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Procedia Chemistry 19 ( 2016 ) 406 – 409
5th International Conference on Recent Advances in Materials, Minerals and Environment (RAMM) & 2nd International Postgraduate Conference on Materials, Mineral and Polymer (MAMIP), 4-6 August 2015 The Characteristics of the Smart Polymeras Temperature or pH- responsive Hydrogel
B. Hilmia, Z.A. Abdul Hamida*, H. Md Akila, and B.H. Yahayab
aBiomaterials Niche Group, School of Materiasl and Mineral Resources Engineering, Universiti Sains Malaysia, 14300, Nibong Tebal, Pulau Pinang, Malaysia bRegenerative Medicine Cluster, Advanced Medical and Dental Institute, Bertam,13200 Kepala Batas, Pulau Pinang Malaysia
Abstract
Hydrogels have unique swelling behaviour and three-dimensional structure and can be applied in biomedical and tissue engineering fields. Hydrogels also can be prepared by several methods. They are called as smart hydrogels as they able to undergo transitional changes in response to environmental stimuli. One of the stimuli is temperature. The temperature can create changes to the smart polymer as they have a very sensitive balance between the hydrophobic and the hydrophilic groups in their structure. Some hydrogels exhibit a separation from solution and solidification above a certain temperature. This threshold is known as the lower critical solution temperature (LCST). The other stimulus is pH which also can give effect to hydrogel in order to be further applied as drug delivery agents. The main feature of this kind of smart polymer is an ability to receive or release protons which responding to the pH changes. These polymers are polyelectrolyte where containing acid groups or basic groups. Temperature and pH responsive hydrogel is essential and currently investigated in many various applications specifically in drug delivery system due to their unique responsive characteristics. ©© 20162016 Published The Authors. by Elsevier Published B.V. Thisby Elsevier is an open B.V. access article under the CC BY-NC-ND license (Peerhttp://creativecommons.org/licenses/by-nc-nd/4.0/-review under responsibility of School of Materials). and Mineral Resources Engineering, Universiti Sains Malaysia. Peer-review under responsibility of School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia Keywords:Smart polymer; Lower critical solution temperature; Solidification
* Corresponding author. Tel.: +6-017-332-0778; fax: +6-04-5941011 E-mail address:[email protected]
1876-6196 © 2016 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia doi: 10.1016/j.proche.2016.03.031 B. Hilmi et al. / Procedia Chemistry 19 ( 2016 ) 406 – 409 407
1. Smart hydrogel
Recently, hydrogels have been a topic of many researches due to their unique swelling behaviour and three- dimensional (3D) structure and have resembles the physiological environment properties which make them applicable in biomedical and tissue engineering fields. Hydrogels are composed of hydrophilic homopolymer or copolymer crosslinked networks and can swell in the presence of water or physiological fluids1. Hydrogels also can be prepared by allowing the polymer solution to gel in situ after photopolymerization, chemical crosslinking, and ionic crosslinking or in response to an environmental stimulus like temperature, pH or ionic strength of the surrounding medium2. The stimulus-responsive hydrogels are also known as smart polymers which means they have shown an active response to small signs and changes in the surrounding environment by showing the significant changes in their microstructure as well as in the physiological and chemical properties3,4. There are several important features of these smart polymers. Firstly, their macroscopical changes are reversible which means they can change back to their original state with the absent of stimuli5.Besides that, the others features of smart polymers are listed as below6: x biocompatible x non-thrombogenic x flexible x easy shaping x easy manufacturing
Generally, hydrogels are regarded as biocompatible materials due to their soft nature and retaining large amount of water which make them similar to natural extracellular matrices7. Furthermore, they also have porous structure and along with the water content, they are suitable to accommodate high loads of water soluble compounds8. This will make the smart hydrogels can be apply for wide range of biomedical and pharmaceutical applications such as drug delivery, tissue engineering, contact lens and many more. An injectable hydrogels also have been developed in order to overcome macroscopic hydrogels which need to be administered by surgical intervention. The injectable hydrogels can be administered in a minimum invasion and less in cost as compared to the surgery which also bring inconvenient to the patient. The injectable hydrogels also can be in the clear form of polymer solutions before the administration and then turn into viscoelastic system or gel after responding to the environmental stimuli9.
