Advances in interpenetrating polymer network hydrogels and their applications

Structural parameters

Structural parameters like number average molecular weight between two cross-links, M_c, cross-link density, ρ, and mesh size, ξ, defined as the maximum size of a solute that can diffuse through the network, of single network hydrogels are estimated either from the equilibrium degrees of swelling based on the Flory-Rehner equilibrium swelling theory, or by the mechanical measurements of the elastic modulus, G [15, 125–127]. As with other multicomponent polymeric systems [8], phase separation often accompanies the formation of IPN hydrogels. The phase structure and morphology of IPN hydrogels determine their physical properties and applications, and therefore it is important to control the phase-separated structures in a scale ranging from nano- to micrometer [60]. The extent of phase separation in IPN depends on several factors, some of them being very important: concentration of each polymer component, cross-linking density, and the ratio between components [5, 36, 42]. Cross-linking density strongly influences the phase morphology of IPN owing to the competition between the tendency of phase separation and its limitation by the chemical cross-links [5]. In the case of IPN hydrogels, differential scanning calorimetry (DSC) measurements could evidence two T_gs, with the T_g of individual components often shifted toward each other, this indicating a partial mixing of the networks [128]. Only one T_g has been observed in the case of PNIPAAm/Biomer [copoly(etherurethane-urea)] semi-IPN hydrogels supporting the mixing at molecular level of the components [128]. Thimma Reddy and Takahara have modulated the compatibility between components in a simultaneous IPN by the ratio between components [42]. Hernandez et al. have found experimental T_g values for PAA/PVA IPN hydrogels higher than those calculated based on the weight fractions and the T_g values of the component polymers [36]. At low PVA concentrations, the PVA and PAA in semi-IPN hydrogels have been compatible, but at high concentrations of PVA phase separation occurred [36]. The high T_g values have been ascribed to interactions between the constituent polymers of IPN which act as physical cross-linkers, in consequence reducing the segment mobility. The presence of two peaks in the DSC traces has indicated phase separation in the case of porous P(VP-co-MAA)/PNIPAAm semi-IPN hydrogels [37]. The presence of only one and narrow transition indicates molecular compatibility between chitosan and PVP in the IPN composite beads [129].

However, the evaluation of the structural parameters in a complex system like IPN hydrogels, containing three components (two polymers and water) with their interactions, has been also performed both for semi-IPN [106, 128, 130–133], and full-IPN [29, 134], mainly in the systems with a high level of compatibility, demonstrated by the presence of only one T_g.

Estimation of structural parameters from equilibrium swelling

The number average molecular weight between two cross-links, M_c, was calculated with Eq. (1) [15], taking the PEG/PAAm IPN hydrogel as an example [29]:

(1)1M¯c = 2M¯n − (υ / V1)[ln(1 − υ2, s) + υ2, s + χ1υ2, s2]υ2, r((υ2, s/υ2, r)1 / 3 − (υ2, s/2υ2, r)) (1)

where: _M¯c is the number average molecular weight between cross-links, _M¯n is the number average molecular weight before cross-linking, ν is the specific volume of the polymers (0.893 cm³ g^–1 for PEG and 0.741 cm³ g^–1 for PAAm), V₁ is the molar volume of water (18 cm³ mol^–1), and χ is the Flory–Huggins interaction parameter of polymer-water (χ_PEG = 0.426, χ_PAAm = 0.48). For the calculation of _M¯c for the IPN hydrogel, values of each parameter were obtained using the weight average of values for PEG and PAAm. υ_2,r and υ_2,s are the volume fractions of polymer in the relaxed gel and swollen gel, respectively, and are defined as: υ_{2, r}=υ_p/υ_r, υ_{2, s}=υ_p/υ_s where υ_p is the volume of polymer, υ_r is the volume of relaxed gel immediately after the gel preparation, and υ_s is the volume of swollen gel.

