Self-healing materials with microvascular networks

Self-healing materials

Self-healing materials are able to partially or completely heal damage inflicted on them, e.g., crack formation; it is anticipated that the original functionality can be restored. This article covers the design and generic principles of self-healing materials through a wide range of different material classes including metals, ceramics, concrete, and polymers. Recent key developments and future challenges in the field of self-healing materials are summarised, and generic, fundamental material-independent principles and mechanism are discussed and evaluated.

Self-healing biomaterials

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.

Light-induced olefin metathesis

Light activation is a most desirable property for catalysis control. Among the many catalytic processes that may be activated by light, olefin metathesis stands out as both academically motivating and practically useful. Starting from early tungsten heterogeneous photoinitiated metathesis, up to modern ruthenium methods based on complex photoisomerisation or indirect photoactivation, this survey of the relevant literature summarises past and present developments in the use of light to expedite olefin ring-closing, ring-opening polymerisation and cross-metathesis reactions.

Healable polymeric materials: a tutorial review

Given the extensive use of polymers in the modern age with applications ranging from aerospace components to microcircuitry, the ability to regain the mechanical and physical characteristics of complex pristine materials after damage is an attractive proposition. This tutorial review focusses upon the key chemical concepts that have been successfully utilised in the design of healable polymeric materials.

Characterization and analysis of carbon fibre-reinforced polymer composite laminates with embedded circular vasculature

A study of the influence of embedded circular hollow vascules on structural performance of a fibre-reinforced polymer (FRP) composite laminate is presented. Incorporating such vascules will lead to multi-functional composites by bestowing functions such as self-healing and active thermal management. However, the presence of off-axis vascules leads to localized disruption to the fibre architecture, i.e. resin-rich pockets, which are regarded as internal defects and may cause stress concentrations within the structure. Engineering approaches for creating these simple vascule geometries in conventional FRP laminates are proposed and demonstrated. This study includes development of a manufacturing method for forming vascules, microscopic characterization of their effect on the laminate, finite element (FE) analysis of crack initiation and failure under load, and validation of the FE results via mechanical testing observed using high-speed photography. The failure behaviour predicted by FE modelling is in good agreement with experimental results. The reduction in compressive strength owing to the embedding of circular vascules ranges from 13 to 70 per cent, which correlates with vascule dimension.

Bioinspired engineering study of Plantae vascules for self-healing composite structures

This paper presents the first conceptual study into creating a Plantae-inspired vascular network within a fibre-reinforced polymer composite laminate, which provides an ongoing self-healing functionality without incurring a mass penalty. Through the application of a 'lost-wax' technique, orthogonal hollow vascules, inspired by the 'ray cell' structures found in ring porous hardwoods, were successfully introduced within a carbon fibre-reinforced epoxy polymer composite laminate. The influence on fibre architecture and mechanical behaviour of single vascules (located on the laminate centreline) when aligned parallel and transverse to the local host ply was characterized experimentally using a compression-after-impact test methodology. Ultrasonic C-scanning and high-resolution micro-CT X-ray was undertaken to identify the influence of and interaction between the internal vasculature and impact damage. The results clearly show that damage morphology is influenced by vascule orientation and that a 10 J low-velocity impact damage event is sufficient to breach the vasculature; a prerequisite for any subsequent self-healing function. The residual compressive strength after a 10 J impact was found to be dependent upon vascule orientation. In general, residual compressive strength decreased to 70 per cent of undamaged strength when vasculature was aligned parallel to the local host ply and a value of 63 per cent when aligned transverse. This bioinspired engineering study has illustrated the potential that a vasculature concept has to offer in terms of providing a self-healing function with minimum mass penalty, without initiating premature failure within a composite structure.

Self-healing at the nanoscale

The design of self-healing materials is a very important but challenging topic in nanotechnology. Self-healing strategies, also inspired by natural processes, allow the fabrication of auto-repairing systems, and in recent years, materials engineering at the nanoscale has allowed further advances in this emerging field. In this mini review, we recall some interesting self-healing systems found in natural processes and others created by man-made activity with special emphasis on the role played in this field by nanostructures. Finally, the self-healing of gold nanoparticles during laser irradiation is considered in more detail since it is a rare example of a functional nanomaterial with self-repairing properties.

