Conventional polymer hydrogels are three-dimensional network structures formed by the polymerization of hydrophilic monomers. Hydrophilic monomers commonly used to prepare hydrogels are hydroxyethyl methacrylate (HEMA), hydroxypropyl methacrylate (HPMA), N-vinylpyrrolidone (NVP), N, N-dimethylacrylamide (DMA), methacrylic acid (MMA), butyl methacrylate (BMA) and so on^{[17][18][19][20][21][22]}. Compared with other monomers, the hydrogel prepared by HEMA has moderate water content, good mechanical properties and light transmission properties. Therefore, the hydrogel for corneal contact lens is usually prepared by free radical thermal polymerization or photopolymerization with HEMA as the main monomer and a small amount of crosslinking agent. The hydrogel has a water content of about 38%, has good hydrophilicity and flexibility, can bring less foreign body sensation to users, and has low pressure on the cornea. However, the hydrogel contact lens has poor oxygen permeability, with a Dk value of only about 10 barrer, which is difficult to meet the needs of long-term wear.

Because the oxygen permeability coefficient of hydrogel is proportional to the water content, researchers have been exploring ways to improve the water content of hydrogel. Young et al. Copolymerized HEMA with phosphocholine (MPC) containing zwitterions to obtain a hydrogel with a water content of 58% and a Dk value of 33 barrer^[17]. Some researchers polymerized HEMA with 1% ~ 10% acrylic acid (MA) and alkyl methacrylate in bulk, and the water content of the hydrogel materials produced in the equilibrium swelling state was more than 45%^[23]. If the copolymer is further converted to its alkali metal salt, the equilibrium swelling ratio of the hydrogel continues to increase. However, when this ionic water-soluble monomer is used in the field of corneal contact lens, it is easy to cause the deposition of proteins and lipids in human tears on the corneal contact lens.

The molecular structure of NVP is similar to that of protein, and it has good water solubility and biocompatibility. When NVP is copolymerized with HEMA, the water content and oxygen permeability of hydrogel materials can be significantly improved. At present, most of the widely used hydrogel contact lens materials are random copolymers formed by HEMA, NVP and crosslinking agents. Li et al. Copolymerized NVP and HEMA to prepare hydrogel soft contact lens materials, and studied the relationship between NVP content and water content and oxygen permeability of hydrogel materials^[19]. It was found that the water content and oxygen permeability of the hydrogel increased with the increase of NVP content. When the NVP content was 25%, the water content of the material was 60% and the Dk value was 25 barrer. When the volume fraction of NVP is more than 40%, the water content of the hydrogel material reaches 84. 5%, the Dk value increases to 35 barrer, but the tensile strength of the material decreases.

In addition, the preparation of porous hydrogel is also an effective method to improve the water content of hydrogel. The introduction of pore structure in hydrogel, on the one hand, can make water quickly enter the gel through capillary action; On the other hand, the pore structure in the hydrogel greatly increases the specific surface area inside the gel, so that more hydrophilic groups can quickly contact with water molecules, which further promotes the water absorption rate of the gel. Hydrogel with large porosity can be prepared by foaming method, phase separation method, template method, pore-forming agent method and interpenetrating network method^{[24][25][26][27][28]}. Compared with the traditional non-porous or less porous hydrogel, the water absorption of the porous hydrogel is significantly improved, and the oxygen permeability is also increased. However, with the increase of porosity, the elastic modulus of the gel decreases, and the mechanical properties decrease, which is difficult to meet the needs of bearing pressure in practical applications, so it is not suitable for corneal contact lenses.

Previous studies have found that although increasing the water content can improve the oxygen permeability of hydrogels to a certain extent, these hydrophilic polymer networks hardly transfer oxygen.The carrying and transmission of oxygen mainly depends on the water molecules swollen in the polymer network, while the Dk value of pure water is only 40 barrer, which is also the theoretical upper limit of the oxygen permeability coefficient of hydrogel^[15]. In addition, excessive moisture content will not only reduce the mechanical properties of the material, but also increase the rate of surface water loss, which is easy to absorb water in tears during contact with the eyes, resulting in dry eyes.