The mesh sizes of the networks, ξ, has been estimated with the Eq. (2) [15]:

(2)ξ = (r¯02)1/2V2, s−1/3 (2)

using the values of end-to-end distance of the polymer chain in the unperturbed (solvent-free) state calculated with Eq. (3)

(3)(r¯02)1/2 = l(2(M¯c/M¯r))1/2Cn1/2 (3)

where l is the C–C bond length, typically 1.50 Å, M_r is the molecular weight of the repeat unit, and C_n is the Flory characteristic ratio (C_n,PEG = 4.0, C_n,PAAm = 8.5) [29, 126].

Estimation of structural parameters from equilibrium shear modulus

The theory of polymer networks predicts that the equilibrium shear modulus G, for polymer gels obtained by cross-linking copolymerization can be expressed as [127, 133]:

(4)G = (1 − 2f)ρpRTM¯cυ2, s1/3υ2, r2/3 (4)

where: _M¯c —the average molecular weight between cross-links, f-the functionality of the cross-links (f = 4 for tetrafunctional cross-linkers), ρ_p—the polymer network density, R—the gas constant, T—the temperature, υ_2,s—the swollen polymer volume fraction (polymer volume fraction after swelling), and υ_2,r-the initial polymer volume fraction (polymer volume fraction immediately after cross-linking but before swelling). For diluted systems (Phantom networks), the front factor, (1–2/f), is equal to 0.5 [127, 133]. υ_2,s and υ_2,r have been calculated with Eq. (5) and (6), respectively [125]:

(5)υ2, s = (wp/ρp)/[((wT, s − wp)/ρ2) + (wp/ρp)] (5)

(6)υ2, r = (wp / ρp) / [((wT, r − wp)/ρ2) + (wp/ρp)] (6)

where ρ₂—the density of water at room temperature, ρ_p—the polymer network density (ρ_p was equal to 1.35 g/cm³ for PAAm gels and 1.25 g/cm³ for semi-IPN PAAm/DxS composite gels), w_p—the weight of dry hydrogel, w_T,r and w_T,s—the total weight of polymer after synthesis and at equilibrium swollen states, respectively.

For homogeneous network of Gaussian chains, the cross-ink density, v_e, which is the effective number of flexible network junctions per unit volume (mol/cm³), can be estimated by Eq. (7) [132, 133]:

(7)νe = ρp / M¯c (7)

The values of the structural parameters estimated by the above equations are helpful in understanding which factors most contribute to the hydrogel properties [134]. Cross-link density, v_e, reflects the effect of swelling on the modulus and provides a better basis for comparison the network structures than the modulus alone [134]. Also, as stated in the literature, the thermodynamic swelling experiments provide more reliable results for the M_c than the viscoelastic modulus [127].

Porosity

Swollen state porosity, P_s, and total porosity, P, (%) of macroporous hydrogels have been evaluated with Eq. (8), and Eq. (9), respectively [135–138].

(8)Ps = 1− qv[1+(qw−1) d2/d1]−1  (8)

(9)P = (1 − d0/d2) × 100 (9)

where: q_v=(D_w/D_dry)³ is the equilibrium volume swelling ratio, q_w=(m_w/m_dry) is the equilibrium weight swelling ratio, D_w and D_dry are the diameters of the equilibrium swollen and dry gels, respectively, m_w and m_dry are the weight of gels after equilibrium swelling in water and after drying, d₁ is the density of solvent (water), d₂ is the density of polymer [135, 137], d0 = mdry/(π Ddry2 ldry/4) is the density of porous network, where l_dry is the length of cylindrical gel in the dry state.