Carbon nanotube reservoirs for self-healing materials

A novel nanoreservoir made of carbon nanotubes (CNTs) is proposed for realizing tougher and automated self-healing materials. The advantages of the approach are that CNTs have the potential to play the role of reinforcing elements prior to and after sealing a crack and that the number of voids is reduced after the material and the CNTs themselves are healed. The focus of this paper is on investigating the feasibility of using CNTs as a nanoreservoir by analyzing the dynamics of a fluid flowing out of a ruptured single-walled CNT (SWNT), where the fluid resembles an organic healing agent. With this in mind the escaping mechanism of organic molecules stored inside a cracked SWNT was investigated through a molecular dynamics study. The study shows that, when a SWNT wall suffers the formation of a crack, a certain amount of organic molecules, stored inside the SWNT, escape into space in a few picoseconds. This phenomenon is found to depend on the temperature and on the size of the cracks. The results of this study indicate that CNTs have the potential to be successfully used to realize the next generation of stronger, lighter and self-healing materials.

Mechanical behavior of highly cross-linked polymer networks and its links to microscopic structure

Highly cross-linked polymer (HCP) networks are becoming increasingly important as high-performance adhesives and multifunctional composite materials. Because of their cross-linked molecular architectures, HCPs can be strong but brittle. One key goal in improving the performance of an HCP is to increase toughness without sacrificing strength. Using large scale molecular-dynamics simulation, we compare and characterize the mechanical behavior of two model HCPs under tensile deformation. In the first case, bond angles among any three connected monomers are unconstrained and in the second case we impose harmonic tetrahedral bond angle constraints. We perform a detailed microstructural analysis that establishes a unique correlation between macroscopic mechanical behavior and the microscopic structure of an HCP. While, in the unconstrained system, strain-hardening behavior is observed that is attributed to the formation of microvoids, the void growth is completely arrested in the constrained system and no strain hardening is observed. Moreover, after the initial strain-hardening phase, the unconstrained system displays the same stress-strain behavior as that of a constrained network. Strain hardening makes the unconstrained system ductile while it retains the same tensile strength as the constrained system. We suggest that bond angle flexibility of cross-linkers might be a possible means to control ductility in an HCP network at a constant cross-linker density. We have also studied the effect of temperature, strain rate, and intermonomer nonbonded interaction strength on the stress-strain behavior. Interestingly at a strong intermonomer nonbonded interaction strength, no strain hardening is observed even in the unconstrained system and fracture sets in at around 1% strain, similar to what is observed in an experimental system such as epoxy and vinyl-ester based thermosets. This indicates that strong nonbonded interactions play a key role in making an HCP strong but brittle.

Anomalous ductility in thermoset/thermoplastic polymer alloys

Mechanical properties of highly cross-linked polymer (HCP) networks, e.g., thermosets, can be significantly modified by adding linear polymer chains, e.g., thermoplastics. In this work, we study thermoset/thermoplastic polymer alloys by means of large scale molecular dynamics simulations (MD) of a coarse-grained model. We focus here on the effect of the linear chain fraction, Gammal, on the mechanical properties of HCP network for a fixed chain length. Our MD simulations show that the ductility (measured by the strain-to-fracture) of an alloy decreases with increasing Gammal up to a threshold fraction, Gammal*, beyond which it increases with Gammal. We find that for Gammal<Gammal* the fracture is predominantly cohesive, while for Gammal>Gammal* adhesive failure occurs. We suggest that the possible origin of this unexpected non-monotonic behavior is due to a competition between (a) growth of microvoids which stores mechanical energy and is compromised as Gammal increases, and (b) reduction of cross-linker density with increasing Gammal.

A thermoplastic/thermoset blend exhibiting thermal mending and reversible adhesion

In this paper, we report on the development of a new and broadly applicable strategy to produce thermally mendable polymeric materials, demonstrated with an epoxy/poly(-caprolactone) (PCL) phase-separated blend. The initially miscible blend composed of 15.5 wt % PCL undergoes polymerization-induced phase separation during cross-linking of the epoxy, yielding a "bricks and mortar" morphology wherein the epoxy phase exists as interconnected spheres (bricks) interpenetrated with a percolating PCL matrix (mortar). The fully cured material is stiff, strong, and durable. A heating-induced "bleeding" behavior was witnessed in the form of spontaneous wetting of all free surfaces by the molten PCL phase, and this bleeding is capable of repairing damage by crack-wicking and subsequent recrystallization with only minor concomitant softening during that process. The observed bleeding is attributed to volumetric thermal expansion of PCL above its melting point in excess of epoxy brick expansion, which we term differential expansive bleeding (DEB). In controlled thermal-mending experiments, heating of a cracked specimen led to PCL extrusion from the bulk to yield a liquid layer bridging the crack gap. Upon cooling, a "scar" composed of PCL crystals formed at the site of the crack, restoring a significant portion of the mechanical strength. When a moderate force was applied to assist crack closure, thermal-mending efficiencies exceeded 100%. We further observed that the DEB phenomenon enables strong and facile adhesion of the same material to itself and to a variety of materials, without any requirement for macroscopic softening or flow.