Wijmans et al. Proposed the dissolution and diffusion mechanism of gas through polymers in 1866, which laid the theoretical foundation for gas permeation membranes^[29]. The transport process of oxygen through the hydrogel membrane follows the mechanism of adsorption-dissolution-diffusion-desorption^[30]. The hydrogel film contains a large amount of water, and when the oxygen-rich gas is in contact with the hydrogel film, the oxygen is adsorbed and dissolved in the water on the surface of the hydrogel film; Under the action of concentration difference, the oxygen dissolved in the water on the surface of the hydrogel film will diffuse from the high concentration side to the low concentration side; Finally, the oxygen on the lower surface of the membrane is desorbed and released from the membrane. In general, the gas adsorption and desorption rate is fast, while the diffusion rate in the membrane is slow, which is the controlling step of the gas permeation rate through the membrane.

Morgan et al. Tested the oxygen permeability of several different hydrogel contact lenses, plotted the relationship between the Dk value of the hydrogel and the equilibrium water content (EWC) (Fig. 1), and found that Dk was linearly related to EWC, which was described by the formula Dk=1.56e^{0.0397EWC[31]}. Tan Guoxin et al. Found that there are three States of water molecules in hydrogels: free water, bound water and bound water between the two^[32]. Free water can diffuse freely in the gel, and its thermodynamic behaviors such as melting temperature, crystallization temperature and enthalpy change are similar to those of pure water. The bound water forms hydrogen bonds with the hydrophilic groups on the macromolecular chain of the hydrogel, and the water molecules are difficult to move freely and can only vibrate within a fixed lattice range; The properties of bound water are between the two. The diffusion of oxygen in the hydrogel is mainly through the "free water" in the hydrogel. Pedley et al. Found that the higher the free water content in the hydrogel, the higher the oxygen permeability^[33]. The amount of free water depends not only on the free volume between polymer chains, but also on the network crosslinking density and polymer properties, especially on the hydrogen bonding ability of polymer polar groups. The number of hydrogen bonds between water molecules and polymer matrix increases with the increase of network crosslinking density. Li et al. Studied the oxygen permeability of HEMA-NVP copolymer hydrogel membrane by isobaric osmosis, and found that the diffusion behavior of oxygen molecules in the hydrogel was mainly affected by the random thermal motion of free water.The effect of the random thermal motion of bound water and polymer chains on the diffusion behavior of oxygen molecules is negligible, but the cross-linked network structure of the hydrogel membrane filters the diffusion process of oxygen^[30]. Within a certain range, the higher the temperature, the greater the water content, and the lower the crosslinking density, the higher the oxygen permeability of the hydrogel material.

These results indicate that the diffusion process is the key factor affecting the transport properties of oxygen in hydrogels, and this process is mainly affected by the random thermal motion of water molecules, the existence state of water and the density of crosslinked network structure of hydrogels. Therefore, it is possible to improve the oxygen permeability of hydrogels by increasing the temperature, increasing the free water content and reducing the crosslinking density of hydrogels.

Silicone polymer is widely used as gas permeable membrane and gas separation membrane because of its large silicon atom volume, loose structure and good gas permeability^{[34⇓⇓~37]}. Aiming at the defect of poor oxygen permeability of traditional hydrogel materials, the introduction of silicone chain segments with high oxygen permeability into the hydrogel system is the most commonly used method to prepare silicon-containing hydrogel materials with high oxygen permeability. Silicone hydrogel is a polymer network formed by copolymerization of hydrophobic organosilicon monomer and hydrophilic monomer, which usually has a biphasic structure of hydrogel phase and organosilicon phase^[38]. The siloxane part mainly improves the oxygen permeability of the material, and the hydrogel part reduces the wear between the material and the tissue and enhances the comfort of use^[39]. Silicone hydrogel combines the advantages of high oxygen permeability of silica gel materials and strong hydrophilicity of hydrogel materials, and can meet the dual requirements of high air permeability and hydrophilicity of materials in the biomedical field.

According to the structural characteristics of silicone monomer molecules, the silicone monomers used to prepare silicone hydrogels can be divided into two types: small molecular silicone monomers and large molecular silicone monomers. Small molecular silicon monomer is a silicon-containing compound with silicon atom as the center and various functional substituents, such as methacryloxypropyltris (trimethylsiloxy) silane (TRIS), γ-aminopropyltriethoxysilane (KH550), γ- (methacryloxy) propyltrimethoxysilane (KH570), propylglycerylmethacrylatebis (trimethylsiloxy) methylsilane (SIGMA), etc^{[40,41][42][43][44]}. This kind of silicon monomer has small molecular weight and good compatibility with hydrophilic monomer, so it is usually used to prepare silicone hydrogel by direct copolymerization of small molecular silicon monomer and hydrophilic monomer. The related research works on conventional and silicone hydrogels are summarized in Table 1.

