Self-healing biomaterials

Abstract

The goal of this review is to introduce the biomaterials community to the emerging field of self-healing materials, and also to suggest how one could utilize and modify self-healing approaches to develop new classes of biomaterials. A brief discussion of the in vivo mechanical loading and resultant failures experienced by biomedical implants is followed by presentation of the self-healing methods for combating mechanical failure. If conventional composite materials that retard failure may be considered zeroth generation self-healing materials, then taxonomically speaking, first generation self-healing materials describe approaches that "halt" and "fill" damage, whereas second generation self-healing materials strive to "fully restore" the prefailed material structure. In spite of limited commercial use to date, primarily because the technical details have not been suitably optimized, it is likely from a practical standpoint that first generation approaches will be the first to be employed commercially, whereas second generation approaches may take longer to implement. For self-healing biomaterials the optimization of technical considerations is further compounded by the additional constraints of toxicity and biocompatibility, necessitating inclusion of separate discussions of design criteria for self-healing biomaterials.

Figure 1

First generation self-healing mechanism. The undamaged matrix is shown in (a). Embedded catalyst is exposed as microcracks are generated (b). Microcapsules containing a healing agent are fractured by the microcracks, causing the release of healing agent and its subsequent reaction with the exposed catalyst (c). Following this polymerization, propagation of the microcrack is inhibited (d). Adapted from various works by M. Kessler, S. White, N. Sottos, et al., , , , [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

Figure 2

Potential design for a self-healing bone cement utilizing the first generation self-healing approach. The catalyst and encapsulated healing agent to be embedded are packaged with the PMMA powder component of the bone cement. Mixture of the two bone cement components. Mixture of the two bone cement components will disperse the solid catalyst and encapsulated healing agent within the bone cement. The cement can then be applied to the implant following current protocols.

Figure 3

Autonomous second generation materials. Following the application of a force, the reversible crosslinks dissociate while the covalent crosslinks remain intact. When the force is removed, the material returns to its original structure.

Figure 4

Second generation materials requiring energy input to achieve healing. These materials are capable of restoring the original material structure following the application of an external force such as heat or light. As depicted, a polymer matrix is damaged and following the application of heat, the polymer chains are able to flow and restore matrix integrity. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

Figure 5

Second generation materials based on DNA crosslinks. The toehold region of the crosslinker DNA strand can be used to eliminate the crosslinks. Addition of a DNA strand complementary to this toehold region results in competitive binding between the removal strand and the crosslinker DNA strand without requiring the application of further external forces. Adapted from Lin et al.[Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]