Polymeric hybrid aerogels and their biomedical applications

Zongjian Liu a, Yuanyuan Ran a, Jianing Xi *a and Jin Wang *bc
aDepartment of Rehabilitation, Beijing Rehabilitation Hospital, Capital Medical University, Beijing 100144, P. R. China. E-mail: xijn999@ccmu.edu.cn
bSuzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, P. R. China. E-mail: jwang2014@sinano.ac.cn
cKey Laboratory of Multifunctional Nanomaterials and Smart Systems, Chinese Academy of Sciences, Suzhou 215123, P. R. China

Received 9th July 2020 , Accepted 3rd August 2020

First published on 3rd August 2020

Aerogels are a class of porous materials that possess extremely high specific surface area, high pore volume, high porosity, and variable chemical structures. They have been widely applied in the fields of aerospace, chemical engineering, construction, electrotechnics, and biomedicine. In recent years a great boom in aerogels has been observed, where various new aerogels with novel physicochemical properties and functions have been synthesized. Nevertheless, native aerogels with a single component normally face severe problems such as low mechanical strength and lack of functions. One strategy to solve the problems is to construct hybrid aerogels. In this study, a comprehensive review on polymer based hybrid aerogels is presented, including polymer–polymer, polymer–carbon material, and polymer–inorganic hybrid aerogels, which will be introduced and discussed in view of their chemical structures and hybrid structures. Most importantly, polymeric hybrid aerogels are classified into three different composition levels, which are molecular-level, molecular-aggregate-level, and aggregate-level, due to the fact that hybrid aerogels with the same chemical structures but with different composition levels might show quite different functions or properties. The biomedical applications of these hybrid aerogels will also be reviewed and discussed, where the polymeric components in the hybrid aerogels provide the main contribution. This review would provide creative design principles for aerogels by considering both their chemical and physical structures.

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Zongjian Liu

Zongjian Liu received BS degree in applied chemistry from Shandong Normal University in 2000. In 2013, he obtained his PhD in applied chemistry from Beijing Institute of Technology under the supervision of Prof. Yulin Deng. Then he joined Prof. Xuning Ji's group at Xuanwu hospital, Capital Medical University as a postdoctoral researcher. He worked in Beijing Luhe Hospital, Capital University as an assistant professor. In 2019, he moved to Beijing Rehabilitation Hospital, Capital Medical University as an associate professor. He published more than 20 peer-reviewed papers. His current research interests include immune responses, and synthesis of functional biomaterials for stroke rehabilitation.

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Yuanyuan Ran

Yuanyuan Ran received BS degree in Bioscience from Liaocheng University in 2008. In 2015, she obtained her PhD degree in Biochemical Engineering from Institute of Technology. Then she worked in Beijing Luhe Hospital, Capital Medical University from 2015 to 2019. In 2019, she moved to Beijing Rehabilitation Hospital, Capital Medical University as an assistant researcher, collaborating with Jianing Xi, Zongjian Liu. She published more than 10 peer-reviewed papers. Her interests mainly focus on immune responses in stroke rehabilitation.

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Jianing Xi

Jianing Xi received BS in clinical medicine from Qingdao Medical College in 1988. In 2009, he obtained his MS in pathophysiology from Liaoning Medical University. During 1998–2012, he is the director of Medical and Research Department in Aerospace Center Hospital, Peking University. In 2013, he worked as the director of Beijing Rehabilitation Hospital and Medical School, Capital Medical University. He is currently a professor and PhD supervisor in rehabilitation medicine and physiotherapy. He published more than 50 peer-reviewed papers, 9 books. His research interests include basic and clinical research in respiratory and stroke rehabilitation, and synthesis of functional biomaterials for stroke rehabilitation.

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Jin Wang

Jin Wang received his PhD in materials science from Beijing Institute of Technology in 2012 under the supervision of Prof. Zeng-guo Feng. Then he joined Prof. Mitsuru Ueda and Tomoya Higashihara's group at Tokyo Institute of Technology and Yamagata University as Postdoctoral fellow from 2012 to 2014. He is currently an associate professor at Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), Chinese Academy of Sciences (CAS). He published 44 peer-reviewed papers and 22 patents. His current research interests include synthesis of porous materials, hydrogels, and aerogels, and their application in biomedicine, environment, and energy.

1. Introduction

Aerogels are porous materials with various attractive properties such as extremely low density, low thermal conductivity, high specific surface area (SSA), high porosity and high pore volume. They have been used in potential applications in the fields of aerospace, chemical engineering (catalysis, filtration, etc.), construction (thermal insulation, acoustic devices, lightweight materials, etc.), electrotechnics (Cherenkov counters, supercapacitors, batteries, electronics, sensors, etc.), water purification (absorption, oil separation, etc.), and biomedicine (tissue engineering, wound care, drug delivery, etc.).1–13 The first aerogel was synthesized by Kistler14 in 1931 and he defined aerogels as “gels in which the liquid has been replaced by air, with moderate shrinkage of the solid network”. However, there is no universally accepted definition for these porous materials. A definition given by Hüsing and Schubert that “aerogels are materials in which the typical pore structures and networks are remarkably maintained when the pore liquid of a gel is replaced by air” is the most accepted definition.15–17 Impressively, Du et al. defined aerogels as a “state of matter” due to their qualitative differences in bulk properties, transitional density and enthalpy between liquid and gas phases regardless of their chemical composition, which means that any material could be transferred to aerogels regardless of their chemical components.18

Nevertheless, wet gels could not be directly transferred to aerogels by ambient pressure drying (APD) because shrinkage is inevitable due to the capillary force formed in the liquid/air meniscus when evaporation takes place.1,2 Supercritical liquid drying (SCLD) is a method to replace the solvent in wet gels with a SCL, in which the SCL is transferred into vapor phase by which the capillary force can be eliminated and aerogels could be obtained by this method.14 Recently, APD has been accepted as a possible route to synthesize aerogels by the introduction of spring-back effects, and freeze-drying has also been used as an effective approach because it can bypass the triple points of solid/liquid/gas, in which the solvent is frozen and then sublimated under vacuum.4,16

There is a long slow-development-period of aerogels, more than a half century, since its first discovery in 1931,14 due to the synthetic difficulty and lack of applications of aerogels. However, aerogels are booming in the past decade, and a large number of new aerogels have been designed and synthesized. Fig. 1 shows the percentage of the most studied aerogels. Graphene aerogels,19–28 carbon nanotube (CNT) aerogels,29–35 and carbon aerogels36–39 are the most investigated topics in the past decade; more than 60% of the literature studies are titled by these carbon materials. Polymer based material and biomaterial (e.g. cellulose) aerogels40–45 is another hot topic, which account for 17.8% of the total literature studies. Among the polymer aerogels, polyimide aerogels,46–50 supramolecular aerogels,51–56 and conducting polymer aerogels57–60 have gained increasing interest in recent years due to their exceptional properties and attractive applications in the field of energy and sensors. These new aerogels significantly improve the application of aerogels in the field of energy and biomedicine. Nevertheless, remarkable improvement of the most traditional silica aerogels has also been achieved and they have received continuous attention,61–71 and silica aerogels are the only large scale commercially available aerogel materials.72

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Fig. 1 Percentage of aerogels based on chemical components, data are summarized on web of science from 2010 to May 2020.

