trials. Particularly, polyacrylamide‐based synthetic hydrogels are extensively explored for the treatment of knee osteoarthritis via intra‐articular injection. Among natural hydrogels, HA and hydroxyethyl cellulose are currently being investigated for the same indication. However, despite being highly effective and biodegradable, there are still adverse events such as inflammation, joint pain and joint effusion that limit the clinical translation of these synthetic matrixes. 34 Additionally, apart from alginate, gelatin‐based hydrogels are widely investigated with cellular components for heart and kidney diseases due to improved interaction with natural biological matrices.
4.2. Hydrogel scaffolds in cancer care
It is not surprising that hydrogels are also being investigated for improved cancer care. In particular, synthetic hydrogels including TraceIT® and SpaceOAR® are being widely investigated for imaging of cancerous cells and protecting healthy cells from radiation induced damage. The TraceIT® hydrogel tissue marker consists of polyethylene glycol (PEG) hydrogel micro particles containing covalently bound iodine, which allows visualization of cancerous tissue up to 3 months using magnetic resonance imaging (MRI), computed tomography (CT), and/or ultrasound for future surgical procedures. While the SpaceOAR® hydrogel is primarily designed to protect normal tissues from radiation injury during radiation treatment of cancerous tissues. However, only one of the natural hydrogels is currently being investigated in the clinic for cancer therapy which could be attributed to their limited structural integrity in comparison to the synthetic hydrogels. Thus, improving rigidity and persistence of natural hydrogels might hold significant promise in cancer applications.
4.3. Hydrogels for urinary incontinence
Likewise, synthetic polyacrylamide‐based Bulkamid® hydrogel has dominated the investigations in the clinical trials and has already been approved in 25 countries including Europe. It is currently being investigated in multicenter clinical trials across 33 sites in the United States and Canada. Currently there are no clinical trials that are exploring natural injectable hydrogel therapies presumably due to their achieving suitable rigidity for use as a spacer for incontinence indications.
4.4. Hydrogels for other indications
Injectable hydrogels for ocular applications have remained one of the most investigated areas in the preclinical space, 35 but surprisingly there are no approved products in the clinic. Ocular Therapeutix's OTX‐TKI (tyrosine kinase inhibitor microcrystals in PEG hydrogel) is the only candidate currently in the clinical trials for the treatment of age‐related macular degeneration (AMD) following intravitreal injection. Intra‐tympanically administered FX‐322, a combination of two proprietary, small‐molecule drugs in poloxamer hydrogel has able to grab fast track designation by the FDA following successful phase 1/2 trials for the treatment of sensorineural hearing loss. Additionally, while there are numerous natural hydrogel (e.g., HA‐based, collagen‐based) aesthetic products in the market, polyacrylamide‐based hydrogels are currently being investigated for facial lipoatrophy. The broad scope of the hydrogel‐based aesthetic products in clinical trials could warrant a separate, and more focused, review.
5. DESIGN CHALLENGES
A hydrogel must meet application‐specific design criteria to suitably treat a medical condition. Broadly, these design criteria can be defined as either physical, chemical, or biological. Here, we will discuss the current status of hydrogel design and fabrication, as it pertains to each design challenge.
5.1. Mechanical robustness
Injectable hydrogels must have a sufficiently low viscosity to be introduced via a needle and syringe and a sufficient elasticity in situ to maintain their injected volume and sustain repetitive load. Overcoming this paradox is a major design challenge. One approach is to use a shear thinning polymer, such as HA.36, 37 As described in the previous section HA is used currently as a dermal filler, and is being investigated as an injectable replacement for cartilage. These materials gel by physical mechanisms (i.e., intermolecular interactions between polymer chains) which are disrupted by the shear of injection. 27 Other clinically approved hydrogels that form via a physical mechanism are hydroxyapatite‐carboxymethylcellulose and collagen.
Another approach is to use in situ crosslinking to inject a hydrogel precursor which gels via a chemical reaction either within a mixing tip or within the physiological environment.38, 39, 40 Depending upon the application of interest, these chemical crosslinking reagents can be biodegradable or nonbiodegradable. For example, the Bulkamid (polyacrylamide) hydrogel employs a nondegradable crosslinking agent, while the SpaceOAR and TraceIT (polyethylene glycol) hydrogels each employ a hydrolytically degradable crosslinker.