Controlling structural parameters

Mesh size and mechanical properties are essential characteristics considered in evaluation of hydrogels used as scaffolds in cell culture or tissue engineering. The mesh size values estimated by the methods presented above are an average of the statistical distributed pores in conventional hydrogels prepared by chain-growth polymerization. Generation of hydrogels with tailored mesh size by thiol-ene photopolymerization has been lately developed [126, 139–142]. Radical mediated step-growth reaction between thiol and norbornene moieties has been first utilized by Anseth and coworkers to create uniform and degradable PEG-peptide networks [139]. The thiol-ene coupling chemistry is cytocompatible, controllable both spatially and temporally, with the advantage of a high conversion of functional groups, being also a facile means to control the structural and mechanical properties of hydrogels [139, 140]. To better understand the chemistry-structure relationship, Yang et al. have recently prepared a library of PEG hydrogels using the benign UV initiated thiol-ene coupling strategy [126]. PEGs of different length functionalized with diallyl, dithiol, and dimethacrylate have been cross-linked with complementary trifunctional compounds. The M_c increased proportional with the PEG chain length (M_c was 3104 g/mol for PEG of 8 kDa, being about four times lower for PEG of 2 kDa). The mesh size increased with the increase of PEG molar mass, ranging from 3.7 nm for PEG of 2 kDa to 9.5 nm PEG of 8 kDa. The hydrogels prepared from PEG-diallyl, with molar mass ranging from 2kDa to 8 kDa, cross-linked with trimethylolpropane tris(3-mercaptopropionate) (TMP-tris-thiol) have been used as scaffolds for the preparation of a library of sequential IPNs [141, 142]. For generation of the second network, the precursors have been allowed to diffuse into the first network for a certain time, followed by the exposure at UV light for cross-linking [141]. It has been observed that IPN hydrogels had a lower swelling degree than the primary networks, the decreasing being higher for lower PEG chain length. This behavior showed that shorter PEG yielded higher cross-linking density of the secondary network and can be used as means to manipulate the water content in IPN hydrogels. The structural parameters and mechanical properties of the IPN hydrogels have been further manipulated by the time diffusion of the secondary network PEG precursors [142].

IPN cryogels

In addition to the IPN strategy, several techniques have been proposed to increase the mechanical strength and the response rate of hydrogels, among them the generation of interconnected pore structures within the hydrogel matrices being of interest. Macroporous hydrogels are usually prepared by: cross-linking polymerization in the presence of pore-forming agents, when a microphase separation occurs [143], porogen leaching [144], cross-linking in the presence of substances releasing porogen gases [145], and lyophilization of the hydrogel swollen in water [21, 103]. Another technique is cryogelation, in which the cross-linking polymerization reactions are conducted below the freezing point of the reaction solutions, when the most part of the solvent (water) forms crystals, the bound water and the soluble substances (monomers, initiator, polymers) being concentrated in a non-frozen liquid microphase, where the gel is formed [146–150]. Advantages of cryogelation consist of the absence of any organic porogen, the ice crystals playing the role of inert template, the microstructure of the gel being the negative replica of the ice crystals. Moreover, compared with the other macroporous hydrogels, cryogels are endowed with a high mechanical stability. The unique feature of cryogels consists of their interconnected macropores (with sizes between 1 and 100 μm), which allow rapid and non-restricted mass-transport of any solute. Cryogels are endowed with a capillary network through which the solvent can flow by convective mass transport, and a high osmotic stability, which make them adequate materials for various biomedical applications and bioseparations [148, 149]. IPN cryogels based on two synthetic polymers [151], or composite IPN cryogels based on synthetic polymers and either polysaccharides such as: dextran (Dx) [137, 152], dextran sulfate (DxS) [133, 138], CS [153–155], potato starch [156, 157], and salecan [158], or protein [159] have been recently reported. IPN composite cryogels have remarkable mechanical stability under compression [138, 153, 158], being suitable materials for biomedical applications [153, 158, 159], and separation of various ionic species [154, 155, 157].

The porosities of semi-IPN PAAm/Dx [137] and semi-IPN PAAm/DxS [138] cryogels, synthesized at –18 °C, have been evaluated and compared with the values found for conventional semi-IPN hydrogels (preparation temperature +20 °C). It was found that the swollen state porosity, P_s, estimated with Eq. 8, was about 44 % for the hydrogels with a cross-linker ratio of 1/80 formed at +20 °C, while it rapidly increased with decreasing the preparation temperature being 94 %, for cryogels with the same cross-linking degree. The dry-state porosity, P, calculated with Eq. (9), gave results similar with those found for P_s only in the case of the composite cryogels, while for conventional hydrogels the values were much lower (∼33 %). These results support the stable porous structure of cryogels, which did not collapse during deswelling or drying [138].

