Crosslinked Hyaluronic Acid Hydrogel Networks Designed as Mechanical Actuators

Bioengineers are in constant pursuit of solutions to problems facing the medical and pharmaceutical field by designing biomaterials that closely mimic the target natural systems. A unique collection of polymers, known as polymeric actuators, have been devised with the ability to convert an external stimulus to a change in shape, size or permeability. The current options within polymeric biomaterials with multi-functionality include matrices that are biocompatible, biodegradable, quick transitioning / shape changing, and mechanically tunable. These properties have been harnessed for application such as stents, valves, semi permeable membranes, and dynamic cell culture substrates. For such applications quick and uniform actuator response that does not need to be sustained for more than a few hours is desired. However there exist other areas of biomedical applications, such as wound closure/healing and nerve regeneration, where polymeric actuators have been underutilized. These applications however call for a polymer system that can actuate at controlled slow speeds and sustain this actuation for several days. At present there is a lack of such slow actuating polymer system. Each year over 50,000 peripheral nerve repair procedures are performed (National Center for Health Statistics, 1995). The total annual costs in U.S alone exceed $ 7 billion (American Paralysis Association, 1997). The treatment of a nerve transection is dependent on the size of the injury gap. Similarly, the extent of regeneration and re-innervation in the PNS is also governed by the size of the gap. For a smaller gap (<10 mm) the surgeon can pull the severed nerve ends closer and suture them to repair the injury. For larger gaps autologous nerve transplant is the gold standard treatment despite the inherent disadvantages. Over the past decades biomaterial researchers have tested several polymeric nerve conduits as an alternative to autologous nerve grafts. However none have been able to match the success rates of autologous grafts. There is a lack of an effective biomaterial solution to the problem of a large gap nerve injury. For many years there has been a hypothesis that nervous tissue can be successfully elongated via application of an external mechanical force alone which could be used to treat peripheral nerve gap injuries. Mechanical actuation studies have been shown to produce successful stretch growth in individual axons and axonal bundles. This phenomenon is at play in nature during embryonic growth and development of the body of organisms to adulthood. Applying tensional forces at appropriate rates (< 100 μm/hr) causes sustained axonal stretch growth. The solution we propose in this work is a biomaterial that can be programmed to perform the function of a mechanical actuator at rates suitable for axonal stretch growth. We designed, fabricated, and characterized a novel hyaluronic acid based hydrogel that shrinks over time along a pre-defined axis thereby providing the source for tension that could be used for sustained axonal stretch growth. The shear thinning property of hyaluronic acid (HA) enabled us to test if we could store a retractive stress in a rapidly crosslinked network under shear flow and then controllably release this stress and achieve shrinkage of the network scaffold along one desired axis. We investigated two strategies to achieve this goal. The retractive stress trapped in the crosslinked network was released either by manipulating the main backbone HA chains or by selectively breaking the crosslinks. The shrinkage rates obtained were within the range of stretching rates that have successfully stretched neuronal cells. We also confirmed that the material’s cytocompatibility was unaffected by the chemical modifications that HA was subjected to. This polymer system is a novel addition to the existing polymeric actuators and is a step towards filling the void of a slow, long term actuating