Cai Libin et al. Prepared KH570-HEMA-NVP terpolymer silicone hydrogel material by bulk copolymerization. The material has good oxygen permeability, optical transparency and refractive stability. The oxygen permeability of the material decreases first and then increases with the increase of KH570 content^[43]. When the content of KH570 is less (5 wt% and 10 wt%) and the content of NVP is 15%, the oxygen permeability coefficient of the hydrogel material is lower, only 15 barrer, which is much lower than that of the HEMA-NVP binary copolymer hydrogel without KH570. However, with the increase of KH570 content to 20 wt%, the oxygen permeability coefficient of the hydrogel material increased rapidly, and the Dk value reached 35 barrer, indicating that the addition of silicon monomer effectively improved the oxygen permeability of the hydrogel. Jiang et al. Hydrolyzed SIGMA with HEMA and NVP to obtain transparent silica hydrogel, and analyzed in detail the relationship between the oxygen permeability of silica hydrogel and the content of EWC and silicon-containing monomer^[44]. The results showed that EWC decreased with the increase of SIGMA content, and the relationship between Dk value and SIGMA content showed a U-shaped curve distribution. As the content of SIGMA increases from 0% to 20 wt%, the EWC decreases, and the Dk value decreases from 22.3 barrer to 17.0 barrer. After that, the Dk value began to increase with the increase of SIGMA content, and when the SIGMA content exceeded 40 wt%, the Dk value increased sharply, and reached 34.5 barrer at 50 wt% SIGMA. According to this study, two factors, the silicon-oxygen bond in the organosilicon phase and the water molecule in the hydrogel, jointly promote the permeation of oxygen and follow a competitive mechanism of mutual inhibition. These findings indicate that the introduction of small molecular silicon monomer improves the oxygen permeability of the material, while maintaining the hydrophilicity of the hydrogel material. However, the distribution of small molecular silicon monomers in the network structure is relatively dispersed after copolymerization, which is not easy to form a continuous channel, and the improvement of oxygen permeability is limited.

Macromolecular silicon-containing monomer is an oligomer containing a large number of silicon atoms in its molecular structure, and its oxygen permeability is better than that of small molecular silicon monomer. Polydimethylsiloxane (PDMS) with polymerizable functional groups is an early and most widely used macromolecular silicon monomer with high oxygen permeability. The Si — O — Si bond in the PDMS molecule is long and the bond angle is large, which can rotate in the range of 104 ° ~ 180 °, so the PDMS molecule is in a highly compliant state^[45]. At the same time, there is repulsion between the methyl group and the main chain, which makes the molecular structure of PDMS loose, and the gap structure provides a good channel for material transmission, so PDMS has excellent oxygen permeability and ion permeability. It has been reported that the Dk value of silicone rubber prepared by pure PDMS is as high as 600 barrer, which is far beyond the maximum oxygen permeability of traditional hydrogel materials^[46,47]. However, due to the high hydrophobicity of PDMS and its poor compatibility with hydrophilic monomers, it is impossible to obtain transparent and uniform materials by direct copolymerization.

In order to improve the compatibility of PDMS with hydrophilic monomers, many researchers have modified PDMS by introducing hydrophilic units into its end groups or side chains. Lin et al. Grafted hydrophilic HEMA onto PDMS segments to form a grafted modified PDMS oligomer with isofluoroketone diisocyanate (IPDI) as a crosslinking agent, and then copolymerized it with hydrophilic monomer polyethylene glycol methacrylate (PEGMA) to form a silicone hydrogel^[48]. The obtained silica hydrogel not only has high oxygen permeability and optical transparency, but also has strong hydrophilicity. When the content of PEGMA is 20%, the Dk value of silicone hydrogel is 92 barrer. Fang et al. Prepared a multiblock copolymer with PDMS/polytetrahydrofuran (PTMO) as the soft segment and 4,4 '-cyclohexylmethane diisocyanate (HMDI) as the hard segment. The results showed that the oxygen permeability of the copolymer membrane depended on the continuous phase component in the material, and the Dk value changed with the PDMS content in the soft segment^[49]. Tao et al. Introduced hydrophilic polyethylene glycol (PEG) segments into both ends of PDMS to prepare block-type amphiphilic macromolecular silicon-containing monomers, and then copolymerized with hydrophilic monomers to obtain silicone hydrogel materials with good comprehensive properties such as light transmission and hydrophilicity, with a Dk value as high as 110 barrer, which is expected to be used for corneal contact lenses^[18]. Junichi Iwata et al. Introduced PEG chain segments and fluorinated hydrocarbon groups into the side chain of PDMS to improve the hydrophilicity of PDMS, and the Dk value of the obtained silicone hydrogel could be as high as 196 barrer, and the corneal contact lens made from this material had good comprehensive performance^[50].