PAAm/CS IPN cryogels, prepared by selective cross-linking of CS entrapped in PAAm as the 1^st network, demonstrated a high efficiency in separation of dyes oppositely charged (methylene blue, MB, and methyl orange), a high capacity to adsorb MB (up to 750 mg/g cryogel, at 25 °C), and a high level of reusability in consecutive sorption/desorption cycles [154]. Multiresponsive semi-IPN composite cryogels were prepared by the radical copolymerization of AAm with N,N′-methylenebisacrylamide (BAAm) in the presence of native PS or anionically modified PS (PA), under the freezing point of the solvent (–18 °C) [156]. All composite cryogels presented a super-fast swelling, the main difference consisting of the time necessary to attain the equilibrium swelling, this being around 30 s in the case of PAAm/PS gels and 15 s in the case of PAAm/PA gels, at the same cross-linker ratio [156]. Semi-IPN PAAm/PA composite cryogels adsorbed MB from aqueous solutions up to 444 mg/g gel. The sorption capacity has been further increased up to 667 mg MB/g gel, at 25 °C, by controlled hydrolysis of PAAm matrix [157]. No loose of the sorption capacity was observed after six consecutive sorption/desorption cycles, behavior which differentiate them on the conventional semi-IPN hydrogels having the same components.

IPN hydrogels with mechanically enhanced properties

Mechanically enhanced IPN hydrogels as “double networks” (DN), promoted by Gong et al. have attracted great attention last decade due to their potential for biomaterials, mainly as replacement of natural cartilage [160–166]. The particular feature of this new type of IPN hydrogels, characterized by high resistance to wear and high fracture strength, consists of the preparation first of a densely cross-linked polyelectrolyte network (rigid skeleton, minor component) (PAMPS), the second network being a neutral and loosely cross-linked network (major component) [160, 162]. The molar ratio between the first and the second network, and their cross-linking degrees are the crucial structural parameters which determine the mechanical properties of these gels. The molecular weight of the second network (PAAm) has a determinant role in the enhancement of the mechanical properties of DN hydrogels, a molecular weight of 1 × 10⁶ g/mol being found as optimum for self-entanglement of the PAAm chains [160]. The toughness of the DN hydrogels has been further increased by creating spherical voids in the 1^st network (silica nanoparticles which have been removed), the 2^nd network (PAAm) being generated in the presence of void PAMPS gels to obtain void-DN gels, which have a hard body (PAMPS/PAAm), and the soft spherical PAAm [163]. The excellent mechanical performances of DN hydrogels originate from the synergistic effect of the networks: the first network serves as source of sacrificial bonds. It breaks into small clusters, which disperse the stress around the crack tip into the surrounding damage zone, while the PAAm ductile chains act as hidden length, which extend to sustain large deformation [162]. The cross-linker concentration in the first network is a critical parameter, a small amount of residual double bonds would remain in the PAMPS network when the concentration of BAAm has been ∼0.01 mol%, and interact with AAm to form chemical cross-links between the two networks [165]. Among various DN hydrogels developed by Gong et al., which have been tested for biomedical applications, the DN gel consisting of PAMPS and poly(N,N-dimethylacrylamide) is the most promising material for artificial cartilage, because it has exhibited an exceptional wear property [162].

Unlike the DN promoted by Gong et al., in the case of the PEG/PAA DN hydrogels developed by Myung et al. the 1^st network is a tightly cross-linked neutral network (PEG-DA), interpenetrated with a loosely cross-linked ionic network (PAA) as the 2^nd network [17, 167, 168]. The interaction between the independently cross-linked networks within the IPN has been varied by changing the molecular weight of PEG macromonomer, the ionization degree of PAA by changing the pH, and increasing the polymer content in the PAA network [167]. The hydrogel physical properties have been tuned by the network parameters and the swelling conditions, materials with water content between 58 and 90 %, tensile strength between 2.0 MPa and 12 MPa, and initial Young’s modulus between 1.0 MPa and 19 MPa being thus prepared. Under physiological pH and salt concentration, these DN hydrogels presented “biomimetic values for Young’s modulus, being very promising candidates for artificial cornea and cartilage [167, 168].