Challenges still remain for the large scale production and application of aerogels. Though various new synthetic approaches including ambient pressure drying73–76 and freeze-drying have been widely used,77–80 pure silica aerogels still face severe mechanical problems and in short of functions. Thus, various hybrid aerogels or organic–inorganic hybrid aerogels have been designed and synthesized in hope of tailoring their physical behaviours and functions.81–85 On the other hand, polymers enjoy a variety of chemical structures and variable molecular weights,86–90 thus they are mechanically strong, conductive, stimuli-responsive, self-healing, and bio-degradable, which are possible functional building blocks for aerogels. However, the porous structure of polymer aerogels, such as pore volume, thermal conductivity and SSA, do not reach the high standard of inorganic aerogels.91–94 Interestingly, composite aerogels comprising different polymers or polymer–inorganic components always show impressive properties when compared to their native counterparts. Polymer based hybrid aerogels may exhibit an integration effect and synergy effect endowed by the hybrid components, such as biodegradability and biocompatibility. Besides, aerogels are highly porous materials similar to hydrogels, and they possess extremely high pore volume and SSA. Thus polymeric hydride aerogels would be ideal candidates for various biomedical applications. This study will focus on the most recent progress in polymer based hybrid aerogels and their applications in biomedicine. In order to give a clear insight into these hybrid aerogel materials, they are reviewed from the composition levels such as molecular-level (all the components are molecularly hybridized), molecular-aggregation-level (one component is in the aggregated state while the other is molecularly hybridized), and aggregation–aggregation-level (all the components are in the aggregated state), as illustrated in Scheme 1.

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Scheme 1 Schematic description of the polymeric hybrid aerogels with different hybridized levels.

2. Polymeric hybrid aerogels with molecular-level composition structures

Hybrid aerogels have been developed for several decades, and the hybridization process can solve the critical problems faced by single component aerogels. For instance, silica–metal oxide hybrid aerogels with a molecular-level composition structure significantly had their heat resistance enhanced from ca. 800 up to 1300 °C.95,96 Organic groups modified silica aerogels, such as Si–CH3, not only gave birth to superhydrophobic silica aerogels, but also introduced a spring-back effect (the –CH3 groups show mutual repulsion and the shrunk pores are recovered) that led to the discovery of APD methodology, which greatly promote the development of aerogel materials and it became one of the most frequently applied method to produce aerogels.73–76,97,98 Small molecular bridged silica aerogels were also prepared by in situ modification of the silica backbone or by using bridged silica precursors, leading to the production of robust silica based hybrid aerogels.99–103 The polymer based hybrid aerogels, however, possess the probability to show functions derived from all the components.

Polymeric hybrid aerogels with molecular-level composite structures could be synthesized simply by mixing two or more polymers together. For instance, alginate–chitosan aerogels were prepared by a water-in-oil emulsion gelation process, followed by supercritical drying;104 the molecular weight and content of chitosan were varied and could influence the textural and in vitro characteristics of the aerogels. The polymeric hybrid aerogels were build up with nanofiber networks and possessed mesoporous structures with a high SSA (162–302 m2 g−1) and pore volume (1.41–2.49 cm3 g−1). By using CaCl2 or CuSO4 as physical cross-linkers, alginate–metal ion hybrid aerogels were synthesized.105 These aerogels showed a nanofibrous morphology and possessed a precise cylindrical shape. The same method was used to synthesize alginate–chitosan hybrid aerogels, which showed a hybrid morphology characterized by the chitosan and alginate and were homogeneously distributed in the gel structure.105 The BET SSA of the Ca–alginate aerogel was up to 296 m2 g−1, while that of Cu–alginate and alginate–chitosan aerogels was in the range of 19–111 m2 g−1 and 127–192 m2 g−1, respectively. The merits of molecular-level hybrid aerogels are as follows: (1) there is no observable interface between two or more components (no phase separation), (2) the morphology of the aerogels could be tailored by one of the composites, and (3) functions could be introduced by the components.

Cyclodextrin (CD) based supramolecular aerogels could be considered as another hybrid aerogels with molecular-level composite structures, in which CDs are threaded on guest molecules in the molecular-level. Organic branched CD polymer gels could be transferred to hybrid aerogels with multi-scale porous structures and the Young's modulus reached as high as 166 MP.53 Furthermore, by using CD-based polyrotaxane as stimuli responsive building blocks,106–110 where the polyrotaxanes were end-capped by thermal responsive polymers, thermal responsive CD – Pluronic F127 – poly(N-isopropylacrylamide) (PNIPAAm) hybrid aerogels were synthesized. Fig. 2 shows the synthetic approach of the hybrid aerogels and their stimuli-responsive behaviour. The aerogels were hydrophilic (with a contact angle smaller than 90°) at low temperature (20 °C) and they became hydrophobic (with a contact angle higher than 90°) at high temperatures (50 °C); the reversible transition in hydrophobicity was introduced by the PNIPAAm component.55 Poly(ethylene oxide) (PEO)-CD hybrid aerogels possessed binary crystal structures with a channel-type crystalline domain and a PEO domain. The porous structure endowed the hybrid aerogel with excellent thermal insulation properties, while the PEO is a phase change material with high latent heat, and thus the hybrid aerogel exhibited multi-functions such as super thermal insulation and phase-change-materials for energy storage.54

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Fig. 2 CD-Pluronic F127-PNIPAAm hybrid aerogels with thermal responsive behaviour introduced by PNIPAAm.55 Copyright©2018 American Chemical Society.

