Nanosponges

Nanosponges are a type of nanoparticle, often a synthesized carbon-containing polymer. They are porous in structure and can therefore be targeted to absorb small amounts of matter or toxin. Nanosponges are often used in medicine as targeted drug delivery systems or as a way of damage control after a injury. Their small size allows them to move quickly through the body, efficiently finding and attacking toxins. They can also be used in environmental applications to clean up ecosystems by performing tasks like purifying water or metal deposits. Nanosponges can be found in nature but are more often synthetically manufactured. Nanosponges are superior to microsponges in application as the smaller size allows less disruption into the system in which it is implemented therefore imposing less risk of failed or detrimental application.

History

Nanosponges were first referred to as “cyclodextrin nanosponges” by DeQuan Li and Min Ma in 1998. This term was used because there is a cross-linked β-cyclodextrin with organic diisocyanates. An insoluble network is present in this structure, which shows a high inclusion constant. These polymers are formed through the reaction of native cyclodextrins with a cross-linking agent, the latter influencing the behavior and properties of the entire unit.

Cyclodextrin nanosponges were not discovered to have potential in being drug carriers until work done by Trotta and colleagues. They performed syntheses of new kinds of cyclodextrin nanosponges that revealed many potential applications that had not been previously considered.

Mechanisms

Molecular cyclodextrin nanosponge structure

Structure

Cyclodextrins are a class of cyclic glucopyranose oligomers, with common structures of α, β, and γ. α-cyclodextrins comprise six glucopyranose units, β- cyclodextrins comprise seven, and γ comprise eight. Cyclodextrins are biological nanomaterials whose molecular structure greatly influences their supramolecular properties. To synthesize cyclodextrins, enzymatic action occurs on hydrolyzed starch.

Cyclodextrin nanosponges are made of a three dimensional cross-linked polymer network. They can be made with α, β, and γ cyclodextrins. The inclusion capacity and the solubilizing capacity of the nanosponges can be tuned according to how much of the cross-linking agent is used.

Functions

Cyclodextrins have a toroidal shape, which allows them to have a cavity inside which can fit other molecules. This useful structure allows them to act as drug carriers in the body, as long as the compounds to be delivered have compatible geometry and polarity with the cavity. To determine when these compounds are delivered, the structure of the cyclodextrin nanosponge can be modified to release its contents sooner or later. Several ligands can be conjugated on the surface of the nanosponge to determine where it will target in the body.

Environmental Application

Purification of Wastewater

Some nanosponges are made to be eco-friendly and have a high concentration of carboxyl groups. They are used to remove metal deposits in wastewater in the oceans, where organisms can absorb these deposits which leads to detrimental build up in their tissue. The concentration of heavy metals grows while going up the food chain as organisms eat other organisms. Being at the top of the food pyramid, humans are most at risk for the detrimental effects of these metals in our food. These effects include allergic reactions, insomnia, vision problems, and can be as extreme as to cause mental disability, dementia, and kidney disease. Unlike many organic pollutants, heavy metals can be removed and destroyed by using nanomaterials like nanosponges. These nanomaterials act as sustainable filtering materials by binding to metals and removing them from wastewater before they disperse into the ecosystem. Using nanosponges for this results in a higher efficiency and lower cost than alternative cleaning methods like ion exchange resins, activated carbon, or other biological agents. Porous materials produced from renewable and low cost sources, like cellulose, chitin, or starch, are one of the most promising classes of absorbents in terms of effectiveness.

Cyclodextrins (CDs) and amylose are derived from starches and are well known for their peculiar structural features and complex properties. The internal cavities in these CDs serve as sites for hydrophobic or very weakly hydrophilic molecules. In order to properly bind metal to these CDs, dextrins must be chemically changed by adding an acidic functional group. These functional groups are allowed to undergo deprotonation in an aqueous media so the reaction of these with the hydroxyl groups in dextrin allows negatively charged insoluble polymers to be created . These polymers are known as nanosponges for their porous characteristic; they are able to bond to both organic molecules and metal deposits. After cleaning, these nanosponges can easily be separated from the water through simple filtration since they are insoluble in all solvents.

One type of nanosponge being researched is prepared with β-cyclodextrins and a linear pea starch derivative called linecaps. β-cyclodextrins are used due to low cost and medium sized pores allowing for a broad range of guest molecules to be collected. Additionally, β-cyclodextrins are favored over dextrin polymers, as they can interact with transition metals also. Primary and secondary hydroxyl groups can act as coordination sites with some metal ions, and CDs can coordinate more than one ion at a time. These two components are reacted with citric acid in water to create the nanosponges using sodium hypophosphate monohydrate as a catalyst for the reaction. These nanosponges were compared to the performance of nanosponges synthesized in the same manner substituting PDMA (pyromellitate substance) for citric acid.

A high number of cross-links were introduced into the synthesis process in order to create a maximum amount of carboxyl groups. This allowed for a higher complexation ability of these nanosponges to other molecules. A high degree of cross-linking generally leads to low swellable polymers which are more suited for water treatment, as the water will not take up the space meant for metal waste and can more easily be filtered from water after cleaning. Higher contact time leads to higher efficiency of the cleaning of nanosponges in wastewater.