Microvoid formation and strain hardening in highly cross-linked polymer networks

Using molecular dynamics simulations of a generic model, we observe strain hardening in highly cross-linked polymer glasses under tensile deformation. We show that formation of microvoids, without bond breaking, constitutes the microscopic origins of strain hardening. A well-defined functional form is observed for the void size distribution that is consistent with voids in dense equilibrium Lennard-Jones particle packings, independent of strain. Microvoid-based strain hardening is not observed in a separate model with tetrahedral bond angle constraints, indicating that flexible cross-linkers are the key factor in the development of strain hardening behavior.

Electrospinning of three-dimensional nanofibrous tubes with controllable architectures

This paper reports a novel static method to fabricate three-dimensional (3D) fibrous tubes composed of ultrafine electrospun fibers. By using this unique technique, micro and macro single tubes with multiple micropatterns, multiple interconnected tubes, and many tubes with the same or different sizes, shapes, structures, and patterns can be prepared synchronously. Parameters that could influence the order degree of patterned architectures have also been investigated. It is expected that electrospun tubes with controllable patterned architectures and 3D configurations may be attractive in many biomedical and industrial applications.

Biomimetic reliability strategies for self-healing vascular networks in engineering materials

Self-healing via a vascular network is an active research topic, with several recent publications reporting the application and optimization of these systems. This work represents the first consideration of the probable failure modes of a self-healing system as a driver for network design. The critical failure modes of a proposed self-healing system based on a vascular network were identified via a failure modes, effects and criticality analysis and compared to those of the human circulatory system. A range of engineering and biomimetic design concepts to address these critical failure modes is suggested with minimum system mass the overall design driver for high-performance systems. Plant vasculature has been mimicked to propose a segregated network to address the risk of fluid leakage. This approach could allow a network to be segregated into six separate paths with a system mass penalty of only approximately 25%. Fluid flow interconnections that mimic the anastomoses of animal vasculatures can be used within a segregated network to balance the risk of failure by leakage and blockage. These biomimetic approaches define a design space that considers the existing published literature in the context of system reliability.

A new self-healing epoxy with tungsten (VI) chloride catalyst

Using self-healing materials in commercial applications requires healing chemistry that is cost-effective, widely available and tolerant of moderate temperature excursions. We investigate the use of tungsten (VI) chloride as a catalyst precursor for the ring-opening metathesis polymerization of exo-dicyclopentadiene (exo-DCPD) in self-healing applications as a means to achieve these goals. The environmental stability of WCl6 using three different delivery methods was evaluated and the associated healing performance was assessed following fracture toughness recovery protocols. Both as-received and recrystallized forms of the WCl6 resulted in nearly complete fracture recovery in self-activated tests, where healing agent is manually injected into the crack plane, at 12wt% WCl6 loading. In situ healing using 15wt% microcapsules of the exo-DCPD produced healing efficiencies of approximately 20%.

Self-healing materials: a review

The ability of materials to self-heal from mechanical and thermally induced damage is explored in this paper and has significance in the field of fracture and fatigue. The history and evolution of several self-repair systems is examined including nano-beam healing elements, passive self-healing, autonomic self-healing and ballistic self-repair. Self-healing mechanisms utilized in the design of these unusual materials draw much information from the related field of polymer-polymer interfaces and crack healing. The relationship of material damage to material healing is examined in a manner to provide an understanding of the kinetics and damage reversal processes necessary to impart self-healing characteristics. In self-healing systems, there are transitions from hard-to-soft matter in ballistic impact and solvent bonding and conversely, soft-to-hard matter transitions in high rate yielding materials and shear-thickening fluids. These transitions are examined in terms of a new theory of the glass transition and yielding, viz., the twinkling fractal theory of the hard-to-soft matter transition. Success in the design of self-healing materials has important consequences for material safety, product performance and enhanced fatigue lifetime.