As illustrated in Figure 2, there are a number of hydrogel parameters that can be optimized to achieve specific mechanical properties. In particular, the molecular weight of polymer chains, extent of crosslinking, and crosslinking mechanism. In general, the viscosity of a polymer solution scales linearly with the molecular weight.41, 42 The hydrogel elastic modulus scales inversely with the molecular weight between crosslinks.43, 44 The crosslinking mechanism is determined by the chemical functionality of the polymer and any crosslinking agents.
FIGURE 2.
Design of hydrogels to overcome biophysical and biochemical challenges. When designing a new hydrogel, one determines the chemical functionality and chain rigidity by either selecting or synthesizing a proper backbone material (e.g., hyaluronic acid, polyethylene glycol, polyacrylate). The molecular weight of that linear backbone, the mechanism of crosslinking/gelation, as well as the molecular weight between crosslinks (i.e., extent of crosslinking) will determine the physical properties of the system. The combination of these chemical and physical identities will determine the gels’ mechanical integrity, solute transport properties, and interactions with host cells. Shown above are the clinical applications of (top) intra‐articular or subcutaneous injection, (middle) drug elution from an injected hydrogel depot, and (bottom) cell infiltration of an injected hydrogel scaffold
5.2. Loading and release of therapeutic agents
Therapeutic agents, which can be delivered to the surrounding environment by an injectable hydrogel carrier, can include small molecules, macromolecules (i.e., peptides, proteins, nucleic acids), or engineered cells. Cargo release from injected hydrogels is determined by the (a) size of the cargo, relative to the mesh size of the hydrogel and (b) affinity of the cargo–gel interaction.
Injectable hydrogels currently used in the clinic that include a therapeutic agent deliver a small molecule drug or a biologic (microparticle depot systems). The most common therapeutic agent is lidocaine, an anesthetic drug that reduces the pain associated with the subcutaneous hydrogel injection. Lidocaine is included as a therapeutic agent within a number of hyaluronic acid hydrogels approved for facial correction applications. As a small molecule, lidocaine's release from the injected hydrogel is minimally perturbed by the hydrogel mesh. Lidocaine's release is likely quite rapid from these gels, similar to what has been seen in the published literature. 45 For other drug‐eluting hydrogel applications, such as active wound dressings, a sustained release of protein drugs is needed. In these cases, one must either reduce the hydrogel mesh size via crosslinking, to perturb solute elution, or increase the hydrogel‐drug affinity with tailored gel compositions to increase retention.
5.3. Hydrogel bioactivity
For advanced tissue regeneration purposes, host cells must infiltrate, modify, and degrade bulk hydrogel materials. To achieve this aim, cells must adhere to the material. Some natural polymers, such as fibrin, 46 collagen, gelatin, 47 and HA, 40 are naturally adhesive. 48 For example, current clinical trials (NCT04115345, NCT04115345, and NCT00981006) uses a gelatin hydrogel with cells and/or growth factors. The hydrogels facilitate tissue healing following an injury (to the kidney or myocardium) by generating a suitable microenvironment, within which cell adhesion is paramount. Other common hydrogel materials, such as polyethylene glycol and polyacrylamide, are non‐adhesive and must be modified chemically with adhesive ligands to facilitate cell infiltration. 49 However, for other applications discussed in this paper, such as marking tumor margins (e.g., TraceIT), rapid biodegradability is a design limitation, and therefore it is advantageous to use a hydrogel that degrades slowly over weeks to months.
Immune responses to injected biomaterials have been an active area of research for many years.50, 51 In seemingly all applied cases, it is important to minimize any immune response to the hydrogel injection. Immunological responses are responsible for adverse outcomes of biomaterial injection or implantation, including inflammation and fibrosis. 52 While these responses are deleterious in their own right, their associated physicochemical shifts (i.e., changes in the local pH or temperature) can also alter the hydrogel material, impairing function further. Therefore, minimizing the host immune response to hydrogel injection is a critical biological design parameter.
