Elastomer–Hydrogel Systems: Comparison
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Please note this is a comparison between Version 2 by Peter Tang and Version 1 by Gokhan Demirci.

Elastomers are a class of polymeric materials that can repeatedly and easily undergo large, reversible deformations with complete recovery. They are usually composed of long-chain molecules, extremely flexible due to their ability to reconfigure themselves and dissipate an applied force. Hydrogels (HGs) are macromolecular structures consisting of polymer networks with the ability to absorb water without any dissolution. By applying sophisticated design and engineering methods, various elastomer–hydrogel systems (EHS) with outstanding performance have been developed in the last decades. These systems composed of elastomers and hydrogels are very attractive due to their high biocompatibility, injectability, controlled porosity and often antimicrobial properties. Moreover, their elastomeric properties and bioadhesiveness are making them suitable for soft tissue engineering.

  • elastomers
  • hydrogels
  • elastomer–hydrogel systems
  • injectable biomaterials
  • adhesive surfaces
  • tissue engineering

1. Introduction

The structure of biological materials has been fine-tuned over millions of years of evolution. The study of live organisms to derive new principles and technologies and then apply them to man-made materials, including polymers and combined systems such as composites and hybrids, to emulate biological functions and performance, is in a central place of biomimetics (bioinspiration). Following the principles of designing materials with defined biological properties, such as bioactivity, adhesiveness, self-healing, etc., the use of polymeric materials in medicine is one of the most important application areas in restoring health, interacting therapeutically or, ultimately, replacing diseased tissues or organs. The diversity of the chemical structures, synthesis possibilities and the monomers’ origins/sources make polymers important in the controlled release of drugs, implants for tissue reconstruction, medical devices and many other areas [1,2,3,4,5][1][2][3][4][5]. Naturally occurring polymers were the first to be used in medical applications around the 1960s. The most common representatives of natural polymers are proteins, polysaccharides and polynucleotides [6]. The main advantage of these natural polymers is high biocompatibility due to their chemical structure and natural origin. Nevertheless, these polymers can cause an immune response due to the possible impurities remaining after processing. Natural material fabrication typically involves decellularization and the removal of antigenicity, to obtain a material that will not cause an immune response after implantation in the human body. Therefore, synthetic materials which are obtained via precision macromolecular engineering or well-established polymerization processes often have better properties, both physical and biological, and are therefore frequently developed and used in medical applications. They can be engineered as very complex supports for tissues and organs that exhibit organized heterogeneity within multiple cell types [7].
Elastomers are a class of polymeric materials that can repeatedly and easily undergo large, reversible deformations with complete recovery [8]. They are usually composed of long-chain molecules, extremely flexible due to their ability to reconfigure themselves and dissipate an applied force. The main feature of elastomers, also called “biomedical elastomers” or “bioelastomers” is their viscoelasticity combined with biodegradation, thus making them suitable for medical applications as drug delivery systems [9,10][9][10], biosensors [11,12][11][12], artificial organs [13[13][14],14], materials for regenerative medicine [15[15][16],16], tissue engineering [17,18,19][17][18][19] and veterinary medicine [20]. They are suitable for medical applications due to their great attributes such as a 3D-crosslinked network structure, good mechanical properties and the possibility of tailoring degradation by introducing functional groups within the structure [18].
Hydrogels (HGs) are macromolecular structures consisting of polymer networks with the ability to absorb water without any dissolution [21]. Water is held between the polymer chains, which gives the material elastic features; however, hydrogels become a rigid macromolecular tangled structure when the water evaporates. The features of HGs such as their hydrophilic character, a similar structure to tissues, ability to degrade, chemical stability and the ability to absorb an immense amount of water or biological fluids, make HGs eligible for tissue engineering and regenerative medicine (TERM) [22].
Synthetic materials were designed as structural components of medical devices with tailored stability/degradation time. However, rapid development of materials engineering has triggered the new trend to incorporate the natural polymers’ sophistication into the synthetic polymer structures, to introduce biological cues that are necessary to support or replace the targeted tissue or organ and to better understand how these features can be created more effectively [23]. Advances in manufacturing strategies have ensured additional contributions to biomaterial design [24].

