Stimuli responsive smart nanosystems

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1 Stimuli responsive smart nanosystems Dr. K. Uma Maheswari Professor, School of Chemical & Biotechnology SASTRA University Joint Initiative of IITs and IISc Funded by MHRD Page 1 of 10

2 Table of Contents 1 IONIC STRENGTH RESPONSIVE SYSTEMS TEMPERATURE RESPONSIVE SYSTEMS SELF-OSCILLATING SYSTEM REFERENCE ADDITIONAL READING Joint Initiative of IITs and IISc Funded by MHRD Page 2 of 10

3 This lecture will deal with ionic strength responsive systems, thermo-responsive systems and self-oscillating gel systems 1 Ionic strength responsive systems These systems are similar to the ph responsive systems except for the fact that they respond to the concentrations of ions other than H +. These polymeric systems are generally charged (polyelectrolytes) and their charge status in the system will determine their solubility characteristics. An increase in the cationic or anionic charges on the polymer will keep them swollen. When the ratio between the cationic and anionic charges approach unity, the Coulombic attractive forces increase causing the polymer to precipitate. Let us consider an example highlighting this property. A polymer matrix of poly(nisopropyl acrylamide-co-vinyl imidazole) was used to immobilize cupric ions. When a protein mixture was added into the system, those proteins that had a binding affinity to the cupric ions were retained. This interaction is favoured because it results in keeping the cationic polymer chains separated due to the presence of the proteins. When the ionic strength of the surrounding medium was increased, the electrostatic repulsive forces between the individual polymer chains (due to the cationic imidazole rings in the polymer) will be neutralized resulting in dominance of hydrophobic interactions between the polymer chains resulting in their precipitation and expulsion of the proteins. 2 Temperature responsive systems Thermo-responsive polymers have been widely explored for a plethora of applications ranging from drug delivery, microfluidics and tissue engineering.an important characteristic of these systems is the existence of a critical solution temperature (T c ) beyond which the properties of these systems undergo an abrupt change resulting in alterations in the drug release profiles. The properties that may change beyond T c are solubility in aqueous solution and hydrophobicity. There are two categories of polymers LCST and UCST polymers. A LCST (lower critical solution temperature) polymer exhibits hydrophilic nature below its T c, above which it turns hydrophobic. In other words, it exists in a swollen or sol state below its T c and in a deswollen or gel state above its T c. Typical examples of LCST polymers are poly(n-isopropyl acrylamide) and hydroxyl propyl methyl cellulose. A UCST polymer, on the other hand, exists in the gel phase below its T c and in a sol phase above its T c. Examples for the UCST polymer systems include poly(n-acetyl acrylamide), poly(allyl urea) and poly(citrulline). Joint Initiative of IITs and IISc Funded by MHRD Page 3 of 10

4 Why would a thermo-responsive polymer undergo this sol-to-gel transformation above its T c? Let us consider a UCST polymer like poly(n-acetyl acrylamide). Below the T c, the polymer chains are associated through extensive hydrogen bonding leading to aggregation. However, as the temperature is increased beyond its T c, the hydrogen bonds break leading to more mobility to the polymer chains. This results in a swollen state. In the case of a LCST polymer like poly(n-isopropyl acrylamide) (PNIPAM), as the temperature increases beyond the T c, the mobility of the hydrophobic chains will only lead to greater contact with the aqueous environment. As this is an unfavourable condition, the hydrophobic forces tend to dominate beyond the T c, driving the polymer chains to associate excluding water molecules to form a gel phase. The T c of PNIPAM is around 32 o C, which makes it amenable for biological applications after suitable modifications and hence it remains the most widely used thermo-responsive polymer. The encapsulation of a drug into a thermo-responsive polymer can bring about triggered release of the drug. For example, let us considerthe example of a drug loaded PNIPAM carrier. When the temperature is below 32 o C, the T c of the polymer, the polymer chains are in a swollen state leading to maximum release of the drug. As the temperature is increased above T c, the polymer chains collapse due to deswelling and the drug release drastically drops. The opposite trend is expected in the case of a UCST polymer. A major issuewith the use of PNIPAM and similar kind of LCST polymers is that their T c does not match the body temperature and thus it becomes difficult to regulate the drug release in vivo. But it is possible to exploit their stimuli response through smart combination of materials as well as design of the drug delivery system. It is possible to alter the T c of a polymer by blending it with another polymer with a differentt c. Alternatelythe design of the release system could be intelligently developed to exploit the sol-gel transformation of the thermo-responsive component. There are two types of drug release systems negative temperature response systems and positive temperature response systems. In a negative temperature response system, the quantity of drug released decreases with increase in temperature while in a positive temperature response system the amount of drug released increases with increase in temperature. If a nanoparticle made of a LCST polymer is loaded with drug, it will display negative temperature response. Instead, if the thermo-responsive gel is loaded inside the pores of a thermo-inertpolymer that also contains the drug (which does not show any abrupt change in properties with temperature), then the release profile will be different. When the temperature is below T c, the PNIPAM gel is in the swollen state. As this blocks the pores, the diffusion of the drug will be retarded. As the temperature crosses the T c, the PNIPAM gels collapse within the pores. This results in reappearance of the porous pathways that enhances the diffusion of the drug from the polymeric system. Thus, a positive temperature response can be achieved with the PNIPAM gels serving as thermo-responsive gates. Joint Initiative of IITs and IISc Funded by MHRD Page 4 of 10