5.4. Technological challenges
Despite the success of hydrogel‐based delivery systems, key technological challenges including chemistry, manufacturing and controls, defined regulatory guidelines, and practical adaptability remain as major roadblocks in their successful clinical translation. Since hydrogel fabrication is complex and varies between hydrogel systems, the development costs through clinical translation range in estimation from $50 million up to $800 million. 30 In this section, we discuss the major translational barriers that must be addressed in order to improve healthcare with an inflating arsenal of injectable hydrogel‐based scaffolds and delivery systems.
5.5. Scale‐up strategies and GMP processes
A major hurdle in the clinical translation and integration of biomaterials‐based hydrogels is their compatibility with current good manufacturing practices (cGMPs). Since most hydrogel systems are synthesized in small batches at preclinical stage, efforts to translate fabrication/synthesis strategies to scaled systems are required. Batch variations, robustness, safety and efficiency issues are inevitable when performed at a larger scale. Natural polymer hydrogels may face additional difficulties, since natural polymers are heterogeneous and can exhibit different properties or characteristics at the molecular scale and potentially after synthesis into hydrogels. Additionally, the high‐water content of hydrogels makes the sterilization, storage and fabrication processes even more demanding.
5.6. Regulatory approvals
The diversity of crosslinking agents and biomaterials employed to develop hydrogel scaffolds, renders their regulatory classification and approval challenging. Unlike drugs which are broadly classified, hydrogels are classified under the “devices” category which according to the Section 201(g) of the FD&C Act covers “any product which does not achieve its primary intended purposes through chemical action within or on the body”. Furthermore, other than few exceptions, majority of the hydrogel‐based products are required to undergo additional FDA review of a 510(k) Pre‐Market Notification submission for obtaining legal marketing rights in the United States. In case of hydrogel scaffolds encompassing a drug or drug‐secreting cells, they are considered as a combination product, and thus their regulatory approval takes up to 7–10 years, which further limits their commercial viability. 30
6. CONCLUSIONS
Injectable hydrogel systems provide a tunable platform to enable sustained release of small molecules or biologics and can also serve as a bulk material to interface between biological surface for application in tissue engineering or regeneration. Many examples of clinically and commercially successful injectable hydrogels exist (Table 1 ) and the newer injectable hydrogel systems that are being investigated in the clinic (Table 2 ) expand on these indications and likewise highlight the continued interest in development of these system. Still, injectable hydrogels face challenges unlike other therapeutic platforms, given their reliance on polymers, mechanical and solute transport properties, degradability, compatibility and scale‐up approaches. Altogether, the application of hydrogels toward improving healthcare remains a highly active area of research with a growing number of technologies being evaluated in humans.
ACKNOWLEDGMENTS
Image templates made freely available by Servier Medical Art (http://smart.servier.com) were used for the preparation of Figure 2. SM is a shareholder/board member/consultant of Fount Biosciences. SM is an inventor on a patent application (20180186900) filed by the University of California.
Mandal A, Clegg JR, Anselmo AC, Mitragotri S. Hydrogels in the clinic. Bioeng Transl Med. 2020;5:e10158 10.1002/btm2.10158
Contributor Information
Aaron C. Anselmo, aanselmo@email.unc.edu.
Samir Mitragotri, Email: mitragotri@seas.harvard.edu.
