Self-healing materials: the patent landscape behind the materials of the future

Human skin can often heal without leaving a trace. However, if a car crashes or its paint is scratched, it requires an often costly repair. While nothing lasts forever, materials can degrade due to time, environmental factors or damage occurred during operation.

A solution to this issue is man-made materials which behave like a human body – that is, a material which can sense a problem, prevent it from getting worse and repair itself as quickly as possible. This is the concept on which self-healing materials are based.

Self-healing materials
Self-healing materials are now a reality, with such materials able to restore their structural integrity if damaged (eg, components used in mechanical processes, the scratches on automobile bodies and cracks in buildings, which can close on their own). There are two types of self-healing:            

• automatic, without interference; and
• non-automatic, which requires external stimulation (eg, heating).

Self-healing can be used for different types of material (eg, polymers) but is not limited to metals, alloys, ceramics and composites. The design strategies for self-healing materials include:

• the release of a healing agent;
• reversible cross-link;
• electrohydrodynamics;
• the shape-memory effect;
• nanoparticle migration; and
• co-deposition.

Patent landscape
Most of the related patents have been filed in China, followed by the United States, Japan and Korea. After 2013 patent filings in China increased significantly. In 2016 there was almost a 50% increase in filings in areas such as:

• construction;
• automotive and aerospace;
• healthcare;
• electronics and semiconductors;
• industrial manufacturing; and
• packaging.

The global self-healing materials market is expected to reach $4.1 billion by 2025, according to a new report by Grand View Research, Inc.

Major players in the field include:

• Saint-Gobain;
• IBM;
• Safran;
• Hitachi;
• Siemens; and
• SUMITOMO.

New entrants in the field include:

• Nanjing Forestry University;
• Jiangnan University;
• the Institute of Process Engineering, Chinese Academy of Sciences;
• Guangzhou Tianhe Rubber Hose Products; and
• Binzhou College.

Pre-2001
Before 2001 the inventions mainly focused on:

• treating slow-healing wounds;
• forming self-repairing flexible waterproof material;
• electrical cables with self-repairing protection;
• fluid self-healing seals;
• healing cracks in concrete to prevent leakage; and
• preventing oxidation.

Notable patents include the following:

• Self-repairing fibre reinforced matrix materials, including organic and inorganic matrices. This matrix composite material can repair microcracks and release corrosion inhibitors or permeability modifiers in concrete and polymer-based shaped articles.
• The device and manufacturing method for electric cables with self-recovery protection.

From 2001 to 2005
During this period, the filed patents refer to:

• self-repairing reinforced matrix materials;
• systems and methods of facilitating network diagnostics and self-healing;
• protection of parts made of composite materials with self-healing properties against oxidation; and
• the method for applying self-repairing metallic materials to high-power diesel engines.

Aside from the above, most of the patents disclose a self-repairing method for concrete, polymers, composite and other materials.

Notable patent literatures during this period include the following:

• The brazing process of repairing a part with a base material with an oriented microstructure – Siemens owns the rights for this patent for turbine components.
• A self-restorable material which comprises a resin with a hydrazone bond. The material has no limitation regarding restorability and can repair itself even in regular-pressure room-temperature air, without the need of a specific external factor such as heating – Mitsui Chemicals and the University of Strasbourg own the rights to this patent.
• A self-healing process for vehicular brake gear. This is predominantly used for motor vehicles with a friction lining and brake disc, in which the friction lining forms a reservoir for a protective material (preferably an additive) which is transferred to the brake disc when the lining and disc are in contact – in 2004 AUDI AG filed an application for this patent in Europe. 

From 2006 to 2010
There was almost a 55% increase in patent filing between 2006 and 2010 compared to the previous period. Most patent applications were filed to mitigate the technical challenges posed by self-healing materials, including improvements to:

• the anti-leakage properties of healing or solidifying material;
• the strength of the bond between a healing agent and material matrix;
• the healing speed;
• the manufacturing method of self-healing polymers, composites, metal, alloys and other materials;
• the method of encapsulation; and
• the stability of healing materials.

Post-2010
After 2010 most patents were filed for solutions to the previous problems, which include the following:

• Plastics are used widely in various industrial fields, but are not as hard as glass and metals; this means that they can easily suffer surface damage (eg, scratches).
• It is difficult to repair undetectable cracks. When cracks are not restored quickly and effectively, it leads to macro-crack formation which triggers brittle fractures and shortens the life of concrete, consequently threatening the safety of a structure. Moreover, inspection and maintenance are difficult and expensive, with labour costing up to 50% of the repair cost.
• Polymeric materials are used for, among other things, paint, upholstery, pipes and circuit boards. They can undergo degradation due to a number of factors, including heat, chemicals and mechanical forces.
• Conventional carbon fibre composite materials crack easily, the mechanical performance of the material is reduced and the material is difficult to repair.

Comment
Polymers and polymeric composite materials are widely disclosed in the patent filings in the field of self-healing materials, as they are cost effective and readily available. Significant advances have been made in recent years, but the field is still in its infancy. Many concerns – including environmental degradation, oxidation and the effects of temperature – are complex and must be analysed from different areas such as material science and electrochemistry. Accordingly, a large number of educational and industrial organisations have come forward to facilitate new advancements, and therefore patent filings, in this promising field.