Since most of the native polymers could not be directly gelled or there was no phase separation in their hydrogels, their corresponding aerogels could not be produced either by supercritical drying or by APD.54 The design of polymeric hybrid aerogels by using one polymer as support could not only solve the problem, but also introduce functions using the other components. For instance, conductive hybrid aerogels based on poly(aniline-co-m-phenylenediamine) have been synthesized by Milakin by using poly(vinyl alcohol) (PVA) as support.111 The conductivity, SSA, and tensile modulus of the hybrid aerogels were 1 × 10−3 S cm−1, 23.1 m2 g−1, and 255 kPa, respectively. Conducting polymer, poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) based hybrid aerogels supported by cellulose were designed and synthesized by Fabiano et al.112 The PEDOT:PSS played the role of an efficient infrared solar absorber, and a 2 mm thick aerogel was sufficient to absorb over 90% of energy in the solar spectrum, which led to a high water evaporation rate up to 1.61 kg m−2 h−1. Fig. 3 shows hybrid aerogels containing three different types of polymers with the molecular-level composition structure, including cellulose, polyvinylpyrolidone (PVP), and polypyrrole (PPY).113 Cellulose nanofibers with diameter ca. 5 nm were used as support to form the porous network. The electrical conductivity of the aerogels ranged from 0.1 to 6.23 S cm−1 with an optimal value of 5.21 S cm−1 and a 98.7% desirability. FT-IR results confirmed the formation of PPY and PVP on the surface of the cellulose.

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Fig. 3 Fabrication reaction of conductive polymer hybrid aerogels comprising cellulose–polypyrrole–polyvinylpyrrolidone.113 Copyright©2020 Elsevier Ltd.

Polymer–inorganic hybrid aerogels could also be prepared with a molecular-level composite structure. One of the impressive studies is proposed by Kanamori et al. using vinyl group functionalized silica precursors such as vinylmethyldimethoxysilane (VMDMS) and vinylmethyldiethoxysilane (VMDES).114–120 These monomers could be polymerized by radical polymerization to obtain polyvinylpolymethylsiloxane, which possessed plenty of alkoxy groups that could further undergo hydrolytic polycondensation to form a polymer and silica interpenetration network. This network was robust and could be APD dried directly from alcohol without any modification or additional solvent-exchange process. The resulting hybrid aerogels showed a homogeneous, tunable, highly porous, and doubly cross-linked nanostructure. The synthetic process and molecular structure of the precursors and the network are illustrated in Fig. 4. These methods were of low-cost and are highly scalable, and the hybrid aerogels exhibited excellent mechanical properties and superhydrophobicity. The aerogels were super-elastic, super-insulating, transparent, super-compressible, and superhydrophobic.114 Most recently, the hybrid aerogels have been further modified with conducting polymers such as PPY,118 and the tri-component hybrid aerogels have been prepared by ambient pressure drying. These hybrid aerogels exhibited a tunable bulk density of 40 to 210 mg cm−3, pore sizes of 10 nm to 15 μm and particulate sizes of 20 nm to 3 μm. Besides, the incorporation of PPY allowed the resulting hybrid aerogels to be electrically conductive, which could be used to light up a light-emitting diode (LED) in a circuit connected with a battery, showing strain-sensitive conductivity.

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Fig. 4 Polymer–silica hybrid aerogels prepared by ambient pressure drying.114 Copyright©2018 American Chemical Society.

Polymer–silica hybrid aerogels could also be obtained by alternative approaches. Jung et al. recently synthesized hybrid aerogels containing three different polymers, namely poly(styrene) (PS), poly(methyl methacrylate) (PMMA), and PS-co-PMMA, by a two-step process including a sol–gel reaction to form a vinyl silica network, and radical polymerization to form a silica network. The introduction of polymers into the silica network not only enhanced the thermal stability, but also increased the static dielectric constant.121 Silica–silk fibroin hybrid biopolymer aerogels were synthesized via a versatile one-pot aqueous-based sol–gel process, where tetraethyl orthosilicate (TEOS) was hydrolysed and assembled with silk fibroin, followed by unidirectional freeze-casting and supercritical drying.122 The hybrid aerogels possessed a honeycomb-shaped micromorphology with a hierarchically organized porous structure. The porosity of the hybrid aerogels ranged from 91 to 94%, and the Young's modulus reached up to 7 MPa (higher than 3 orders of magnitude improvement compared to their native aerogel counterparts); thus the aerogels are potential candidates for biomedical applications. Besides monolithic hybrid aerogels, polymer–silica hybrid aerogels in the form of nanofibers123 and microspheres124 have also been synthesized by electron spun and emulsion gelation processes, respectively. These low dimensional aerogels could be utilized in interesting applications in would healing or drug delivery, which will be discussed in the following sections.

3. Polymeric hybrid aerogels with molecular-aggregate-level composition structures

Hybrid aerogels with molecular–aggregate-level composition structures are the ones in which one of the components is in the aggregated state while the others are incorporated in the molecular level. Therefore, there is no obvious large phase separation between two or more components, and they form uniform frameworks for aerogels similar to that of hybrid aerogels with a molecular-level composition structure.

One of the significant recent progress in this field is polymer–silica hybrid aerogels via surface initiated atom transfer radical polymerization (ATRP), in which a silica porous network has already been formed (the aggregated domain is formed) and the monomers are polymerized by initiation from the activated surfaces of the silica pores. For instance, Khezri et al.125 first synthesized a 3-(trimethoxysilyl)propyl methacrylate (MPS) modified silica aerogel network, and then PMMA was incorporated by grafting through polymerization of methyl methacrylate monomers via in situ simultaneous reverse and normal initiation technique for ATRP. However, there was no information about the mechanical properties of the hybrid aerogel, but a decrement in Tg values from 85.6 to 76.8 °C was observed for the hybrid aerogels. Similar PMMA modified silica aerogels were prepared by Loy et al.,126 and the hybrid silica aerogels were strengthened by surface initiated ATRP of MMA and ethylene glycol dimethacrylate (EGDMA). Tetramethoxysilane (TMOS) was copolymerized in methanol with 2.67 mol% of the ATRP initiator, 3-(triethoxysilyl)propyl-2-bromo-2-methylpropanoate, to afford the formation of a wet gel network. The molar ratio of MMA and EGDMA was 1[thin space (1/6-em)]:[thin space (1/6-em)]1, highly cross-linked poly(MMA-co-EGDMA) was grown from the surface of the aggregated silica particles, which results in the wet-gel. After supercritical carbon dioxide drying, the resulting hybrid aerogels were significantly stronger than native silica aerogels or PMMA–silica hybrid aerogels of the same density. Other polymers, such as poly(styrene-co-MMA), poly(styrene-co-butyl acrylate), and PS, have also been incorporated with silica via the ATRP process to obtain modified aerogels.127–129

