Bioinspired High Mechanical Performance Artificial Extracellular Matrices
INTELLECTUAL MERIT: This project will prepare and characterize hydrophilic networks that bridge the gap between multicomponent, hierarchically ordered, hydrated biological structures like the extracellular matrix (ECM) of living tissues and the relatively simple composition and organization of single-component synthetic hydrogels. The goal is to gain insight into the structure-property relationships of hydrogels, in particular the ECM, and to generate a new class of hydrogel scaffold materials for applications in tissue engineering. Three-dimensional scaffolds suitable for tissue engineering typically need to support cell growth and differentiation, be permeable to nutrients and various signaling molecules, and to be either resorbable, or degradable, or a benign component of newly formed tissues. A key physical feature to be examined in this study is toughness, the ability to resist failure under stress, a characteristic lacking in many synthetic tissue engineering hydrogels. The proposed work will develop high strength, moderately extensible hydrogel networks based on chondroitin sulfate or hyaluronic acid networks interpenetrating with networks of polyethylene glycol or elastin-like polypeptide. In a second phase, highly extensible, tough networks will be prepared using nanogels as unconstrained, multifunctional crosslink junctions in synthetic polymer networks. It is expected that these new scaffold materials will have the right properties to provide the mechanical cues needed to enable cartilage cell growth into functional cartilage tissue.
BROADER IMPACTS: The project will include at least two undergraduate students each year, and appropriate funding has been included in the budget; one graduate student will be trained in the course of this project. The project will also provide shorter term laboratory projects for graduate students in at least one bioengineering course taught by one of the Co-PIs. At the K-12 level, the PIs have established participation over the past three years with the KU Project Discovery program by developing hands-on experiments for the participants that introduce them to aspects of tissue engineering. The Project Discovery program involves a week long summer camp for high school girls from diverse demographic groups.
Intellectual Merit: The extracellular matrix (ECM) of mammalian tissues is comprised of multiple high molecular weight compounds that are linked together at multiple length scales by multiple interactions. This complex structure leads to mechanical properties that are superior to synthetic materials of comparable water contents. In this project, water-swollen networks known as hydrogels were synthesized both from biopolymers taken from the mammalian extracellular matrix and from synthetic compounds and linked together in several ways to produce hydrogels of exceptional mechanical properties. Specifically, a chemically modified ECM polymer chondroitin sulfate was linked with low molecular weight compounds to improve the properties of these materials in several different ways. First, it was shown that the mechanical strength of the chondroitin sulfate networks could be significantly improved simply by copolymerization with small amounts of ethylene glycol derivatives. This method was adapted to the encapsulation of cells for the regeneration of tissues such as cartilage. Second, it was shown that by making a physically entangled network of chondroitin sulfate with common synthetic polymers such as polyacrylamide (to create what are known as "double networks") could dramatically increase the stresses required to chondroitin sulfate gels. Double network hydrogels produced by this method have the highest yield strength – the stress required to cause tearing – ever reported for hydrogels. This is shown in the accompanying figure which shows the stress required to stretch single network gels and double network gels to different strains (the ratio of the stretched length to the original length) until they break. A gel made of a single network of modified chondroitin sulfate (MCS) is shown as green triangles; it shows that the gel is brittle and breaks at a low stress after only about 20% elongation. In contrast, a single network gel of polyacrylamide (red squares), a commonly used gel in many technologies, is ductile and stretches to large deformations, but does not support a significant stress. However, when the MCS gel is interpenetrated with the polyacrylamide gel to form a double network (DN), a significant synergistic effect of the brittle-ductile polymer combination – the DN gel is far stiffer than either gel, and can be stretched much farther than the original MCS gel. The plateau observed in the DN is highly unusual for a hydrogel, and occurs at a stress higher than ever reported previously for a hydrogel. This is the point at which the gel is deforming irreversibly but without breaking, known as ‘yielding.’ This behavior mimics that of many biological tissues such as the ECM, which are highly resistant to fracture. The ECM is stabilized not only by cross-linking of biopolymers with low molecular weight materials and by forming entangled networks, but also by ionic interactions. A new type of ionically-complexed network was produced from a novel gel created in this work known as poly(N-vinyl formamide) (PNVF) combined with the common gel polyacrylamide (PAAm). When soaked in alkaline solution, the originally non-ionic PNVF gel is converted to a positively charged form and the PAAm is converted to a negatively charged form. The oppositely charged networks then bind together significantly increasing the mechanical strength of the networks as a result. Broader Impacts: This project has added to the technological base of the United States. It supported the degrees of 3 graduate students, ten undergraduates, and engaged multiple other students through collaboration. Elementary school teachers worked on these gels and carried this experience back to their classrooms. Numerous journal publications and presentations distributed these results to other experts. The results of this work have significant potential practical application. The double network gels are under investigation for use in cartilage regeneration, as the properties of the gels developed in this work approach that of native cartilage. These gels could potentially be used as a cartilage replacement or seeded with cells which could regenerate native cartilage while providing the physical support needed during the regeneration. Polyacrylamide is widely used in many different technological applications, but it is synthesized from highly toxic precursors. The PNVF gel developed in this work is a chemical analog of polyacrylamide but its precursors are much less toxic. This work demonstrated that PNVF gel may be substituted for PAAm gels without loss of mechanical performance.
