Attachment of fibrous materials in nature and surgical repair
Mother nature is the ultimate problem solver. Often by brute force and necessity for survival, the organism that has the best solution to a problem is the one to proliferate. One of these problems for which nature provides an elegant solution is the attachment of dissimilar materials. This dissertation explores the strengthening and relaxation of interactions between soft and hard materials, typically with one of the two being fibrous in character, with the goal of identifying strategies for improving the surgical reattachment of tendon to bone. The work begins with a study of the plant Harpagonella palmeri, which the dissertation reveals to use a discrete, graded attachment to spread their fruit more successfully via “hitchhiking” onto the fur or socks of passing animals. Using judiciously and biomimetically chosen sutures and suture spacing may halve the peak stresses that arise at a sutured reattachment of tendon to bone. The dissertation continues with the analysis of python jaws, which present arrays of hooked teeth to hold onto the soft, fibrous tissue of prey. Additional modeling revealed that python teeth reduce peak stresses associated with grasping, relative to other teeth like those in sharks, and motivated development of a new device to reattach tendon to bone. The dissertation additionally presents optimization of the size and spacing of such teeth for application in surgical repair. A key feature of the attachment of both hitchhiker plant fruits and python teeth to fibrous tissues is that at least one of the tissues may show viscoelasticity over timescales relevant to attachment. To explore the timescales of viscoelastic relaxation in a model plant cell wall, a system was developed for relaxation testing of Arabidopsis thaliana hypocotyls held under isometric mechanical stress. These tests revealed a previously unreported active response, whereby live hypocotyls actively relaxed for 2000 s, and thereafter relaxed passively with behavior similar to that of dead (frozen and thawed) hypocotyls. Testing of mutant A. thaliana hypocotyls (CLSC4 and GFP-SAUR19) and testing in media of controlled pH revealed that the fibrous character of the cell wall and its mediation by pH and auxin signaling are key factors in these responses. Analysis of a sutured connection to a viscoelastic substratum then revealed that the effects of viscoelastic relaxation can be harnessed to improve soft/hard reattachment via discrete connections such as sutures or H. palmeri grappling hooks. Taken together, the results provide a comprehensive platform for modeling and designing interfaces between hard and soft fibrous materials.