Researchers have also explored other ways to improve the compatibility of silicon-containing macromers and hydrophilic monomers. Liu et al. Prepared silicone hydrogels by copolymerizing small molecular silicon monomer TRIS and large molecular silicon monomer methacryloxypropyl-terminated polydimethylsiloxane with hydrophilic monomers^[51]. It was found that the introduction of small molecular silicon monomer could not only increase the compatibility between macromolecular silicon monomer and hydrophilic monomer, but also help to improve the oxygen permeability coefficient of silicone hydrogel materials, and the water content and equilibrium solute distribution coefficient of the materials were closely related to the amount of silicon monomer. Yang et al. Prepared silicone nanoparticles (SiNPs) by sol-gel method using PDMS and tetraethyl orthosilicate (TEOS) as raw materials^[52]. Silicone hydrogel was then prepared by photopolymerization after mixing SiNPs as crosslinker with HEMA and NVP. The results show that the oxygen permeability of silica hydrogel increases with the increase of SiNPs content, while the effect on EWC is not significant. When the content of SiNPs is 1.2 wt%, the obtained hydrogel EWC is 73% and the Dk value is 71 barrer. The results showed that the oxygen permeability of conventional hydrogel was significantly improved by adding a small amount of SiNPs. Shimizu et al. Prepared silicone hydrogels with interpenetrating network structure using 2-methacryloylphosphocholine (MPC) and TRIS as raw materials. The experimental results showed that the silicone hydrogels with interpenetrating network structure had better optical and mechanical properties, and had super-hydrophilic surface^[53].

In addition, in order to improve the oxygen permeability of rigid contact lens material (RGP, the main component of which is polymethyl methacrylate), the researchers copolymerized PDMS with methacrylate and fluorine-containing monomers to prepare fluorosilicone acrylic resin^[54]. Such materials typically have Dk values above 100 barrer, but contain little water, have poor surface wettability, and have a high modulus, making them unsuitable for use as soft contact lenses. At present, it is often used as a rigid orthokeratology lens material to temporarily correct the refractive errors of patients in clinic, and has the function of delaying the deepening of myopia in children and adolescents^[55].

Kraus et al. Studied the gas permeation mechanism of silicone hydrogel, and considered that the gas transport in silicone hydrogel also followed the dissolution-diffusion mechanism^[56]. Silicone hydrogel contains organosilicon phase and hydrogel phase, and its oxygen permeability is the superposition of the amount of oxygen permeated in the two phases. However, the diffusion ability of oxygen in these two phases is different, and the oxygen permeability of silicone hydrogel is not only related to the silicon-containing component, but also affected by the microstructure of the polymer. In the 1980s, some researchers have studied the relationship between the phase separation structure and the gas permeability in polymer membranes, but it is difficult to prove the relationship between the two by experiments, so many studies have demonstrated the effect of the phase separation structure on the gas permeability and its mechanism by establishing molecular models^[57]. Pozuelo et al. Compared the transport properties of oxygen molecules in HEMA-based hydrogels and silicone hydrogels, and found that the oxygen permeability of silicone hydrogel materials was about one order of magnitude greater than that of hydrogel materials, and the oxygen solubility coefficient of silicone hydrogel materials was about six times that of hydrogel materials^[58]. They established a microphase separation structure model of silicone hydrogel materials through molecular dynamics simulation (Fig. 2), and considered that in silicone hydrogel materials, the hydrogel phase and the hydrophobic siloxane phase were microphase separated and formed a co-continuous structure, and oxygen mainly moved through the free volume of the hydrophobic siloxane phase. The value of the oxygen diffusion coefficient obtained by the simulation is roughly similar to that obtained by the experiment.