Similar polymer–silica hybrid aerogels prepared by surface initiated living polymerization, such as reversible addition fragmentation chain transfer (RAFT), have been synthesized. To an already formed silica network, poly(butyl acrylate) (PBA) and PBA-co-poly(ethyleneglycoldimethacrylate) (poly(BA-co-EGDMA)) were grafted from the silica network via the surface-initiated RAFT technique.130 The hybrid aerogels, poly(BA-co-EGDMA) silica aerogels, synthesized in this work show several advantages over the native and PBA reinforced silica aerogels, a 36 times increase in compressive strength with only a 2-fold increase in density. The changes in thermal insulation and microstructural morphologies were also negligible. The polymer–silica hybrid aerogels prepared by traditional radical polymerization do not guarantee chemical linkage between the polymer and silica network because polymers may self-polymerize in the solution, and they may not be considered as aerogels with the aggregate–molecular-level composition structure. However, the silica aerogel/PMMA composites reported by Li et al.131 showed static electrical phase interfaces and they investigated load transfer across the interfaces. The load transfer through the static electrical phase interfaces in the hybrid aerogels was found to be effective, and the static electrical interfacial interaction between PMMA and the silica network was strong enough to support the aerogel skeleton. The thermal insulation of hybrid aerogels was also found to be more excellent than that of other composite aerogels, which the author attributed to the static electrical phase interfaces with high thermal insulation.

Other polymer–silica hybrid aerogels, such as nanofibrous silica–polymer hybrid aerogels for sustained drug deliveries, have been synthesized by the incorporation of methyl group functionalized KCC-1 nanofibrous silica microparticles into a PVA and poly(acrylic acid) (PAA) aqueous solution, and finally dried by the freeze-drying method.132 The solid-state esterification reaction between PAA and PVA produced a highly entangled structure, which could hold the silica microparticles in the network. This methodology did not affect the chemical groups on the silica surface, and as a result the adsorption features of the silica network were not much affected after the formation of the hybrid aerogels. The aerogel materials have been shown to deliver hydrophobic drugs in a controlled manner. Opposite approaches are also possible for the construction of polymer-silica hybrid aerogels. For example, silica–polymer hybrid aerogels were prepared by a two-step approach involving a co-hydrolysis/co-condensation process of TEOS and a cross-linking poly(butyl methacrylate (BMA)-co-BA) nanoparticles process. The average diameters of the polymer nanoparticles ranged from 32 to 132 nm. They acted as nucleation sites for the silica network. The porous structure of the polymeric hybrid aerogels was significantly modified by the presence of polymer nanoparticles.133

Polymer based hybrid aerogels with other components in the form of aggregate–molecular-level such as graphene oxide, clay, and nanoparticles have also been widely synthesized and applied. For instance, graphene oxide (GO)–collagen hybrid aerogels with different GO concentrations for enhanced mechanical properties and for in situ bone regeneration were synthesized using a sol–gel process. The hybrid aerogels exhibited a unique, folded microstructure, and their elastic modulus was enhanced with the increment of GO concentration.134 GO–gelatin hybrid aerogels were synthesized by a two-step process, where a GO aerogel was first prepared and gelatin solution was then filled in the GO aerogels followed by a thermal-induced phase separation process. The SEM images indicated that no obvious phase separation between GO and gelatin was observed, they formed the porous framework together, and the mechanical strength of the gelatin aerogel and the GO–gelatin hybrid aerogel was approximately 1.3 MPa and 22.6 MPa, respectively. The results indicated that a significant increase in the elastic modulus of the hybrid aerogels has been achieved.135 Chitosan–carboxymethyl cellulose–GO triple hybrid aerogels were synthesized by using calcium ions (Ca2+) as the cross-linker. Due to the pH-sensitive behaviour of the polysaccharides, the hybrid aerogels were potential pH-stimuli responsive drug delivery carriers.136

Polymer–gold nanoparticle (AuNP) hybrid aerogels were prepared by a simple freeze-drying method as shown in Fig. 5. By taking advantage of the high SSA of the highly porous structure and the conductivity of AuNPs, the hybrid PNIPAAm–AuNP hybrid aerogels showed high sensitivity to water molecules. Interestingly, the hybrid PNIPAAm–AuNP aerogel could be used as a humidity sensor to detect human breath in many different situations.137 Chitosan–polybenzoxazine-clay hybrid aerogels were prepared by freeze-drying of a hybrid aqueous solution containing various sodium montmorillonite (Na-MMT) colloidal dispersions and chitosan–benzoxazine polymer blends. These aerogels possessed ultra-low densities (<0.073 g cm−3) and high porosities (higher than 93%). The decomposition temperatures of the hybrid aerogels were significantly enhanced with the introduction of clay, implying superior thermal stability of these hybrid aerogels. Due to the thermal ring-opening polymerization of benzoxazine, the effect of heat treatment on dimensional stability and water absorption of the chitosan–benzoxazine hybrid aerogels has been investigated. The results demonstrated that the polymerized hybrid aerogels were highly stable not only in neutral water, but also in acidic medium. The obtained hybrid aerogels were efficient water absorbents with relatively high water absorption up to 2900 wt% under ambient conditions.138

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Fig. 5 Schematic illustration of the fabrication of hybrid PNIPAm/AuNP aerogels and their application in humidity sensors.137 Copyright©2018 American Chemical Society.

4. Polymeric hybrid aerogels with aggregate-level composition structures

Polymeric hybrid aerogels with aggregate-level composition structures are those hybrid aerogels with large phase separation structures that could be observed between the different components. These structures can be easily prepared as compared to the aforementioned two types of polymer based hybrid aerogels. Most of the silica composite aerogels could be classified into this type. Polymer based composite aerogels, however, enjoy a wide range of possibilities because two different polymers may also form such a composite structure,47 in which two different polyimides, poly[4,4′-(4,4′-isopropylidenediphenoxy)-bis(phthalic anhydride)-co-p-phenylene diamine] and poly[biphenyl-3,3′,3,4′-tetracarboxylic dianhydride-co-2,2′-dimethylbenzidine], were segregated and self-assembled to give a phase separated microstructure similar to that of a lotus structure, and the hybrid polyimide aerogels were superhydrophobic.

