Biomimetic Materials

Primary Limitation of Hydrogels

A primary limitation of hydrogels for use in a variety of applications such as cartilage tissue engineering scaffolds is relatively poor mechanical properties. As the polymer content is diluted by hydration as the hydrogel swells, the mechanical properties of the hydrogel also decline. However, there are many biological hydrogels such as the lens of the eye, cartilage, certain insect cuticles and so on which have much better mechanical properties than those typically observed in synthetic hydrogels of comparable water contents. Biological structures typically must avoid the catastrophic failure of brittle fracture above all. Most biological materials have a complex hierarchical structure that builds in steps from the molecular level up to the macroscopic tissue or structure.  Thus the most notable difference between biological hydrogels and synthetic hydrogels is that biological hydrogels generally have two or more macromolecular components organized in well-defined nanostructures that are responsible for distributing load and inhibiting crack propagation. Hence the key to understanding how biological hydrogels function and how to design synthetic hydrogels with improved properties is to understand how to select mutually reinforcing components and control their molecular organization.

We have been studying the elytra (wing covers) of the beetles Tribolium castaneum and Tenebrio molitor to provide insight into the design of hierarchically ordered, multicomponent biomaterials. Immature, hydrated beetle elytra have been shown to have superior strength and toughness relative to synthetic gels. In parallel to these insect studies, novel materials that capture key structural features of cuticle are being developed, notably semi- interpenetrating (sIPN) and interpenetrating networks (IPN) based on the biocompatible polymers polyethylene glycol (PEG), agarose, and methacrylated chondroin sulfate (MCS). The mechanical performance of these gels are comprehensively evaluated to determine a number of different mechanical properties, including the Young’s modulus (E), shear modulus (G), fracture stress, fracture strain and toughness. The goal is to develop biocompatible, high strength networks and to correlate the mechanical properties with the underlying nanostructure. 

Single Network Gels

Mechanical properties of IPN and semi-IPN hydrogels were shown to be superior to those of single network gels. A diverse array of properties could be obtained with the composite PEGDA-agarose semi-IPN gels by varying polymer composition and concentration, suggesting such networks can be easily tailored to a variety of biomedical applications.  IPN gels of MCS and polyacrylamide displayed a distinct yielding zone, very unusual for a hydrogel, and the yield stress was more than twice the highest value previously reported in the literature.

Historically, I have been involved with development of biomimetic materials, especially those based on recombinant proteins since a sabbatical in David A. Tirrell’s lab at the University of Massachusetts-amherst in 1992-1993.  However, I became most active in this area of the past ten years with a series of NSF-funded studies in collaboration with a team of scientists at Kansas State University led by Michael R. Kanost to study insect cuticle, a composite material which comprises the insect exoskeleton.  We are among the first groups to exploit insect cuticle as motif for biomaterials development, and the only group in the world to do so by combining molecular biology techniques with physical polymer science.  The citation rate of our papers in this area are significantly increasing as researchers are becoming aware of the significance and potential of this work.