Smart inorganic polymer

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.[6] 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.[6] 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.

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.[7] 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.[8] 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.[9]

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.[10] This makes it a potentially useful electrochromic smart glass.

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⁺.[11] 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 to which it is salted.[11] This smart ability to attenuate the kind of tissue regenerated in response to different metal cations makes it a promising polymer for biomedical applications.

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.[12] 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.[12]

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.[13]

A generic polystannane

Polystannane, a unique polymer class with a tin backbone, is the only known polymer to possess a completely organometallic backbone.[15] 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.[16] However, polystannane is very unstable to light, so any such advancement would require a method for stabilizing it against light degradation.[16]

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.[17]

Polyferrocenylsilanes are a group of common organosilicon metallopolymer with backbones consisting of silicon and ferrocene.[14] Variants of polyferroceylsilanes have been found to exhibit smart self-assembly in response to oxidation and subsequent smart self-disassembly upon reduction, as well as variants which can respond to electrochemical stimulation.[14] One such example is a thin film of a polystyrene-polyferrocenylsilane inorganic-organic hybrid copolymer that was found to be able to adsorb and release ferritin with the application of an electrical potential.[18]

A number of ferrocene-organic inorganic-organic hybrid polymers have been reported to have smart properties that make them useful for application in biosensing.[19] Multiple polymers with ferrocene side-chains cross-linked with glucose oxidase have shown oxidation activity which results in electrical potential in the presence of glucose, making them useful as glucose biosensors.[20] This sort of activity is not limited to glucose, as other enzymes can be crosslinked to allow for sensing of their corresponding molecules, like a poly(vinylferrocene)/carboxylated multiwall carbon nanotube/gelatin composite that was bound to uricase, giving it the ability to act as a biosensor for uric acid.[21]