Silica-polyester hybrid aerogel blanks are the most typical example for the aggregate-level composite structures, as shown in Fig. 6; the phase separation between the silica and polyester is obvious and could be clearly identified. The polyester phase was represented by interconnected fibers and the silica aerogels by irregular microparticles.139 The polyester fibers were available in industrial scale and they were processed on a laboratory scale to produce nonwoven fabrics. Silica–polyester hybrid aerogel blankets were prepared by a two-step sol–gel process derived from TEOS, which was followed by APD drying. The porous structure and properties of the hybrid blanket could be controlled by various synthesis conditions. It was shown that synthesis conditions had a strong effect on the morphology of the resultant hybrid aerogels. Uniform distribution of the fibers coated with silica aerogel microparticles was observed for the sample prepared with higher catalyst and water contents. Improvement in the coating ratio on the fiber surface was observed with an increase in the sol volume from 14 to 24 ml. Silica–polybutadiene (PB) hybrid aerogels using silane functionalized PB latex nanoparticles were also prepared in order to enhance the physical and mechanical properties of the native silica aerogels. This can be controlled by varying the synthesis conditions.140 The effect of the silica sol concentration, pH of the mixture, and the silane grafting degree of the polymer latex on the structure, morphology and mechanical properties of the hybrid aerogels were investigated. Silane-modified PB particle hybrid aerogels were prepared by using TEOS as the silica precursor through a two-step acid–base catalyzed sol–gel process followed by APD drying. The hybrid aerogels with different PB contents (0–70 wt%) were synthesized. The hybrid aerogels containing up to 30 wt% of PB showed lower densities (0.19 to 0.24 g cm−3) and higher SSA (763 to 461 m2 g−1) with little volume shrinkage. The hybrid aerogels possessed compression strength varying from 262 to 14[thin space (1/6-em)]450 kPa, and the results indicated that increasing the silane grafting degree of PB improved the compression strength while the porosity could be well preserved.

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Fig. 6 SEM images of fabrics and aerogel blankets.139 Copyright©2019 Elsevier Ltd.

Polypropylene (PP)–silica hybrid aerogels in order to overcome the brittleness of the native silica aerogels were designed and produced by thermally induced phase separation and a supercritical CO2 drying method.141 Silica aerogels were formed onto a PP scaffold using a two-step sol–gel process with methyltrimethoxysilane (MTMS) as the silica precursor. Enhancement of the mechanical properties of the PP–silica hybrid aerogels was obtained compared with a native MTMS-based silica aerogel. While compared with the PP monolith, enhanced surface-chemical and microporous–structural properties, such as higher hydrophobicity up to 135°, pore volume up to 0.18 cm3 g−1, small average pore diameter of 12.55 nm, and larger SSA up to 57.2 m2 g−1, were observed for the hybrid aerogels. Polymer–silica hybrid aerogels with one-dimensionally aligned pores were synthesized by trapping mesoporous silica particles within a hyperbranched polymer network made from PVA and PAA.142 The synthetic approach included dispersing mesoporous silica nanoparticles in a polymer solution, freeze-drying, and high temperature activated solid-state condensation reactions between the PVA and PAA. This method produced water-stable hybrid aerogels with controlled structures such as well-aligned pores. By changing the relative amount of the mesoporous silica particles, the PAA to PVA ratio, the solid to liquid ratio, and the density of the functional groups on the internal surfaces of the mesoporous silica nanoparticles, and the bulk densities of the target aerogels were controllable.

Compared to the all-carbon hybrid aerogels,143 these hybrid aerogels containing polymers show more flexible chemical compositions and variable functions. For instance, CNT–polyimide hybrid aerogels were synthesized by using PVP as a surfactant to promote the uniform dispersion of carbon nanotubes and removal in situ during the polymerization of polyimide. The hybrid aerogels were robust, with compressive strength and compressive modulus up to 240.9 and 323.9 kPa, respectively. However, the density of the CNT–polyimide hybrid aerogel was only 32.1 mg cm−3. The polyimide endowed the aerogel with excellent heat resistance, while the CNT and porous structure endowed the aerogel with excellent electromagnetic interference (EMI) shielding effectiveness up to 41.1 dB.144 Another CNT–polyimide hybrid aerogel with the merits of super elasticity, high porosity, robust, and high-temperature resistance was prepared by freeze drying and the thermal imidization process. Due to the strong chemical interactions between the polyimide and the CNT, the hybrid aerogels exhibited versatile and brilliant sensing performance, such as wide sensing range (80% strain and 61 kPa), low detection limit (0.1% strain and <10 Pa), high sensitivity (11.28 kPa−1), fast response time, quick recovery time, and high stability (1000 cycles).145 Polyimide–MXene hybrid aerogels were prepared by freeze-drying of polyamide acid and MXene mixed suspensions, followed by thermal imidization. The hybrid aerogels showed improved compressive performance, remarkable oil absorption capacity, and efficient oil and water separation. Besides, the hybrid aerogels could completely recover their original height after 50 compression–release cycles, and exhibited good elasticity and fatigue-resistant ability, which were almost impossible for native MXene aerogels.146

Hybrid aerogels containing up to three components have also been prepared. A 3D epoxy/GO nanosheet/hydroxylated boron nitride (EP/GNS/BNOH) hybrid aerogel was prepared by a simple one-pot hydrothermal approach, followed by freeze-drying and high-temperature curing. The hybrid aerogel had a well-organized and defined 3D porous structure. The compressive strength of the polymeric hybrid aerogel was found to be remarkably higher than that of EP/GNS. The hybrid aerogels satisfied a number of critical factors: low cost, superelasticity, high compressive strength, and high thermal resistance. The results suggested that the aerogels could be potentially applied in fields of building, automotive, spacecraft, and mechanical systems.147 Polymer–metal nanofiber hybrid aerogels were synthesized by He et al. by an incipient network conformal growth (INCG) technology.58 Pyrrole (PY) was polymerized in situ on the surface of incipient 3D networks formed by silver nanowire suspension. The microstructures of the produced aerogels, such as the junction joint density and thickness of PPY, can be regulated by changing the concentration of silver nanowires, reacting time and the feeding ratio of PYs. Nearly zero temperature coefficient can be obtained by optimizing the microstructure of the aerogels. The resulting products could be used as piezoresistive sensors with high sensing stability, high sensitivity, short response time and low minimum detectable pressure (4.93 Pa).