REFERENCES
- 1. Peppas NA, Hilt JZ, Khademhosseini A, Langer R. Hydrogels in biology and medicine: from molecular principles to bionanotechnology. Adv Mater. 2006;18(11):1345‐1360. [Google Scholar]
- 2. Peppas NA, Bures P, Leobandung W, Ichikawa H. Hydrogels in pharmaceutical formulations. Eur J Pharm Biopharm. 2000;50(1):27‐46. 10.1016/S0939-6411(00)00090-4. [DOI] [PubMed] [Google Scholar]
- 3. Slaughter BV, Khurshid SS, Fisher OZ, Khademhosseini A, Peppas NA. Hydrogels in regenerative medicine. Adv Mater. 2009;21(32–33):3307‐3329. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 4. Korsmeyer RW, Gurny R, Doelker E, Buri P, Peppas NA. Mechanisms of solute release from porous hydrophilic polymers. Int J Pharm. 1983;15(1):25‐35. [DOI] [PubMed] [Google Scholar]
- 5. Liechty WB, Scheuerle RL, Ramirez JEV, Peppas NA. Uptake and function of membrane‐destabilizing cationic nanogels for intracellular drug delivery. Bioeng Transl Med. 2019;4(1):17‐29. 10.1002/btm2.10120. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6. Clegg JR, Irani AS, Ander EW, et al. Synthetic networks with tunable responsiveness, biodegradation, and molecular recognition for precision medicine applications. Sci Adv. 2019;5(9):eaax7946. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 7. Kimna C, Lieleg O. Engineering an orchestrated release avalanche from hydrogels using DNA‐nanotechnology. J Control Release. 2019;304:19‐28. 10.1016/j.jconrel.2019.04.028. [DOI] [PubMed] [Google Scholar]
- 8. Appel EA, Tibbitt MW, Webber MJ, Mattix BA, Veiseh O, Langer R. Self‐assembled hydrogels utilizing polymer–nanoparticle interactions. Nat Commun. 2015;6(1):1‐9. 10.1038/ncomms7295. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 9. Gawade PM, Shadish JA, Badeau BA, DeForest CA. Boolean biomaterials: logic‐based delivery of site‐specifically modified proteins from environmentally responsive hydrogel biomaterials. Adv Mater. 2019;31(33):1970237 10.1002/adma.201970237. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 10. Shadish JA, Strange AC, DeForest CA. Genetically encoded photocleavable linkers for patterned protein release from biomaterials. J Am Chem Soc. 2019;141(39):15619‐15625. 10.1021/jacs.9b07239. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 11. Yu Y, Lin X, Wang Q, He M, Chau Y. Long‐term therapeutic effect in nonhuman primate eye from a single injection of anti‐VEGF controlled release hydrogel. Bioeng Transl Med. 2019;4(2). e10128. 10.1002/btm2.10128. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 12. Johnson CT, Wroe JA, Agarwal R, et al. Hydrogel delivery of lysostaphin eliminates orthopedic implant infection by Staphylococcus aureus and supports fracture healing. PNAS. 2018;115(22):E4960‐E4969. 10.1073/pnas.1801013115. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 13. Drury JL, Mooney DJ. Hydrogels for tissue engineering: scaffold design variables and applications. Biomaterials. 2003;24(24):4337‐4351. [DOI] [PubMed] [Google Scholar]
- 14. Lee KY, Mooney DJ. Hydrogels for tissue engineering. Chem Rev. 2001;101(7):1869‐1880. [DOI] [PubMed] [Google Scholar]
- 15. Kloxin AM, Kasko AM, Salinas CN, Anseth KS. Photodegradable hydrogels for dynamic tuning of physical and chemical properties. Science. 2009;324(5923):59‐63. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 16. Engler AJ, Sen S, Sweeney HL, Discher DE. Matrix elasticity directs stem cell lineage specification. Cell. 2006;126(4):677‐689. [DOI] [PubMed] [Google Scholar]