2. Preparation of Elastomer–Hydrogel Systems

Elastomer–hydrogel systems (EHS) are the combination of two or more polymeric materials, commonly of natural and synthetic origin, offering remarkable properties and multifunctionalities through the combination of different structural components (Figure 1). These may be systems where a hydrogel is encapsulated by an elastomer matrix to prevent its dehydration or both components are forming interpenetrating polymer networks (IPN) which can be bonded to each other by covalent bonds or non-covalent interactions such as hydrogen bonding, van der Waals and electrostatic interactions.
Figure 1.
Schematic representation of elastomer–hydrogel systems. Hyphen (-) signs represent free electron pairs.
A wide range of EHS preparation methods such as a two-step polymerization, molecular stent, one-spot, extrusion 3D-printing and free-shapeable methods results in diverse properties of the systems, thus widening their applicability in different areas [25]. In the last decades, EHS have triggered more attention due to their specific physicochemical key properties such as enhanced mildness, solubilization, density, permeability, stiffness, low surface tension, stability, mesh size and structure. Moreover, their biocompatibility, biodegradability, non-immune response and structural similarity to the extracellular matrix (ECM) have attracted the researchers to focus on new developments in medicine. The properties of EHS can be structured through selection of their chemical composition, cross-linking strategy, structure stabilization, hydrophobicity/hydrophilicity ratio, etc. Among various groups of polymers, thermoplastic elastomers such as polyurethanes [26], poly(ε-caprolactone) copolyesters [27], poly(ether ester)s [28] and thermoset elastomers such as crosslinked polyesters [29] have been developed for heart valves and muscle applications, skin, cartilage implants, blood vessels, vascular catheter and wound dressings [30,31,32,33,34][30][31][32][33][34]. Simultaneously, hydrogels which show a physicochemical similarity with ECM and provide high-water content are considered as highly biocompatible materials. Therefore, the use of elastomers and hydrogels is increasing rapidly in medical applications [35,36,37,38][35][36][37][38]. The rational design of elastomers and hydrogels could be a solution to obtain highly functional elastomer–hydrogel systems with tailor-made elasticity and wettability while preserving or creating strong adhesion between the components or with the biological tissues. Such systems can be created by physical interactions (topological entanglements) and/or chemical bonds to create a strong interlinks between elastomer and hydrogel. Different chemicals such as silane coupling agents [39], cyanoacrylates [40] and benzophenone [41], have been used to create chemical bonds between distinctly different polymers; for instance, a silane coupling agent (SCA) introduced into the precursors of a cured polydimethylsiloxane (PDMS) elastomer and polyacrylamide (PAAm) hydrogel. After manufacturing, SCA condenses and creates bonds across the interface and improves adhesion. Bonding energy differences were investigated on various addition sequences such as cured hydrogel and cured elastomer, cured hydrogel and uncured elastomer, uncured hydrogel and cured elastomer [39]. Furthermore, there are techniques of combining these distinctly different components inspired by nature and it is proven that these strategies can be applied for the biomimetic devices and machines with a wide selection of elastomers and hydrogels.

23. Biofunctionalities of Elastomer–Hydrogel Systems

EHS are gaining increased interest for medical applications due to their unique combination of properties, often emulating live organisms’ function and performance. Some of the sophisticated properties found in biomimetic materials will be discussed with emphasis on bioadhesiveness, injectability, antibacterial properties, biodegradability and porosity which are important for tissue engineering (Figure 2).
Figure 2.
Functions and performance of elastomer–hydrogel systems.

2.1. Bioadhesiveness

3.1. Bioadhesiveness

Most of the medical applications, especially surgical procedures, require tissue adhesives, sealants, and hemostatic agents. Those bioadhesives are mostly a glue to bind the tissues, seal the gaps or cracks and initiate the formation of blood clots, respectively [121,122][42][43]. Synthetic compounds which show adhesive properties such as poly(ethylene glycol) diacrylate (PEGDA) [123][44], N,N-dimethylaminoethyl methacrylate-co-methyl methacrylate (NDMEM) [124][45], gelatin methacrylate (GelMA) [125][46], tannic acid (TA) [126][47], etc., have been successfully introduced by physical or chemical processes into the patches or scaffolds with the development of materials science. This approach has gained successful outcomes in medical applications thanks to the adhesion ability of those materials to various tissues such as soft tissue, bone and skin [127,128,129,130][48][49][50][51]. However, their lack of robust and reversible adhesion abilities limit their application efficiencies. Therefore, inspiration from nature provides enormous information on how to develop materials with versatile adhesion capacities for both wet and dry surfaces. Determination of the key compounds within the various species has opened the way to introducing these compounds into the structured materials for medical applications. Thanks to these compounds, EHS can act fully or partially as bioadhesives, depending on the functional groups introduced that are inspired by nature. EHS can be structured to achieve the desired, controllable and reusable adhesion strength in wet environments. Recently, bioinspired adhesives have attracted great attention due the combination of natural functionality realized through synthetic approaches. For instance, mussels show extremely good adhesion with high binding strength to various surfaces under wet conditions [131,132,133][52][53][54]. It was found that the catechol unit is the main factor that allows mussels to adhere to a variety of surfaces [134,135][55][56]. Materials containing catechol units can be used to create covalent and non-covalent attachments to various substrates for many medical applications, including drug delivery systems and wound healing [47,136,137][57][58][59].

