Preparation of Organic-Inorganic Molecular Hybrid Materials and Development of Novel EEG Electrodes
Author:Wang Si Zhe
Supervisor:duan zuo wen
Brain science, as one of the most important fields of science in 21st Century, needs the support of the non-invasive EEG recording technology. However, it needs to inject conductive pastes, as the electrolyte, between the electrodes and scalp in the traditional EEG measurement. It makes a lot of inconveniences. Thus, development of EEG technology in the new fields has been limited by this. So it is urgent to develop new types of non-paste EEG electrodes (no conductive paste), which has become a hot research area in the world.Aiming to solve this problem, this thesis attempts to develop two new types of non-paste EEG electrodes to replace the traditional electrodes with conductive pastes (so call "wet EEG electrode"). The first design is to develop a kind of high strength organic-inorganic molecular hybrid hydrogel. Then electrodes combine with the hydrogel that acts as a electrolyte between the electrodes and the scalp. The second idea is to keep the electrodes and electrolyte solution in a cavity structure, connecting the electrode and scalp with the electrolyte solution through columnar pipes in the electrode housing. An in situ polymerize super-macroporous hydrogels is in the columnar pipes and the electrodes can slowly release the electrolyte solution.The mechanical strength of the hydrogel is a key factor to limit the application. In this thesis, a new method that enhance the mechanical strength of hydrogel by using organic-inorganic molecular hybrid material has been developed. And an amino-silsesquioxane hybrid PAAm hydrogel has been prepared. In the process of development of the hydrogel, we have further studied the sol-gel process which is the main method of preparation of organic-inorganic hybrid materials. We have tried to solve two key problems in the course:(1) hydrolysis condensation reaction is influenced by many different factors; (2) it is difficult to drive both the hydrolysis and condensation reactions to completion, which causes stability issues for the products, which restricts the mass production in industry. An improved preparation method is explored, which can be used for large-scale industrial production. It also provides technical support for the preparation of high strength hydrogel for the EEG electrodes.Based on the above analysis, the contents of this thesis are as follows:In the 2nd chapter, a highly efficient synthesis of methacrylate silsesquioxane (MASSQ) based on sol gel process was proposed, and the effect of water/silane feed ratio on the properties of the products was investigated. For the traditional sol-gel process, the water/silane feed ratio has great influence on the structures and homogeneity of the products, and further affects the optical properties and processing abilities of the materials. This research took 3-Methacryloxypropyltrimethoxysilane as an example, to prepare silsesquioxane oligomers via hydrolysis and condensation reactions with hydrochloric acid as the catalyst. The total reaction time of this method is less than 10 hours. It was found that when the ratio of water/silane increased, the viscosity of the products increased, and the optical transparency decreased. Furthermore, the rheo logical behavior showed that the products changed from a Newtonian fluid to a non-Newtonian fluid. All of these changes demonstrated that the products were changed from a homogeneous system to a heterogeneous system. When the molar ratio of water/silane was 3, the product possessed the best processing ability and optical transparency. This product was characterized with FTIR,1H NMR,29Si NMR, MALDI TOF mass spectrometry and electrospray ionization mass spectrometry. And the results showed that the product consists of oligomers with the degree of polymerization (n) ranging from 4 to 13. Among these components, there are fully condensed cage-structure oligomers (n= 6/8/10), and also incompletely hydro lyzed or condensed constituents which contain a few methoxyl or hydroxyl groups.In the 3rd chapter, for the purpose of solving the stability issue of the sol-gel derived products, and for preparing multifunctional organic-inorganic hybrid materials, a method has been proposed. In the method, pre-prepared mono-adducts of isophorone diisocyanate (IPDI) and 2-hydroxyethyl acrylate (HEA) was synthesized with MASSQ, forming MASSQ-UA. The characterization results show that the active silanol groups in MASSQ were effectively reduced and sterically retarded by IPDI-HEA, and the acrylate functional groups were introduced into the products successfully. The study of optical and rheological behavior shows that MASSQ-UA is a homogeneous system and possesses a good processing ability. Both the thermal stability tests and water resistance tests of UV-cured coating showed that the stability of MASSQ-UA was better than that of MASSQ. The above results show that the methodology developed here can effectively improve the stability of the sol-gel derived hybrid materials, and may provide a useful tool for the industrial design and production of organic-inorganic hybrid materials at the molecular level.In the 4th chapter, a new type of organic-inorganic molecular hybrid hydrogel with high mechanical strength was prepared, and a new type of EEG electrode was developed according to a design. Amino-silsesquioxane was synthesized by the method proposed in the 2nd chapter. And the aza-Micheal addition reaction was used to modify AmSSQ, which makes double bonds grafted on its surface. Then the products were polymerized with acrylamide (AAm) as crosslinking agents, forming AmSSQ-c-PAAm organic-inorganic molecular hybrid hydrogels. The hydrolytic condensation degrees of AmSSQ were studied when the molar ratios of water/silane were 1.5 and 3, and the structures of AmSSQ were characterized by FTIR,1H NMR and 29Si NMR. Three hybrid hydrogels were prepared when AmSSQ/monomers weight ratio= 2%,5% and 10% respectively, and PAAm hydrogel was also prepared as contrast. It is found that the amount of AmSSQ had little influence on the water absorption rate and equilibrium water content of the hydrogels, but significantly enhanced the mechanical strength. The hydrogel prepared with the weight ratio of AmSSQ/monomers is 5%, has the largest mechanical strength,100 kPa, which has a 100% increase compared with the PAAm hydrogel (50 kPa). Further, the hybrid hydrogel EEG electrode was prepared by using this hybrid hydrogel (AmSSQ/monomers weight ratio= 5%). electrochemical impedance spectroscopy studies indicate that the electrode-scalp impedance is less than 20 kΩ, and the optimal value is 7 kΩ. which has reached a level close to commercial wet electrode (<10 KΩ). And the impedance of the electrode was slightly decreased after 1 hour continues work. In the eyes open-closed EEG test and steady-state visual evoked potentials (SSVEP) test, signals were obtained by using hybrid hydrogel EEG electrodes, and the peak frequency and amplitude of signal were consistent with commercial wet electrodes. This indicats that the hybrid hydrogel electrodes have the potential to EEG measurement.In the 5th chapter, a new type of EEG electrode has been developed. This EEG electrode has a cavity which contained 0.5 mL electrolyte solution and an Ag/AgCl sintering electrode. Thus, the electrode-electrolyte interface could be quickly established. The front part of the electrode was made into a three-dimensional comb structure, which is able to pass through the barrier of the hair and touch the scalp. In the interior of the comb pipeline, super-macroporous hydrogels were in situ polymerized by cryogelation technology, and played a role in guidance and transfering of electrolyte solution. The electrolyte solution could go through the comb pipeline and wet the scalp directly, and which effectively reduce the impedance between the electrodes and the scalp. The results of impedance experiments show that, at 10 Hz, the average electrode-scalp impedance of each subject was all below 20 kΩ at the beginning and below 27 kΩ after one-hour continues work. EEG tests (eyes open/closed test and SSVEP test) showed that the electrodes were able to acquire EEG signals similar to that of the commercial wet electrodes. It suggests that the super-macroporus hydrogel EEG electrodes have the potential in EEG measurement.