Injectable Alginate Hydrogels for Medical Applications

One of the biggest challenges in medicine is getting a drug to the right part of the body at the right time. Even when the target site in the body is known, like a pain-causing injury or a cancerous tumor, most drugs are given as oral pills or intravenous infusions, which limits their effectiveness. In some cases, the drug is broken down or excreted too quickly to exert its effects. In others, large doses must be injected to deliver enough drug to the target tissues, causing harmful side effects to other body systems.

The Wyss Institute is developing solutions to these problems in the form of flexible, tunable hydrogels made of alginate, a natural biopolymer found in the cell wall of brown algae. These hydrogels can be pre-loaded with a number of different therapeutic agents, including chemotherapies, biologics, antibodies, and/or nanoparticles prior to being injected to the disease site. Once injected, they release their therapeutic cargo in a controlled and sustained manner, often resulting in significant reduction of the required dose, toxic side effects, and the need for multiple/high doses of drugs.

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A number of different alginate hydrogels are in development for different medical applications:

Injectable cryogels for tunable drug release

Cryogels are hydrogels whose shape and other properties can be changed by altering the conditions under which they are frozen, and are thus an attractive option for creating drug delivery systems. They often have mesh sizes larger than the size of typical proteins, and the proteins themselves often do not bind strongly to cryogels. These issues can cause the rapid release of a drug from its cryogel by diffusion, which may be dangerous for the patient. Pre-encapsulation of proteins inside carrier molecules prior to their incorporation into the cryogel is frequently used to try to ameliorate this problem, but that process is time-consuming, involves the use of harsh chemicals, and has low encapsulation efficiency.

We have developed an alternate solution by attaching therapeutic proteins to nanoplatelets of Laponite® ELG, a synthetic clay that can be readily incorporated into cryogels. In the lab, incorporating Laponite®-bound proteins into cryogels prevented unwanted “bursting” of the drug from the matrix, and maintained a controlled release over the course of four weeks. This method also eliminates the need for protein pre-encapsulation.

Permanent or biodegradable click-crosslinked hydrogels

Hydrogels are appealing for their ability to deliver therapeutic proteins and whole cells directly to a target location, such as a cancer patient’s tumor. Most hydrogels are formed by crosslinked ionic bonds, because these bonds are generally benign to any encapsulated cells or proteins. However, these bonds are reversible and weak, and can lead to rapid degradation of the matrix through ion exchange once injected into the body. Covalently bonded alginate hydrogels offer a more robust crosslinking option; however, traditional methods  are often found to damage or degrade encapsulated proteins and/or cells.

We have engineered highly tunable “click” alginate-based hydrogels using click chemistry to covalently crosslink alginate in a manner that does not interfere with proteins or cells contained within the hydrogel itself. As a permanent click hydrogel, these materials are minimally inflammatory, maintain structural integrity over several months, and reject cell infiltration when injected subcutaneously into mice (ionically crosslinked hydrogels broke down after one month).

Click-crosslinked alginate hydrogels can also be made biodegradable by oxidation of the alginate polymer backbone, which then becomes susceptible to hydrolysis in the body. This is a highly tunable process, allowing for biodegradation over a period of days to years. Through the ability to modify the alginate backbone to a high degree using click chemistry, click hydrogels can achieve controlled release of small molecules, biologics, antibodies, and nanoparticles. They are also highly soluble, allowing for further manipulation of the matrix mesh size by varying the click alginate’s concentration.

Our click chemistry system has also been applied to gelatin to enhance cell adhesion, proliferation, and cell viability. Cells can be readily incorporated into these gelatin-based hydrogels, where they can remodel the matrix and allow for cell trafficking, minimizing the need for additional synthesis steps associated with competitive hydrogel approaches. Furthermore, these hydrogels’ formulation can be manipulated to control their degradation rate. These materials are also highly injectable, enabling cell delivery at various therapeutic target sites.

Tunable, hybrid hydrogel-collagen matrices to mimic extracellular matrix

The extracellular matrix (ECM), a three-dimensional network of molecules such as collagen, enzymes, and glycoproteins that support and regulate cells, is of great interest to biologists investigating cell activity and engineers trying to replicate ECM for the production of artificial tissues. However, current systems that mimic ECM as cell-culture substrates allow for limited control of ECM mechanics, often lack native ECM molecules to bind cells, and have difficulty maintaining collagen, which is a major determinant of ECM properties and is therefore desirable in artificial ECM (aECM).

A new system from the Wyss Institute combines collagen with click-crosslinked alginate hydrogels by sequentially crosslinking the two substances both ionically and covalently. Varying the type and magnitude of crosslinking enables the stiffness and viscoelasticity of the aECM to be tuned while altering neither the microscale architecture nor the transport of molecules through the aECM. By timing the sequence of ionic and covalent crosslinking, the collagen can be made to self-assemble into fibrillar structures within the network. This mechanically tunable aECM provides a new approach for investigating the roles of both stiffness and viscoelasticity on cellular behavior.

Refillable drug depots for targeted delivery

A drug depot that is inserted into the body in one procedure inevitably can only deliver as much of the drug as it is pre-loaded with. Such depots, once empty, must either be removed from the body or biodegrade before a new one can be inserted.

We have created a refillable drug depot that consists of an alginate hydrogel to which a bio-orthogonal click chemistry moiety is attached, and which can be loaded with a drug of interest. Once the hydrogel depot is in the body and its payload is depleted, a new dose of the pro-drug attached to a bio-orthogonal click moiety that is complementary to those found on the hydrogel is delivered intravenously. As the pro-drug circulates through the bloodstream, it encounters the existing depot and the two moieties bind to each other, thus concentrating the drug at the depot, from which it can then be released at the specified rate. In studies, this system stopped tumor growth in mice with human breast cancer tumors.

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Sustained release of growth factors for blood vessel and nerve recovery

The molecules vascular endothelial growth factor (VEGF) and insulin-like growth factor-1 (IGF-1) are naturally occurring compounds in the body that are known to help blood vessels and nerves regrow after injury or chronic disease. However, injecting them into the bloodstream has not been shown to improve recovery significantly and is associated with toxicity. By loading both of these factors into an injectable alginate hydrogel, Wyss researchers have been able to deliver them to an injury site in a manner that keeps them localized and improves recovery of both peripheral arteries and peripheral nerves better than injections of VEGF or IGF-1 alone.

In a rabbit animal model of peripheral artery disease (PAD), injection of hydrogels containing VEGF and IGF-1 improved blood flow through injured arteries from ~50% to ~90%, even when given 30 days after injury, suggesting that it could be used to treat arterial damage due to chronic vascular disease. Models of nerve damage and muscle transplantation treated with the hydrogel recovered 97% of their muscle function. Potential application areas include peripheral artery disease, facial nerve loss, transplants, and more. Learn more about regenerating tissues with sustained growth factor delivery.

All versions of the alginate hydrogel are available for licensing.

Laponite is a registered trademark of BYK Additives Ltd.