Jiang et al. Confirmed the existence of microphase separation structure in silicone hydrogel by experimental means, and different silicone monomers will lead to different polymer micromorphology, thus affecting its gas permeability^[44]. They suggested that the complex relationship between organosilicon monomer content, water content, and permeability is related to whether the hydration phase or organosilicon phase forms a continuous phase. When the silica phase forms a continuous channel in the silica hydrogel, it is beneficial for oxygen to pass through, and the oxygen permeability of the silica hydrogel is increased. However, too high silicon content will cause macroscopic phase separation, and the water content, light transmittance and mechanical properties of silicone hydrogel will be significantly reduced. Erdodi et al. Used PDMS and PEG to prepare silicone hydrogel, and also found that there was phase separation in the silicone hydrogel, and different monomer contents led to different proportions of the two phases, which had different effects on the final oxygen permeability coefficient of the material^[59]. Kwon et al. Studied the effects of siloxane monomer structure and hydrophobic/hydrophilic monomer composition on the properties of silicone hydrogels, and found that the phase separation behavior of silicone hydrogels was mainly affected by the hydrophobic/hydrophilic monomer composition^[60].

Due to the complex micro-morphology and structure of silica hydrogel, its oxygen permeability coefficient is no longer proportional to the equilibrium moisture content. In addition to the equilibrium water content, the total oxygen permeability of the material may be affected by the silicon monomer content, crosslinking density, polymer micro-network structure and phase separation morphology, free volume, and polymer chain mobility^[61]. Nicolson and Vogt proposed that silicone hydrogels can be regarded as composites with distinct hydrogel and silicone phases, and that the oxygen permeability of the bulk material depends on the amount of silicon monomers, the intrinsic diffusion rate of silicon monomers, and their spatial distribution^[46]. Seitz et al. Studied the effect of silicon monomer composition on the microphase separation structure and oxygen permeability of silica hydrogels, and found that the oxygen permeability coefficient of silica hydrogels increased nonlinearly with the increase of silicon monomer content, which was not only related to the spatial arrangement of silicon phase regions, but also to their fluidity^[62]. They proposed the possible micromorphology of the model silicone hydrogel and presented the effect of silicon monomer content and distribution on the oxygen permeability value in theory (Fig. 3).

Wu et al. Compared the micromorphology and oxygen permeability of silicone hydrogels containing short linear PDMS and branched TRIS, respectively, and found that the micromorphology of silicone hydrogels played a key role in oxygen permeability^[61]. Under the condition of the same silicon content, the silicon hydrogel prepared by short linear chain PDMS has more obvious phase separation structure than that prepared by branched structure TRIS, thus having higher oxygen permeability and oxygen solubility. But also the mobility of linear siloxanes is much higher, which can be attributed to the interface and topological constraints caused by the molecular structure and different domain structures. Tao et al. studied the effect of the chain length of macromolecular silicon monomer PDMS on the oxygen permeability and micro-phase separation structure of silicone hydrogel, and found that with the increase of the chain length of PDMS, the oxygen permeability of silicone hydrogel increased first and then decreased, and the silicone phase in the material gradually agglomerated, resulting in obvious phase separation^[18]. Sugimoto et al. Found that monomethacrylate-terminated PDMS had higher oxygen permeability and better fluidity than double-terminated PDMS, because the silicone hydrogel prepared from single-terminated PDMS had lower crosslinking density and larger free volume, which was beneficial to gas transport^[63]. Therefore, in PDMS-based silicone hydrogels, the increase of crosslinking density will inhibit the mobility of molecular chains in the polymer, thus inhibiting gas diffusion.

These results show that the oxygen permeability of silica hydrogel is related to many factors, such as oxygen permeability coefficient, silicon monomer structure, silicon monomer content, microphase separation morphology, crosslinking density, free volume and so on. Although researchers have done a lot of exploration, due to the complexity of the influencing factors, the research on the oxygen permeability mechanism of silicone hydrogel is far from mature.In particular, how the structure and composition of silicon monomer affect the phase separation structure of silicon hydrogel, and the quantitative relationship between the phase separation structure of silicon hydrogel and its oxygen permeability need to be further studied.