- 17. Rao VV, Vu MK, Ma H, Killaars AR, Anseth KS. Rescuing mesenchymal stem cell regenerative properties on hydrogel substrates post serial expansion. Bioeng Transl Med. 2019;4(1):51‐60. 10.1002/btm2.10104. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 18. Mahadik BP, Bharadwaj NA, Ewoldt RH, Harley BA. Regulating dynamic signaling between hematopoietic stem cells and niche cells via a hydrogel matrix. Biomaterials. 2017;125:54‐64. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 19. Nguyen EH, Daly WT, Le NNT, et al. Versatile synthetic alternatives to Matrigel for vascular toxicity screening and stem cell expansion. Nat Biomed Eng. 2017;1(7):0096. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 20. Wang X, Maruotti J, Majumdar S, et al. Collagen vitrigels with low‐fibril density enhance human embryonic stem cell‐derived retinal pigment epithelial cell maturation. J Tissue Eng Regen Med. 2018;12(3):821‐829. [DOI] [PubMed] [Google Scholar]
- 21. Annabi N, Tamayol A, Uquillas JA, et al. 25th anniversary article: rational design and applications of hydrogels in regenerative medicine. Adv Mater. 2014;26(1):85‐124. 10.1002/adma.201303233. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 22. Yue K, Liu Y, Byambaa B, et al. Visible light crosslinkable human hair keratin hydrogels. Bio Transl Med. 2018;3(1):37‐48. 10.1002/btm2.10077. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 23. Almeida H, Domingues RMA, Mithieux SM, et al. Tropoelastin‐coated tendon biomimetic scaffolds promote stem cell tenogenic commitment and deposition of elastin‐rich matrix. ACS Appl Mater Interf. 2019;11(22):19830‐19840. 10.1021/acsami.9b04616. [DOI] [PubMed] [Google Scholar]
- 24. Feng Q, Wei K, Lin S, et al. Mechanically resilient, injectable, and bioadhesive supramolecular gelatin hydrogels crosslinked by weak host‐guest interactions assist cell infiltration and in situ tissue regeneration. Biomaterials. 2016;101:217‐228. [DOI] [PubMed] [Google Scholar]
- 25. Afewerki S, Sheikhi A, Kannan S, Ahadian S, Khademhosseini A. Gelatin‐polysaccharide composite scaffolds for 3D cell culture and tissue engineering: towards natural therapeutics. Bioeng Transl Med. 2019;4(1):96‐115. 10.1002/btm2.10124. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 26. Clegg JR, Wagner AM, Shin SR, Hassan S, Khademhosseini A, Peppas NA. Modular fabrication of intelligent material‐tissue interfaces for bioinspired and biomimetic devices. Prog Mater Sci. 2019;100589. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 27. Koetting MC, Peters JT, Steichen SD, Peppas NA. Stimulus‐responsive hydrogels: theory, modern advances, and applications. Mater Sci Eng: R: Rep. 2015;93:1‐49. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 28. Aguado BA, Grim JC, Rosales AM, Watson‐Capps JJ, Anseth KS. Engineering precision biomaterials for personalized medicine. Sci Transl Med. 2018;10(424):eaam8645. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 29. Rosales AM, Anseth KS. The design of reversible hydrogels to capture extracellular matrix dynamics. Nat Rev Mater. 2016;1(2):1‐15. 10.1038/natrevmats.2015.12. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 30. Li J, Mooney DJ. Designing hydrogels for controlled drug delivery. Nat Rev Mater. 2016;1(12).1‐17. 10.1038/natrevmats.2016.71. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 31. Wechsler ME, Stephenson RE, Murphy AC, Oldenkamp HF, Singh A, Peppas NA. Engineered microscale hydrogels for drug delivery, cell therapy, and sequencing. Biomed Microdevices. 2019;21(2):31 10.1007/s10544-019-0358-0. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 32. Maulvi FA, Soni TG, Shah DO. A review on therapeutic contact lenses for ocular drug delivery. Drug Deliv. 2016;23(8):3017‐3026. [DOI] [PubMed] [Google Scholar]
- 33. Kumbar S, Laurencin C, Deng M. Natural and synthetic biomedical polymers. Newnes. 2014;17:402. [Google Scholar]