Other strategies focused toward improving the mechanical strength and the stimuli responsiveness of hydrogels have been lately reported [169–172]. Thus, multi-responsive polyampholyte composite hydrogel with excellent mechanical strength and rapid shrinking rate have been recently synthesized by Xu and coworkers, in two steps [169]. Firstly, microgels of PNIPAAm as core and poly(vinylamine) (PVAm) as shell have been synthesized via surfactant-free emulsion polymerization with N,N-methylenebisacrylamide (MBAAm) as cross-linker. Acrylic acid (AA), acryloyloxyethyl trimethylammonium chloride and AAm have been grafted on the surface of microgels in the presence of MBAAm as cross-linker. The first network, with low mechanical strength, has been constituted from cross-linked ungrafted polyampholyte chains, while the second network consisting of core-shell microgels with grafted polyampholyte chains had excellent mechanical strength (the compress strength of the composite hydrogel being up to 17–30 MPa). Such hydrogels have potential for application as substitutes for cartilage due to their remarkable mechanical strength and in drug-controlled release owing to the rapid response rate [169].

Dai and coworkers have recently reported fabrication by sequential strategy of mechanically strong conducting hydrogels composed of PAAm with a concentration of 5 mol %, as the first network, and the semi-IPN hydrogel, constituted of poly(3,4-ethylenedioxythiophene)-poly(sodium styrenesulfonate) (ionically cross-linked with Fe³⁺) (PEDOT-PSS), as the second network [170, 171]. It was found that to attain optimal mechanical properties a high EDOT concentration, which determined the value of the fracture stress, and an EDOT/PSS molar ratio <1.0, which induced a homogeneous microstructure and a high fracture strain, have been required. The potential application of these DN hydrogels as electrochemical actuators has been discussed. pH and temperature responsive DN composite hydrogels having PNIPAAm as a tightly cross-linked 1^st network, PAA as loosely cross-linked 2^nd network, and graphene oxide (GO) as an additive have been prepared by Li and coworkers [172]. The as prepared composite hydrogels exhibited fast response at swelling/deswelling and much better mechanical properties than conventional PNIPAAm single-network hydrogel. The average pore size in PNIPAAm/PAA/GO hydrogel was about 3 μm, which further decreased with the increase of the PAA content. The composite DN hydrogels did not break even at a stress of 56.3 MPa and a strain of 94.9%, and furthermore, recovered their original shape in a few seconds after the load release [172].

Highly stretchable and extremely tough hydrogels have been recently fabricated by Suo et al. [173–176]. These IPN composite hydrogels, consist of a covalently cross-linked network (PAAm) and an ionically cross-linked Alg network, and have been prepared by a simultaneous strategy. The as prepared hydrogels could be stretched beyond 20 times and achieved fracture energy as high as ∼9 kJ/m² [173]. The hydrogels cross-linked with trivalent cations have been much stronger than those cross-linked with divalent cations [174]. The mechanisms which explain the exceptional properties of this composite gel consist of: (i) the ionically cross-linked alginate component provide the energy dissipation upon straining, (ii) the long PAAm chains ensure the crack bridging and maintenance of mechanical integrity once the ionic cross-links are broken, and (iii) the secondary cross-links formed between Alg and PAAm network force transfer between them [174, 175]. Implantation of these IPN hydrogels into subcutaneous tissue of rats led to mild fibrotic encapsulation and minimal inflammatory response [175]. To simultaneously achieve high stiffness and toughness, Alg with short and long chains have been combined, IPN composite hydrogels with elastic moduli of ∼1 MPa and fracture energies up to ∼16 kJ/m² being obtained at a high ionic cross-link density [176].