2. Temperature stimulus
The temperature can create changes to the smart polymer as they have a very sensitive balance between the hydrophobic and the hydrophilic groups in their structure10. The phenomenon of transition from a solution to a gel is usually referred as sol-gel transition as shown in Fig.1. Some hydrogels exhibit a separation from solution and solidification above a certain temperature. This threshold is known as the lower critical solution temperature (LCST). It is also defined as the critical temperature in which the polymeric solution shows a phase separation from one phase or isotropic phase to two phase or anisotropic phase12. The polymer is in soluble state below the LCST. When above the LCST, they become increasingly hydrophobic and insoluble, leading to gel formation13. In other words, the interaction strengths of hydrogen linkages between the water molecules and the polymer become unfavourable and lead to dehydration and polymer swelling14. On the other hand, hydrogels that are formed upon cooling of a polymer solution have an upper critical solution temperature (UCST)13. The examples of thermoresponsive polymers are poly(N-isopropylacrylamide) (PNIPAAm) and triple blocks of copolymers polyoxyethylene-polyoxypropylene-polyoxyethylene (PEO-PPO-PEO) also known as Pluronic.
408 B. Hilmi et al. / Procedia Chemistry 19 ( 2016 ) 406 – 409
Fig.1. Sol-gel transition with response to the temperature11.
3. pH stimulus
The other stimulus which is pH also has great influence to the hydrogel since the human body has different pH values in each tissues and cell compartments as shown in Table 1. The suitable pH for the hydrogel to react is important in the further biomedical application. These certain pH values make the smart polymer to become specific delivery of drug and therapeutic agents.
Table 1. pH values in several tissues and cell comparments15.
Tissue/Cell compartment pH value Blood 7.4-7.5 Stomach 1.0-3.0 Duodenum 4.8-8.2 Colon 7.0-7.5 Lysosome 4.5-5.0 Golgi complex 6.4 Tumor-extracellular medium 6.2-7.2
The main feature of this kind of smart polymer is an ability to receive or release protons which responding to the pH changes 2,16. Usually, these polymers are polyelectrolyte where containing acid groups or basic groups like ammonium salts17. The smart polymers with acid groups are known as polyacids or polyanions that have a large number of ionisable acid groups in their structure such as carboxylic acid or sulphonic acid2. The carboxylic groups accept protons at low pH values and release protons at high pH values18. Thus, the polymer will swell with the increase of pH because of the electrostatic repulsion of the negatively charged groups12. The examples of polyanions are poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA). On the other hand, the smart polymers with basic groups are called as polybases or polycations. They are usually protonated at high pH values and positively ionized at neutral or low pH values18. The examples of these polymers are poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) and poly(lysine) (PL). In addition, these smart polymers also change the solubility by changing the electrical charge of the polymer molecule19. So, these hydrogels need to decrease the electrical charge in the molecular level in order to transform from a soluble state to an insoluble state. There are several ways that can be used to decrease the polymer’s electric charge which are decreasing pH, neutralizing the electric charge and reducing the hydrophilicity of the polymeric macromolecules20.
B. Hilmi et al. / Procedia Chemistry 19 ( 2016 ) 406 – 409 409
4. Conclusion
The stimuli-responsive polymers or smart-polymers can be developed by several ways including in response to temperature and pH which can make them applicable in several applications such as modified release of drugs, tissue engineering and biosensors.The temperature and pH responsive hydrogel could be precisely tailor made according to the desired properties based on the desired stimulus either temperature or pH or both stimulus. Therefore, it is worthwhile a comprehensive study on these types of smart hydrogelsis carrying out to develop new materials which could be applied in specific area within the physiological environment.
Acknowledgements
This study was supported by Research University Individual Grant (RUI 1001/PBAHAN/814201).
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