2.2. Injectability

3.2. Injectability

Traditional surgeries are increasingly being replaced by less invasive methods that shorten an overall procedure and the patient’s recovery time. Especially, in tissue engineering, the focus is on to improving the materials’ performance by their injectability [138][60]. The injectable systems can efficiently deliver particles such as drugs (antibiotics, anesthetics), biomolecules (fibrin), fillers (silica nanoparticles) or genes (DNA, siRNA) [139][61]. An attractive model, developed by Li et al., is an injectable probe for measuring oxygen in tissues [140][62]. Hydrogels containing N-isopropylacrylamide copolymer macromers for mesenchymal stem cell (MSC) delivery allow the formation of bone bridges, promoting the viability of MSCs, and can be used to create hard tissues, due to gelatin microparticles (GMP) which are enzymatically digestible porogens and sites for cell attachment [141][63]. Another long-term persistent hydrogel is the photo-crosslinked material composed of a double-network of modified sodium alginate and gelatin created by the Schiff base reaction [142][64]. Collectively, different works have clearly demonstrated the huge potential of injectable materials for biomedical applications. Xu et al., produced an injectable EHS consisting of hyperbranched multi-acrylated poly(ethylene glycol) macromers (HP-PEGs) and thiolated hyaluronic acid (HA-SH) and used it as a stem cell delivery system for diabetic wound healing [143][65]. It is also worth noting that new injectable and photocurable elastomers containing fatty acid derivatives can be successfully used for minimally invasive surgical protocols in the repair of small hernia defects [144][66].

2.3. Biodegradation

3.3. Biodegradation

Biodegradable materials are now tending to become the most commonly used materials in medical applications due to their gradual bio-resorption into the human body [145][67]. Biodegradability is one of the key properties for the materials which are used in medical applications. It should be considered that the degradation rate must be consistent with the healing and regeneration process. Various crosslink densities, crosslinking mechanisms and component types were applied to control the degradation rate of such systems [146,147][68][69]. The most commonly used biodegradable materials consist of homo- or copolymers of alpha-hydroxy acids, such as lactic and/or glycolic acids. Biodegradation can be triggered either by water (hydrolytic degradation) and/or enzymes (enzymatic degradation) within the body. The chemical structure of a polymer has the greatest influence on the type of degradation. Other important factors are chemical composition, the type of crosslinking bonds, molecular weight and its distribution, porosity, stereochemistry and chain mobility [148][70]. The elastomeric part of the EHS usually tends towards hydrolytic biodegradation due to its molecular chain structures sensitive to water. The hydrolysis of ester bonds usually leads to the creation of carboxyl and hydroxyl end groups, whereas natural biomaterials tend to degrade enzymatically. The injected and/or implanted EHS can be degraded by oxidative (catalases, horseradish peroxidase and xanthine oxidase) or hydrolytic (protease, hydrolase, phosphatases, lipase and esterase) enzymes when exposed to body fluids and tissues [149,150,151][71][72][73]. Inflammatory cells (e.g., macrophages and leukocytes) create reactive oxygen species such as hydrogen peroxide, superoxide and nitric oxide during the inflammatory response to foreign materials [152][74]. EHS can be cut up by those species which are contributing to material degradation whereas the hydrolytic enzymes hydrolyze the components of the hybrid network to help in the absorption of nutrients and solutes. For instance, a poly(caprolactone) (PCL)/gelatin(Gel) scaffold (sublayer) was electrospun on a dense polyurethane (PU)/propolis(EEP) (top layer) membrane to fabricate a bilayer wound dressing. It was demonstrated that the EHS combining a synthetic polymer with a natural one could enhance the stability of the scaffold. Hydrolytic and enzymatic degradation studies showed that PU/EEP membrane exhibited a slower degradation rate in comparison with a PCL/Gel hybrid structure. In the case of hydrolytic degradation, the total mass loss after 28 days for PU/EEP and PCL/Gel was found to be 1.9 and 76%, respectively [153][75].