5 The same concept can be applied by grafting PNIPAM chains to channel-like structures on a thermo-inert polymer loaded with the drug. The swollen chains will block the channel thereby retarding drug release below the T c, while above the T c, the PNIPAM chains collapse exposing the channels leading to accelerated drug release from the polymer. On the other hand, grafting PNIPAM chains throughout the surface of the polymer containing a drug will result in a negative temperature response. Below the T c, the PNIPAM chains will be fully extended and swollen allowing the diffusion of the drug from the inner polymer core. Above the T c, the PNIPAM chains collapse and coat the outer surface of the polymer preventing the diffusion of the drug. Figure 1 depicts the various designs of drug delivery systems that can be made using temperature responsive polymers to achieve negativee or positive temperature response. Fig. 1: Different designs of thermo-responsive systems for negative and positive temperature response release; low temperature denotes temperatures below T c and high temperatures denote temperatures above T c Joint Initiative of IITs and IISc Funded by MHRD Page 5 of 10

6 Thus it is clear that the design of the delivery system is a key for regulating the drug release. Apart from these properties, the molecular weight of the thermo-responsive polymer might influence the T c values to a small extent. Apart from thermo-responsive drug release applications, tissue engineering applications of the thermo-responsive polymers have gained popularity.tissue engineering and regenerative medicine is a rapidly evolving area that is one of major beneficiaries of the progresses made in the field of nanobiotechnology. The introduction of cells, especially stem cells that possess the ability to different cells into different types of cells, into a damaged tissue, has given encouraging results in partially restoring the functioning of the tissue. (More detailed discussion on the role of stem cells in regenerative medicine is given in Module 10). However, introduction of free cells into the damaged area is not the perfect solution as the cells could migrate away from the damaged area or alternately form isolated islands of cells which might not be sufficient to maintain the functional and structural integrity of the damaged area. Hence, introduction of a monolayer of cells might offer a better optionto overcome this problem. This monolayer of cells is known as a cell sheet and is cultured on a petri plate under sterile conditions before being removed from the culture plate for introduction into the body. The removal of the cell sheet from the culture plate offers a huge challenge. As the cells have adhered to the culture plate and have proliferated, they would have secreted their own extracellular matrix. Hence, trypsinization (addition of the proteolytic enzyme trypsin) or mechanical scraping is employed to remove the cell sheet. Mechanical scraping is not preferred as it can damage the cells due to pressure. Addition of trypsin induces breaking of cell-cell contacts as well as cell-extracellular matrix contacts, which once again damages theintegrity of the cell sheet. Hence, a novel strategy was employed to use the thermo-responsive poly(n-isopropyl acrylamide) polymer for cell sheet engineering. The polymer was coated on the culture plate and the culturing temperature for maintained at 37 o C. The cells were seeded on the polymer film and cultured under sterile conditions to obtain the cell sheet. At 37 o C, which is higher than the critical solution temperature of poly(n-isopropyl acrylamide), the surface of the polymer is hydrophobic, which favours cell adhesion and proliferation. After formation of the cell sheet, the temperature was reduced to 32 o C, the critical solution temperature of the polymer.at this temperature, the polymer becomes hydrophilic and attracts water molecules from the medium and swells. The swelling polymer pushes away the cells from the surface enabling the release of the cell sheet. In this case, the cell-cell and cellextracellular matrix interactions are not disturbed. This technology has made a huge difference in the field of regenerative medicine. Figure 2 depicts the use of thermoresponsive polymer system for cell sheet technology. Joint Initiative of IITs and IISc Funded by MHRD Page 6 of 10