5. Polymeric hybrid aerogels for biomedical applications

5.1 Hybrid aerogels for wound healing

The alginate–chitosan aerogel fibres synthesized by Batista et al.104 presented a highly porous cotton-like structure. All the components are natural biomacromolecules, which are non-cytotoxic and show excellent biocompatibility. Interestingly, the aerogel fibers showed a strong antibacterial activity, promoted cell migration, and exhibited similar performance in cell experiments when compared to the commercial medical device (Kaltostat®), which were good candidates for wound healing applications. Calcium–alginate hybrid aerogels augmented with zinc and silver provided the next generation superabsorbent medical scaffold for wound curing. The hybrid aerogels showed high cell and tissue tolerability, and exhibited controlled drug release rates during wound healing. Raman et al.148 had addressed the synthesis and metal detection of alginate hybrid aerogels. Three cations have been introduced into the alginate aerogel (Fig. 7). Silver addition led to an increase in aerogel zinc content while the calcium incorporation did not affect. All three metal ions were released into supernatants after the swelling of aerogels, comparable to those happened in human and animal injury models. In vitro bioactivity studies showed that Zn-enriched swelling supernatants can suppress NO production in stimulated RAW 264.7 macrophages, suggesting an effective anti-inflammatory performance.
image file: d0sm01261k-f7.tif
Fig. 7 Fabrication of the alginate hybrid aerogels and metal detection.148 Copyright©2019 Elsevier Ltd.

The incorporation of a polymeric support into a hierarchical structure also leads to great performance for biomedical applications. Cai et al.149 designed and synthesized a chitin nanoparticle based freestanding hydrogels via an electro-assembly process. These hydrogels can be further dried to obtain highly porous and tough hybrid aerogels for wound healing. The electro-assembly was a simple, straightforward, and controllable approach, which was found to depend on the pH of the chitin nanoparticle suspension and the degree of deacetylation of the chitin. The electro-assembly of chitin nanoparticles is reversible due to the physical assembly feature. By using supercritical CO2 drying and freeze-drying, chitin based aerogels and cryogels were synthesized. Due to their large SSA, interconnected porous structure, and enhanced hydrophilicity, the chitin aerogels were demonstrated to accelerate the healing of wounds.

Chitosan and chondroitin sulfate (ChS) hybrid aerogels were prepared from aqueous suspension of chitosan and ChS and were designed as healing agents in order to reduce the amount of materials applied to wounds via the reduction of electrostatic potential and avoiding covalent cross-linkers.150 The hybrid aerogels synthesized under these conditions were biocompatible and showed specific characteristics for the induction of wound healing. The aerogels did not affect the metabolic activity of cultured 3T3 fibroblasts and the biochemical parameters of the experimental animals. Because of the hydration properties, rapid adaptation to the wound bed and the early accelerator effect of wound closure were achieved. These chitosan–ChS hybrid aerogels were functional inducers of healing and they were useful as safe, inexpensive, and easy to handle materials for skin chronic wounds.

Wheat grass bioactive-reinforced collagen hybrid aerogels as potential materials for collagen turnover and angiogenesis for wound healing have been demonstrated by Kiran et al.151 The reinforcement of wheat grass bioactivities in collagen aerogels with improved physicochemical and biomechanical performances was observed because of the intermolecular interaction between the wheat grass and collagen fibrils. DSC demonstrated an enhanced denaturation temperature. Besides, the hybrid aerogel possessed increased water absorption ability and substance permeability, which enabled the channels of various agents for cellular growth. Besides, the cumulative effect of the growth factors in wheat grass and the collagen molecule augments the angiogenic ability and collagen production of the hybrid aerogels. The performances were demonstrated by in vivo wound healing assays, and the results indicated that the hybrid aerogels were biocompatible, biodegradable, and nonadhesive, and were suitable for applications in wound healing.

5.2 Hybrid aerogels for bone regeneration

The bone graft substitute is a promising method for repairing large bone loss. Aerogels made from natural polymers were attractive materials for synthetic bone graft due to their high porosity, large pore volume, variable pore size, and excellent biocompatibility. Nevertheless, the mechanical properties of natural polymer aerogels are poor for bone repair. To solve the problem, Xu et al.134 introduced GO to synthesize hybrid aerogels for bone regeneration. In their work, they developed a highly porous aerogel consisted of GO and type I collagen (COL) by a sol–gel process. The results indicated that the GO–COL hybrid aerogels were highly porous and hydrophilic. The compressive modulus of the GO–COL hybrid aerogels was improved with the increase of GO concentration. The In vitro experiment showed that the hybrid aerogels with 0.1% GO–COL exhibited better biomineralization rate and cell compatibility than other compositions. A better bone repair effect was obtained in the 0.1% GO–COL hybrid aerogels than the native COL aerogel in rat cranial defect models in a vivo study.134

Ultralight hybrid aerogels composed of electrospun PLGA–collagen–gelatin and Sr–Cu codoped bioactive glass fibers have been synthesized. Heptaglutamate E7 domain specific BMP-2 peptides were incorporated in the hybrid aerogels. Their potential in cranial bone defect healing has been investigated.152 The hybrid aerogels were made into 8 mm × 1 mm (diameter × thickness) size. A sustained release of E7-BMP-2 peptide from the hybrid aerogels was achieved and it significantly enhances bone healing due to the biodegradable properties. Histomorphometry and X-ray microcomputed tomography confirmed greater bone volume and bone formation area in which the E7-BMP-2 peptide were loaded in the hybrid aerogels. Moreover, histopathology data divulged a near complete degradation of the hybrid aerogels, and improved vascularization of the regenerated tissue was obtained.

PVA–silica hybrid aerogels fibers were synthesized via an electrospinning sol–gel method, and their potential application in bone tissue engineering was investigated in terms of in vitro bioactivity by Wang et al.153 When the PVA concentration was 8%, the resulting PVA–silica hybrid aerogel fibers exhibited a continuous and uniform structure. The average fiber diameter was 542.9 nm, and the silica phase was uniformly distributed in the PVA framework, and its actual composition was close to the theoretical value. After 3 days of immersion in a simulated body fluid, the PVA–silica hybrid aerogel fibers showed the precipitation of lamellar apatite crystals, reflecting their in vitro bioactivity and potential for bone regeneration.