2.4. Porosity

3.4. Porosity

The porosity is an important feature in medical applications, especially in scaffolds [154,155][76][77]. The pore architecture and interconnectivity have a beneficial role in proliferation, cell survival and migration to create functional materials, and secrete ECM. Therefore, scaffold porosity is a must for homogenous cell distribution and interconnection throughout engineered tissues [156,157][78][79]. Additionally, pore size can have an effect on the cell growth, vascularization, nutrients and oxygen diffusion, especially in the absence of a functional vascular system [133,134,135,158,159,160][54][55][56][80][81][82]. Various techniques, components and ratios are used to obtain controlled pore size and architecture scaffolds. For instance, Kanimozhi et al. prepared a chitosan/poly(vinyl alcohol)/carboxymethyl cellulose (CP-CMC) porous scaffold by simple freeze drying and salt leaching techniques. Among scaffolds, 1:1 weight ratios showed significantly high porosity as compared to other ratios. The incorporation of CMC enhanced the scaffold porosity from 50 to 90% by increasing the molar ratio of CMC. However, when comparing the freeze-dried scaffolds and salt-leached scaffolds of 1:1 weight ratio, the 50% CP:50% CMC material showed a higher porosity of 90% in salt-leached and 70% in freeze-dried scaffolds, respectively. The reason was explained thus: with the increase of CMC ratio, the actual volume occupied by the molecules decreased [161][83]. In another study, Morris et al. produced porous elastomer–hydrogel scaffolds of chitosan/polyethylene glycol diacrylate (CS/PEGDA) using 3D bioprinting by a stereolithography method to create internal pore and macroscopic shapes. They achieved varied pore sizes by changing the CS molecular weight ratios. For instance, the average pore size of the pure PEGDA scaffolds increased from 24% to 67% by the addition of low molecular weight CS (LMWCS) (MW = 50–190 kDa) into the scaffold with the ratio LMWCS:PEGDA at 1:7.5. These kinds of studies show that controlled pore size and architecture can be achievable for specific needs in medical applications [162][84].

2.5. Antibacterial Surfaces

3.5. Antibacterial Surfaces

Antibacterial materials, especially surfaces, are playing an important role in protecting from contamination and eliminating bacteria from skin tissue and the surfaces of medical devices and implants. Bacterial adhesion is the main cause of the creation of 3D biofilm complex structures which infect the surrounding tissues. Therefore, new strategies which eliminate biofilm-based issues are applied. Hence, EHS which contain antibacterial components are being developed. For instance, Piarali et al. fabricated a fiber mesh based on the surface modification of polyhydroxyalkanoate, using an electrospinning technique, for tissue regeneration. Here, basically an EHS was created by a synthetic antimicrobial peptide with anti-biofilm and strong bactericidal properties [149][71]. In another study, Muzammil et al. created elastomer–hydrogel scaffolds containing castor-oil-reinforced chitosan with various hydrophilic polymers. The obtained EHS showed antibacterial and hemostatic activities with good mechanical properties. Therefore, such systems could be good candidates for skin tissue regeneration and wound healing applications [163][85].

3. Elastomer–Hydrogel Systems for Soft Tissue Engineering Applications

The development of advanced systems for tissue engineering applications has been widely studied over the last decades. Specific interactions between the components, the combination of raw material advantages and the molecular organization of these systems dictates the direction of the tissue engineering applications. Different EHS systems which combine different classes of elastomers and hydrogels in one material with large yield formulations and many advantages, such as high interaction with targets to enhance their performance have been effectively developed. EHS play an important role in the success of tissue engineering approaches, as they guide the structure of developing tissues, gaining mechanical and physical stability, and migrating cells or delivering the molecules to transplanted areas. Those highly efficient EHS find applications in soft, bone, skin, neural and cardiac tissue engineering [164,165,166,167][86][87][88][89]. For instance, Fischenich et al. has developed a thermoplastic elastomer (TPE) hydrogel system for soft tissues, especially for articular cartilage, the knee meniscus, etc. The created system was based on a blend of unreacted ω-hydroxy-polystyrene-b-poly(ethylene oxide) (SO) and coupled polystyrene-b-poly(ethylene oxide)-b-polystyrene (SOS). The obtained TPE hydrogel system could be a promising candidate for soft tissue replacement with a comparable equilibrium compressive modulus of approximately 0.5 MPa and shear modulus of 0.2 MPa [168][90]. Lewis et al. reported a thermoplastic elastomer–hydrogel system based on the prefabrication of an efficient nanoscale network architecture using the melt-state ω-hydroxy-polystyrene-b-poly(ethylene oxide) (SO) and polystyrene-b-poly(ethylene oxide)-b-polystyrene (SOS) as amphiphilic block copolymers. They proved by physical and mechanical analysis that the obtained systems have relevant moduli and water compositions, subsecond elastic recovery rates, negligible hysteresis, and unprecedented resistance to fatigue over hundreds of thousands of compression cycles. They suggested that such hydrogels may have tremendous promise beyond the synthetic soft tissue engineering applications for which they have been targeted [169][91]. In another study, Remya et al. synthesized EHS by modifying PCL with different molecular ratios of water soluble polymer PEO using the electrospinning technique to create scaffolds. The weight loss for pure PCL was 8.5% whereas for PCL/PEO blends with 50:50 ratios and differing the molecular weight of the PEO (10 k g/mol vs. 60 k g/mol), the weight loss was 41.7 and 48.7%, respectively after 3 months. The study also showed that the properties of PCL scaffolds such as cell viability, mechanical properties and hydrophilicity were increased by the incorporation of PEO and these materials could be possible candidates for bone tissue engineering applications [169][91].

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