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Smart inorganic polymers

Smart inorganic polymers (SIPs) are hybrid or fully inorganic polymers with tunable (smart) properties such as stimuli responsive physical properties (shape, conductivity, rheology, bioactivity, self-repair, etc.). While organic polymers are often petrol-based, the backbones of SIPs are made from elements other than carbon which can lessen the burden on scarce non-renewable resources and provide more sustainable alternatives. Common inorganic backbones utilized in SIPs include polysiloxanes, polyphosphates, and polyphosphazenes, to name a few.

Discoveries in the past decades have revealed the potential of inorganic polymers for broad applicability in diverse fields spanning from drug delivery and tissue regeneration to coatings and electronics.^[1][2][3] In general, Inorganic polymers can provide greater consumer safety owing to improved properties and environmental compatibility (no need for plasticizers, intrinsically flame-retardant properties), and SIPs additionally provide unique technological applicability such as use as solid polymer electrolytes for consumer electronics based on polymers with a low glass-transition temperature, as molecular electronics with non-metal elements in lieu of metal-based conductors and electronic circuits, as electrochromic materials, and as self-healing coatings.

Role of COST action CM1302

COST action 1302 is a European Community "Cooperation in Science and Technology" research network initiative with a budget of over 100,000 euros per year that supported 62 scientific missions in the area of smart inorganic polymers resulting in 70 publications between 2014 and 2018 with the mission of establishing an intellectual framework with which to rationally design new smart inorganic polymers. Coordinated by Evamarie Hey-Hawkins of Leipzig University, it has played a central role in the advancement of the fledgling field of smart inorganic polymers in the 2010's. The culmination of this work can be further explored in her 2019 book, Smart Inorganic Polymers: Synthesis, Properties, and Emerging Applications in Materials and Life Sciences.

Polysiloxane smart polymers

A generic polysiloxane

Polysiloxane, commonly known as silicone, is the most commonly commercially available inorganic polymer. The large body of existing work on polysiloxane has made it a readily available platform for functionalization to create smart polymers, with a variety of approaches reported which generally center around the addition of metal oxides to a commercially available polysiloxane or the inclusion of functional side-chains on the polysiloxane backbone. The applications of smart polysiloxanes vary greatly, ranging from drug delivery, to smart coatings, to electrochromics.

Smart polysiloxane for drug delivery

Synthesis of smart stimuli responsive polysiloxanes through the addition of a polysiloxane amine to an α,β-unsaturated carbonyl via aza-Michael addition to create a polysiloxane with N-isopropyl amide side-chains has been reported. This polysiloxane was shown to be able to load ibuprofen (a hydrophobic NSAID) and then release it in response to changes in temperature, showing it to be a promising candidate for smart drug delivery of hydrophobic drugs. This action was attributed to the polymer's ability to retain the ibuprofen above the lower critical solution temperature (LCST), and conversely, to dissolve below the LCST, thus releasing the loaded ibuprofen at a given, known temperature.

Smart polysiloxane coatings

Commercial polysiloxane coatings are readily commercially available and capable of protecting surfaces from damaging pollutants, but the addition of TiO₂ gives them the smart ability to degrade pollutants stuck to their surface in the presence of sunlight. This particular phenomena is promising in the field of monument preservation. Similar hybrid textile coatings made of amino-functionalized polysiloxane with TiO₂ and silver nanoparticles have been reported to have smart stain-repellent yet hydrophilic properties, making them unique in comparison to typical hydrophobic stain-repellant coatings. Smart properties have also been reported for polysiloxane coatings without metal oxides, namely, a polysiloxane/polyethylenimine coating designed to protect magnesium from corrosion that was found to be capable of self-healing small scratches.

Smart poly-(ε-caprolactone)/siloxane

Poly-(ε-caprolactone)/siloxane is an inorganic-organic hybrid material which, when used as a solid electrolyte matrix with a lithium perchlorate electrolyte, paired to a W₂O₃ film, responds to a change in electrical potential by changing transparency. This makes it a potentially useful electrochromic smart glass.

Phosphorus smart polymers

There exist a sizable number of phosphorus polymers with backbones ranging from primarily phosphorus to primarily organic with phosphorus subunits. Some of these have been shown to possess smart properties, and are largely of-interest due to the biocompatibility of phosphorus for biological applications like drug delivery, tissue engineering, and tissue repair.

Smart polyphosphate

Polyphosphate (PolyP) is an inorganic polymer made from phosphate subunits. It typically exists in its deprotonated form, and can form salts with physiological metal cations like Ca²⁺, Sr²⁺, and Mg²⁺. When salted to these metals, it can selectively induce bone regeneration (Ca-PolyP), bone hardening (Sr-PolyP), or cartilage regeneration (Mg-PolyP) depending on the metal. This smart ability to attenuate the kind of tissue regenerated in response to different metal cations makes it a promising polymer for biomedical applications.

Smart polyphosphazenes

A generic polyphosphazene

Polyphosphazene is an inorganic polymer with a backbone consisting of phosphorus and nitrogen, which can also form inorganic-organic hybrid polymers with the addition of organic substituents. Some polyphosphazenes have been designed through the addition of amino acid ester side chains such that their LCST is near body temperature and thus they can form a gel in situ upon injection into a person, making them potentially useful for drug delivery. They biodegrade into a near-neutral pH mixture of phosphates and ammonia that has been shown to be non-toxic, and the rate of their biodegradation can be tuned with the addition of different substituents from full decomposition within days with glyceryl derivatives, to biostable with fluoroalkoxy substituents.

Smart poly-ProDOT-Me₂

Poly-ProDOT-Me₂ is a phosphorus-based inorganic-organic hybrid polymer, which, when paired to a V₂O₅ film, provides a material that changes color upon application of an electrical current. This 'smart glass' is capable of reducing light transmission from 57% to 28% in under 1 second, a much faster transformation than that of commercially available photochromic lenses.

Metalloid smart polymers

Smart polystannanes

A generic polystannane

Polystannane, a unique polymer class with a tin backbone, is the only known polymer to possess a completely organometallic backbone. It is especially unique in the way that the conductive tin backbone is surrounded by organic substituents, making it act as an atomic-scale insulated wire. Some polystannanes such as (SnBu₂)_n and (SnOct₂)_n have shown the smart ability to align themselves with external stimuli, which could see them become useful for pico electronics. However, polystannane is very unstable to light, so any such advancement would require a method for stabilizing it against light degradation.

Smart icosahedral boron polymers

Icosahedral boron is a geometrically unusual allotrope of boron, which can be either added as side chains to a polymer or co-polymerized into the backbone. Icosahedral boron side chains on polypyrrole have been shown to allow the polypyrrole to self-repair when overoxidized because the icosahedral boron acts as a doping agent, enabling overoxidation to be reversed.

See Also

• Coatings
• Coordination Polymers
• Drug Delivery
• Electrochromism
• Inorganic Polymers
• Smart Materials