Chitosan–silica hybrid aerogel membranes were synthesized by using a sol–gel process and their applications in guided bone regeneration were investigated in terms of their in vitro cellular activity and in vivo bone regeneration ability.154 The silica aerogels were dispersed in the chitosan network in the nanoscale. The hybrid membrane exhibited good mechanical properties and excellent in vitro bone bioactivity. Osteoblastic cells could adhere well and grow actively on the hybrid aerogel membranes. The alkaline phosphatase activity of the cells was also higher on the hybrid aerogel than that on the native chitosan membrane. The in vivo study in a rat calvarial model indicated significantly improved bone regeneration using the hybrid aerogel membrane compared to that using the native chitosan. Histomorphometric analysis performed 3 weeks after implantation showed a fully closed defect in the hybrid aerogel, whereas there was only 57% defect closure in the native chitosan.

5.3 Hybrid aerogels for drug delivery

The methyl group functionalized KCC-1 nanofibrous silica microparticles and PVA–PAA hybrid aerogels can load and deliver camptothecin (CPT), a superhydrophobic anticancer drug with antitumor activity but limited clinical application due to its low water solubility.132 The hybrid aerogels showed a sustained release model of CPT for more than 14 days. The drug release profile was variable by changing the relative amounts of PVA, PAA, and KCC-1. The aerogels were biocompatible, such as with immortalized human epithelial (HaCaT), African green monkey kidney (Vero), and murine fibroblast (L929) cells. When loaded with CPT, they showed potent antitumor activity against various cancer cells such as HeLa (HPV18-positive), SiHa (HPV16-positive) and C33A (HPV-negative), significantly inhibited cell growth.132

pH-sensitive hybrid aerogels composed of chitosan, carboxymethyl cellulose (CMC), and GO showed excellent biocompatible and environment friendly properties, making them potential candidates for drug loading and delivery. Because chitosan and CMC are pH-sensitive polysaccharides, the release of 5-FU from the chitosan/CMC/Ca2+/GO-5-FU is also pH-responsive. The controlled release of 5-FU as well as the swelling ratios (SR) of the hybrid aerogels at different pH values indicated that the hybrid aerogels showed the highest cumulative release (68%) and the largest SR value (20.52) at pH 7.4. The results also showed that GO played an important role in the sustained release of 5-FU from the hybrid aerogels. The release kinetics of the hybrid aerogels was followed by Fickian diffusion. The findings in this work opened a new avenue for the fabrication of drug carriers from polysaccharides for controlled drug carriers.136 The PVA–PAA hybrid aerogels loaded with mesoporous silica nanoparticles can serve as good carriers for hydrophobic drugs. The potential application of the hybrid aerogels as drug carriers was demonstrated by using a hydrophobic, anti-inflammatory agent dexamethasone (DEX) as a model drug. Because of their hydrophobic pores, the hybrid aerogels showed excellent drug loading capacity for DEX. The encapsulation efficiency of the aerogels was higher than 75% and the release behaviour of the DEX was highly tailorable (it can be faster or slower on demand) simply by changing the PVA-to-PAA weight ratio in the precursors. The aerogels also showed a sustained drug release for over 50 days or more.142

Organic–inorganic hybrid aerogel nanoparticles using dextran (Dex) and dextran aldehyde (Dex–CHO) as organic polymers for the coating of inorganic silica aerogels were designed and synthesized. The Dex and Dex–CHO served as enzyme-triggered and colon targeted 5-fluorouracil (5-FU) delivery materials.155 The drug loading and Dex/Dex–CHO coating efficiency could be improved by the functionalization of the silica aerogel surface by 3-(aminopropyl)triethoxysilane (APTES) as shown in Fig. 8. It was confirmed that the release of 5-FU from the Dex and Dex–CHO coated silica hybrid aerogels in simulated gastric and intestinal fluids was 1.7% and 3.4%, respectively, while the amount of 5-FU released from uncoated silica aerogels under the same conditions was 86.4%. The MTT assay results of the unmodified and modified hybrid aerogels did not show remarkable cytotoxic effect on Caco-2 cells, but exhibited a decrease in the viability of Caco-2 cells. The results indicated that Dex and Dex–CHO coated silica aerogels were biocompatible nanoparticles that were not affected by the upper gastrointestinal regions and were ideal enzyme-triggered drug carriers for drug targeting to the colon area.

image file: d0sm01261k-f8.tif
Fig. 8 Schematic representation of synthesis of spherical silica aerogels and the corresponding modification and drug loading process.155 Copyright©2020 Elsevier Ltd.

A multi-layer composite (MLC) consisting of a PVA film substrate and drug loaded silica aerogel powders was prepared. It had been used as a controlled drug carrier for fluconazole. The silica aerogels were synthesized at ambient pressure and loaded with fluconazole before incorporation with PVA. The results of the drug release experiment indicated that the hybrid aerogels had a faster release rather than pure fluconazole, possibly due to the high SSA (>800 m2 g−1) and high porosity (>80%) of the aerogels. The PVA nanofibers were found to control the fluconazole release rate. The silica–PVA hybrid aerogels exhibited the non-Fickian mechanism for the drug release.156

Coated and uncoated silica–alginate hybrid aerogel beads have been prepared by an emulsification/internal setting method.157 The hybrid aerogels combined high SSA and high mechanical strength of the silica component with biodegradability and bioadhesivity of the alginate fraction, which were promising materials for drug delivery. The particle size and SSA of the hybrid aerogel beads were 282 um and 900 m2 g−1, respectively. Preliminary tests demonstrated that the silica–hydroxypropyl methylcellulose or hydrophobic silica coating affected the loading and release rate of the model drug ketoprofen. The coated hybrid aerogel beads showed a prolonged release up to 60 min.

It is worth noting that aerogels can be obtained as 3D bulk monoliths, 2 dimensional (2D) films, 1 dimensional (1D) fibers, and 0 dimensional (0D) particles. It seems that the 0D aerogels particles are more suitable for drug delivery because the diffusion and release of drugs from the micro-size aerogel particles is relatively more efficient than that of macro-size monoliths. Moreover, only the micro- or nano-size aerogel particles can be circulated in the body, which is a prerequisite for controlled and target delivery.157 For the application of aerogels in tissue engineering, wound healing, and regeneration, fibrous aerogels and films of micro-size would be superior to the macro-size bulk aerogels.104,153,156 However, when aerogels are used as a scaffold for tissue engineering, the dimensions and sizes of the aerogels are dependent on which tissue they will be applied to as the matrix.