- 34. Tonbul M, Adas M, Bekmezci T, Kara AD. Intra‐articular polyacrylamide hydrogel injections are not innocent. Case Rep Orthop. 2014;2014:150709‐4. 10.1155/2014/150709. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 35. Wang K, Han Z. Injectable hydrogels for ophthalmic applications. J Control Release. 2017;268:212‐224. 10.1016/j.jconrel.2017.10.031. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 36. Rodell CB, Kaminski AL, Burdick JA. Rational design of network properties in guest–host assembled and shear‐thinning hyaluronic acid hydrogels. Biomacromolecules. 2013;14(11):4125‐4134. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 37. Highley CB, Prestwich GD, Burdick JA. Recent advances in hyaluronic acid hydrogels for biomedical applications. Curr Opin Biotechnol. 2016;40:35‐40. [DOI] [PubMed] [Google Scholar]
- 38. Meng Z, Chao Y, Zhou X, et al. Near‐infrared‐triggered in situ gelation system for repeatedly enhanced photothermal brachytherapy with a single dose. ACS Nano. 2018;12(9):9412‐9422. [DOI] [PubMed] [Google Scholar]
- 39. Varma D, Lin H, Long R, et al. Thermoresponsive, redox‐polymerized cellulosic hydrogels undergo in situ gelation and restore intervertebral disc biomechanics post discectomy. Eur Cell Mater. 2018;35:300‐317. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 40. Li L, Wang N, Jin X, et al. Biodegradable and injectable in situ cross‐linking chitosan‐hyaluronic acid based hydrogels for postoperative adhesion prevention. Biomaterials. 2014;35(12):3903‐3917. [DOI] [PubMed] [Google Scholar]
- 41. Scholte TG, Meijerink N, Schoffeleers H, Brands A. Mark–Houwink equation and GPC calibration for linear short‐chain branched polyolefines, including polypropylene and ethylene–propylene copolymers. J Appl Polym Sci. 1984;29(12):3763‐3782. [Google Scholar]
- 42. Wang W, Bo S, Li S, Qin W. Determination of the Mark‐Houwink equation for chitosans with different degrees of deacetylation. Int J Biol Macromol. 1991;13(5):281‐285. [DOI] [PubMed] [Google Scholar]
- 43. Langley NR, Polmanteer KE. Relation of elastic modulus to crosslink and entanglement concentrations in rubber networks. J Polym Sci. 1974;12(6):1023‐1034. [Google Scholar]
- 44. Yesilyurt V, Ayoob AM, Appel EA, Borenstein JT, Langer R, Anderson DG. Mixed reversible covalent crosslink kinetics enable precise, hierarchical mechanical tuning of hydrogel networks. Adv Mater. 2017;29(19):1605947. [DOI] [PubMed] [Google Scholar]
- 45. Liu D‐Z, Sheu M‐T, Chen C‐H, Yang Y‐R, Ho H‐O. Release characteristics of lidocaine from local implant of polyanionic and polycationic hydrogels. J Control Release. 2007;118(3):333‐339. 10.1016/j.jconrel.2007.01.001. [DOI] [PubMed] [Google Scholar]
- 46. Schek R, Michalek A, Iatridis J. Genipin‐crosslinked fibrin hydrogels as a potential adhesive to augment intervertebral disc annulus repair. Eur Cell Mater. 2011;21:373‐383. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 47. Nichol JW, Koshy ST, Bae H, Hwang CM, Yamanlar S, Khademhosseini A. Cell‐laden microengineered gelatin methacrylate hydrogels. Biomaterials. 2010;31(21):5536‐5544. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 48. Caliari SR, Burdick JA. A practical guide to hydrogels for cell culture. Nat Methods. 2016;13(5):405‐414. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 49. Mann BK, Gobin AS, Tsai AT, Schmedlen RH, West JL. Smooth muscle cell growth in photopolymerized hydrogels with cell adhesive and proteolytically degradable domains: synthetic ECM analogs for tissue engineering. Biomaterials. 2001;22(22):3045‐3051. [DOI] [PubMed] [Google Scholar]
- 50. Bryers JD, Giachelli CM, Ratner BD. Engineering biomaterials to integrate and heal: the biocompatibility paradigm shifts. Biotechnol Bioeng. 2012;109(8):1898‐1911. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 51. Chung L, Maestas DR Jr, Housseau F, Elisseeff JH. Key players in the immune response to biomaterial scaffolds for regenerative medicine. Adv Drug Deliv Rev. 2017;114:184‐192. [DOI] [PubMed] [Google Scholar]
- 52. Mariani E, Lisignoli G, Borzì RM, Pulsatelli L. Biomaterials: foreign bodies or tuners for the immune response? Int J Mol Sci. 2019;20(3):636. [DOI] [PMC free article] [PubMed] [Google Scholar]