Next-generation self-healing polymers
The next generation of polymers that repeatedly self heal are on their way, according to scientists at the University of Illinois at Urbana-Champaign, USA.
The research builds on the group’s self-healing polymer created in 2001, where a microencapsulated healing agent and catalyst are distributed throughout the epoxy matrix. When the material cracks, the capsule ruptures, and the Grubbs ruthenium catalyst polymerises the dicyclopentadiene (DCPD) liquid monomer to bond the crack autonomically.
‘A significant drawback [however] is that once you break the capsules open, there is no more healing agent if the materials are damaged in that same region again,’ explains Nancy Sottos, Professor of Engineering at the University of Illinois. ‘Many engineering materials incur damage in the same high stress region more than once, eventually leading to failure.’
By introducing a 3D microvascular network to the ductile epoxy substrate, emulating biological circulatory systems, researchers have attempted to create a renewable delivery mechanism for the DCPD monomer to extend the life of materials. The structure is fabricated using a direct-write assembly technique developed by Professor of Aerospace Engineering Scott White, and Professor of Materials Science and Engineering Jennifer Lewis.
The process involves depositing a polymer ink in a continuous filament to build a 3D scaffold layer by layer, followed by the infusion of epoxy resin. Once the resin is cured, the ink is liquified and removed under a light vacuum by heating. A brittle aerospace grade epoxy coating is then applied to the substrate.
‘When a crack forms in the coating, it is drawn to the vascular channels in the underlying substrate,’ says Sottos. ‘The healing agent wicks into the crack via capillary forces [where it interacts with catalyst particles]. With each healing cycle, new agent is delivered. If all the agent is used up, it can be refilled either from an internal or external reservoir.’
Vertical channels deliver DCPD to cracks in the coating, while horizontal channels enable the network to be refilled.
At present, the healing process lasts for seven cycles. Kathleen Toohey, a postgraduate student who worked on the project, explains, ‘While we can pump more healing agent into the network, “scar tissue” builds up in the coating and prevents the agent from reaching the catalyst.’
This could be overcome by a dual network in the substrate for a two-part healing chemistry that does not involve a catalyst.
Sottos adds, ‘The healing agent (an epoxide) and polymeriser (curing agent) are delivered to the crack by independent networks. Mixing could be improved through more bio-inspired vascular design and manufacturing.’
The new system has so far undergone 20 cycles of healing.
The team is also looking to extend the design beyond cracks in the coating to repairing lacerations in the substrate. The technique could benefit structural and mechanical engineering. Moreover, alongside healing, polymer composites with microvascular networks could eventually replicate other biological systems such as thermal regulation and chemical sensing through delivery and circulation of active species.
Scientists believe the research could have implications in biomaterials science to promote tissue growth and create fully regenerative materials.