It was found, at high metal concentrations, that pyromellitate was able to absorb more metal deposits. At low concentrations, they both performed nearly identical. However, in the presence of interfering sea water, citrate nanosponges were able to selectively absorb more metal than the PDMA nanosponges, allowing them to be more effective in the cleanup of metal from salt water. Although the research of these citric acid nanosponges is still undergoing revision and development, they show promise for being a sustainable way to clean metal deposits from the ecosystem.

Medical Applications

Drug delivery

Nanosponges are being researched to be used for drug delivery systems to treat cancer and infectious diseases. Although nanosponges are one three-thousandth the size of red blood cells, they each can carry thousands of drug molecules. They can hide in the immune system where immune cells try to destroy and remove foreign material from the body. Particles coated with membranes from circulating red blood cells cannot be detected. Additionally, particles coated with membranes from circulating white blood cells or leukocytes avoid attack from macrophages.

Major concerns regarding recently developed chemical entities include pharmacokinetic issues, poor solubility in water, and low bioavailability. These lead to obstacles when using conventional drug dosage forms. Nanosponges can conquer these problems as their porous structure allows them the unique capability to entrap both hydrophilic and hydrophobic drugs and release them in a highly predictable manor. These small sponges travel throughout the body until they reach the targeted site where they bind to the surface and perform controlled drug release. Nanosponge technology is widely explored for its use in drug delivery using oral, parenteral, and topical administration techniques.This may include substances such as antineoplastic agents, proteins and peptides, volatile oils, and genetic materials. These small sponges travel throughout the body until they reach the targeted site where they bind to the surface and perform controlled drug release. Potential applications in target site-specific drug delivery include the lungs, spleen, and liver.

Fight Antibiotic Resistance

Membrane-coated nanosponges could be used to fight antibiotic resistance because they trap and remove toxins from blood. Toxins that attack red blood cells will cling to nanosponges because the sponges are coated with living cells. The sponges absorb the toxins, so they can no longer harm the cells, and the toxins are taken to the liver and broken down.

Detoxification

A study was conducted to determine nanosponges' ability to absorb pore-forming toxins. Pore-forming toxins (PFTs) are the most common protein toxins found in nature. They disrupt cells by forming pores in cellular membranes which alter the permeability of the cells. Examples of this include bacterial infections and venom. The idea behind this study was that by limiting PFTs the severity of bacterial infections may be able to be reduced. The study was conducted using a nanosponge (polymeric core) wrapped in a natural red blood cell membrane bilayer. The polymeric core stabilizes the membrane shell and the membrane bilayer allows the nanosponge to absorb a wide range of PFTs. Testing was done to determine the ability of the nanosponges to neutralize PFTs. Researchers found that the nanosponge absorbed membrane-damaging toxins and diverted them away from their cellular targets. In mice, the nanosponges significantly reduced the toxicity of staphylococcal α-hemolysin and improved the survival rate. The conclusion of this study was that nanoparticles have the potential to be able to treat a variety of diseases and injuries caused by pore-forming toxins.

Pore Forming Toxin Relief

Nanosponges are often introduced to the body by injection, and will be draped in a red blood cell-like membrane so that the bacteria or venom will attack it. Once it is attacked, it is trapped within the scaffolding of the nanosponge. After the nanosponge is full of toxins and cannot trap anymore it moves to the liver to filter out the toxins. Researchers are posed with is how to tackle all the different types of bacteria and venom, making a lot of different nanosponges for each specific bacteria and venom is nearly impossible. As of now they are focusing on toxins such as; E. coli, MRSA, pneumonia, bee venom, snake venom and sea anemone venom. These venoms all use a pore-forming strategy, in which they create pores in the cells they attack in order for them to leak until they are no longer functional. A single nanosponge can capture many of the bacteria and venoms, instead of being tailored to each individually because when the venom physically tried to induce a whole in the red blood cell membrane, the venom will get stuck inside the sponge.

An obstacle researchers are faced with is the lifespan of the nanosponges. Once nanosponges are injected they can move rapidly through the blood system, and be found in the liver to be filtered out within hours. This means the nanosponge does not have enough time to soak up the maximum amount of toxin that it can hold. Researchers are working on a technique using hydrogel to coat the nanosponges to increase the life of the nanosponge and help them to remain stationary after injection to more efficiently purge the body of toxins. A study done by The University of California found that 80 percent of nanosponges coated with a hydrogel lasted more than two days after injection. Only 20 percent of nanosponges not coated in the hydrogel lasted two hours after injection, and diffused to other locations in the body.

Treating Ischemic Strokes

Mn₃O₄@nanoerythrocyte-T7 (MNET) nanosponges can regulate oxygen and scavenge free radicals in the event of an ischemic stroke, which is a global leading cause of death and disability. These engineered nanosponges can help attenuate hypoxia after a stroke by mimicking red blood cells and increasing the amount of oxygen in the infarct area. This allows for the extension of the survival time of neurocytes, a crucial part of treating an ischemic stroke because their normal functions must be maintained.

MNET works because it contains hemoglobin, which allows for there to be an oxygen sponge effect. This effect works by releasing oxygen in hypoxic areas and absorbing it in oxygen-rich areas. The sponge effect, along with the free radical scavenging, can successfully and efficiently treat ischemic strokes.

Biomimetic nanoparticles, like MNET nanosponges, can easily pass the Blood-Brain Barrier (BBB). The efficiency of the BBB-crossing of MNET is improved by the T7 peptide, which is critical in treating an ischemic stroke. In a study on middle cerebral artery occlusion (MCAO) rats, those treated with MNET experienced a significant attenuation of neurological damage.