5.4 Hybrid aerogels for tissue engineering

The GO–gelatin hybrid aerogels prepared by Zeinali et al.135 could be used as a scaffold for nerve tissue engineering. For the In vitro experiment, P 19 mouse cells were cultured and differentiated into nerve cells on the hybrid aerogels, followed by monitoring the immunofluorescence test. Cell differentiation was about 20 and 87% on the control and the aerogel surface, respectively. The P 19 cells were effectively differentiated into neural cells when the aerogel was used as the scaffold. The results indicated that the hybrid aerogel was a suitable matrix for neural tissue engineering. A highly porous collagen–alginate–GO aerogel was designed and synthesized as an extracellular matrix to promote cell migration, cell attachment, and cell proliferation.158 The hybrid aerogels showed a highly porous interconnected network with a nonporous external wall. The chemical functional groups of collagen and GO were demonstrated to be maintained after the supercritical CO2 drying. The SEM images showed that the collagen–alginate aerogels could promote cell attachment and proliferation, indicating that the alginate–collagen aerogel could be an ideal matrix for tissue engineering (Fig. 9).
image file: d0sm01261k-f9.tif
Fig. 9 Schematic illustration of the preparation of the PDA/rGO aerogel, and its application for skeletal muscle regeneration in vitro and in vivo.159 Copyright©2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

The skeletal muscle is sensitive to electrical stimulation, so materials are required to transmit electrical signals while maintaining elasticity in order to build an efficient cellular communication network. For this purpose, polydopamine (PDA)–reduced GO (rGO) hybrid aerogels were synthesized by a multistep base reduction process including the tannic acid functionalized methacrylate group (TA-MA), dopamine, and hydrothermal reduction.159 This bio-inspired PDA–rGO hybrid aerogel possessed high conductivity, good electromechanical stability, and ideal Young's modulus, which were needed for the growth and differentiation of C2C12 myoblasts. After the cell-free PDA-rGO aerogel-transplanted denervated muscle was loaded with cyclic electrical stimulation for three weeks, the mean muscle fiber size improved by 90%, and the maximum contraction force of the denervated muscle was increased by 50%, while few inflammation infiltrations were observed. The results indicated that the PDA-rGO aerogel was a safe and effective material for retarding the disuse muscle atrophy.

5.5 Hybrid aerogels for other biomedical applications

Polymer–AuNP hybrid aerogels have been prepared by Chen et al.137 by taking advantage of the conductivity of AuNPs and high SSA of the highly porous framework. The introduction of PNIPAAm in the hybrid aerogels showed high sensitivity to water molecules. The hybrid PNIPAAm–AuNP aerogels could be used as humidity sensors to detect human breath in different situations, such as normal, fast, or deep breath. Persons under different conditions such as illness, smoking, and normal can be detected. The system was promising for practical flexible wearable devices for human health monitoring. Besides, the humidity sensors could be used in tune recognition.137

By monitoring the interaction of dorsal root ganglia neurons on coated polyurea–silica hybrid aerogels, Sabri et al.160 demonstrated that the hybrid aerogel showed the ability to promote the growth and differentiation of the root ganglia neurons. From three different coatings tested (e.g. poly-L-lysine, BME, and laminin), laminin was proved to be the most effective surface modification agent for attachment, confinement, and growth of DRG neurons on the hybrid aerogel surface. DRG neurons can attach to the untreated surface of hybrid aerogels. The physical immobilization of DRG neurons on untreated hybrid aerogels is dependent on the 3D and porous surface of the aerogels, leading to attachment and immobilization of the cell body. The porous structure of the aerogel remained intact even after fixing treatment, which enables further studies and image analysis. The results suggest that hybrid aerogels are promising for supporting neuronal differentiation.

Chitosan-based hybrid aerogels containing Au nanoparticles were obtained under microwave-assisted conditions using biocompatible and ecofriendly reagents. These aerogels were studied inter alia over their chemical structure, morphology, biodegradability, antimicrobial activity, and cytotoxicity. The experimental results showed that the aerogels were antibacterial and bioactive, which may be successfully applied in regenerative medicine.161

6. Conclusions

In conclusion, polymer based hybrid aerogels, including polymer–polymer, polymer–carbon material, polymer–inorganic hybrid aerogels and their recent progress are introduced and discussed in view of their chemical structures and hybrid structures, as well as the relationship between the structure–properties. In this study, polymeric hybrid aerogels are classified into three different composition levels, which are molecular level, molecular–aggregate level, and aggregated level. For the molecular level and molecular–aggregate level hybrid aerogels, the components of the hybrid aerogels formed uniform building blocks, while the aggregated level hybrid aerogels showed a significant phase separation between the components. Literature studies indicate that though the chemical structures of the hybrid aerogels may be the same, the different composition levels may result in aerogels with quite different functions or properties, as well as their corresponding physical properties and potential applications. Thus, various types of polymer based hybrids and their applications, especially biomedical applications due to the combination of polymers with functional components, are reviewed. This review article could be a promising platform to provide creative design principles for aerogels by considering both the chemical and physical structures, and examples of potential applications of aerogels in tissue engineering, drug delivery, wound healing, bone regeneration, etc. have been introduced.

Since there are plenty of non-natural polymers, combined with the development of living polymerization techniques (e.g. ATRP and RAFT), well-defined polymeric hybrid aerogels with specific chemical and physical structures could be possibly produced. For instance, if double network hydrogels162–164 were converted to aerogels, a brand new polymeric hybrid aerogel must appear, possibly possessing exceptional flexibility and robustness. Stimuli-responsive, self-healing, and electronic conductive aerogels could also be produced by carefully designing polymer and porous structures. Besides, anisotropic, hierarchically porous, reversible volume change, and reversible hydrophobic–hydrophilic change aerogels could be obtained due to the great potential of polymers. Thus, it could be foreseen that by specifically controlling the microstructure and chemical structure of aerogels, ideal porous materials that can solve the critical problems in biomedical applications may be produced. Moreover, due to the introduction of specific functions and smart behaviours, the hybrid aerogels may find interesting application in wearable devices, green energy production, and smart systems.

Conflicts of interest

There are no conflicts to declare.


This work was financially supported by the National Natural Science Foundation of China (91963124, 51773225, 81671161).


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