7 Fig. 2: Cell sheet technology employing a thermoresponsive polymer 3 Self-oscillating system The main principle of self-oscillatinreaction. The major ingredients of the original reaction comprised cerium (IV) sulphate, gel systems is the Belousov-Zhabotinsky (B-Z) malonic acid, potassium bromate, citric acid and dilute sulphuric acid. The oxidation of the malonicaicd and citric acid moieties by bromate ions in the acidic medium calaysed by the cerium metal ion causes a periodic oscillation of the metal ions between two oxidation states. Looking at the phenomenon from the perspective of the metal ion catalyst, the sequence of events can be explained as follows. During the course of getting oxidized, the malonic acid reduces the yellow colouredcerium (IV) sulphate to cerium (II) sulphate, which is colourless. The bromate ions in the solution re-oxidize the cerium (II) sulphate to cerium (IV) sulphate, which is promptly reduced by malonic acid. Thus the cerium ions oscillate between the divalent and tetravalent oxidation states thus setting up a chemical oscillator. This reaction was also observed in the case of ferroin, a commonly used indicator for iron estimations.when the reaction mixture is observed in a petri plate, the oscillations in the colours produced due to the B-Z reaction appear to travel back and forth and hence this phenomenon is referred to as a chemical wave. In order to exploit this phenomenon to develop a smart self-oscillating gel system, the metal ion catalyst is linked to the polymer chain. Ruthenium (II) tris(2,2 -bipyridine) was Joint Initiative of IITs and IISc Funded by MHRD Page 7 of 10

8 covalently grafted to a poly(n-isopropylacrylamide) polymer chain. The system when placed in a medium containing the other constituents of the B-Z reaction (organic substrate like malonic acid/citric acid, oxidizing agent like bromate ions) will undergo reduction and oxidation reactions continuously. Figure 3 shows the structure of the selfoscillating gel system. Fig. 3: Structure of self-oscillating gel system based on ruthenium tris(bispyridine) and poly(n-isopropyl acrylamide) As the ruthenium ion toggles between the +2 and +3 oxidation states, its affinity to water changes. The Ru(III) has greater hydrophilicity while the Ru(II) exhibits a hydrophobic character. As a result when the ruthenium is in its reduced state (Ru 2+ ), it deswells while in its oxidized state (Ru 3+ ), it swells. Such rhythmic swelling and deswelling can be used to bring about pulsatile drug release. Figure 4 depicts the swelling and deswelling process that occurs due to change in the oxidation state. Joint Initiative of IITs and IISc Funded by MHRD Page 8 of 10

9 Fig. 4: Swelling and deswelling with respect to change in the oxidation state Also, such rhythmic contractions mimic the heart and peristaltic movements of the esophagus and intestine. Hence, such systems might be invaluable in creating biomimetic systems. The following animation (Figure 5) represents the progression of the knowledge of a simple chemical oscillation reaction to interesting intelligent gel-based applications. Fig. 5: Self walking gel based on B-Z reaction Note: Can be viewed only in Acrobat Reader 9.0 and above Joint Initiative of IITs and IISc Funded by MHRD Page 9 of 10

10 The most recent interest using such self-oscillating gels is the development of a selfwalking gel! In this case, the gel is designed in such a way that it has a anisotropic distribution of the ruthenium tris(2,2 -bipyridine) moieties with certain zones rich in the ruthenium complex and the other regions poor in the ruthenium complex. Now, a substrate with closely spaced grooves is used to demonstrate the walking process. The gel is placed perpendicular to the grooves and the chemical wave is initiated by introducing the components of the B-Z reaction. The presence of the grooves restrict backward movement of the gel and propels the gel forward during the swelling phase. The periodic oscillations enable the gel to move during each swelling cycle. The bending and extension phases resemble walking to an onlooker! 4 Reference Smart Nanoparticles in Nanomedicine (The MML series, Vol. 8), Editors: Reza Arshady & Kenji Kono, Kentus Books, Additional reading 1. Stimuli-responsive polymers and their applications in drug delivery, Priya Bawa, Viness Pillay, Yahya E Choonara, Lisa C du Toit, Biomedical Materials, 4 (2009) 2. Future perspectives and recent advances in stimuli-responsive materials, Debashish Roy, Jennifer N. Cambre, Brent S. Sumerlin, Progress in Polymer Science, 35 (2010), Recent advances and challenges in designing stimuli-responsive polymers, Fang Liu, Marek W. Urban, Progress in Polymer Science, 35 (2010), 3-23 Joint Initiative of IITs and IISc Funded by MHRD Page 10 of 10