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The unique optical and physical properties of soft, transparent, stimulus-sensitive nanocomposite gels - art. no. 66540O
Abstract
A new type of polymer hydrogel with a unique organic (polymer)/inorganic (clay) network structure has been synthesized by the in-situ free-radical polymerization of N-isopropyl acrylamide (NIPA) in the presence of exfoliated clay platelets in an aqueous medium. The resulting nanocomposite hydrogels (NC gels) consisting of PNIPA and clay (hectorite) exhibit extraordinary optical, mechanical and swelling properties. NC gels also show a clear phase transition due to the coil-to-globule transition of the PNIPA chains. It was observed that the phase-transition temperature (lower critical solution temperature: LCST), defined as the onset temperature of a steep transmittance drop, shifts to a lower or higher temperature than that of pure water (≅ 34 °C) when conditions are altered. When an inorganic salt, such as NaCl, CaCl2 and AlCl3, was added to the surrounding water, the LCST of the NC gels generally shifted to a lower temperature, in a manner almost inversely proportional to the salt concentration. On the other hand, when the NC gels adsorbed cationic surfactant, e.g. hexadecyl trimethyl ammonium chloride, the LCST shifted toward a higher temperature, although the shift and its profile strongly depended on the adsorption conditions, such as the surfactant concentration and the adsorption time. Consequently, non-thermo-sensitive NC gel was obtained by using a surfactant aqueous solution with a concentration higher than the critical micelle concentration.
... 40 Furthermore, the transition temperature decreased or increased markedly on adding an inorganic salt (for example, NaCl, CaCl 2 , AlCl 3 ) or a cationic surfactant (for example, hexadecyltrimethylammonium chloride) to the gel, respectively. 50 To ensure that NC gels showed outstanding swelling/deswelling behavior in response to both temperature and pH, while retaining their remarkable tensile mechanical properties, we prepared NC gels with a semi-interpenetrating organic/inorganic network structure by using linear poly(acrylic acid) (PAAc). 51 The PAAc content (C PAAc ) required to achieve good mechanical properties, as well as good temperature and pH sensitivities, changed with C clay , and the upper critical value of C PAAc /C clay was B2.5À3. ...
We have fabricated new types of polymer hydrogels and polymer nanocomposites, that is, nanocomposite gels (NC gels) and soft polymer nanocomposites (M-NCs), with novel organic/inorganic network structures. Both NC gels and M-NCs were synthesized by in situ free-radical polymerization in the presence of exfoliated clay platelets in aqueous systems and were obtained in various forms and sizes with a wide range of clay contents. Here, disk-like inorganic clay nanoparticles function as multifunctional crosslinkers to form new types of network systems. NC gels have extraordinary optical, mechanical and swelling/deswelling properties, as well as a number of new characteristics relating to optical anisotropy, polymer/clay morphology, biocompatibility, stimuli-sensitive surfaces, micropatterning and so on. The M-NCs also exhibit dramatic improvements in optical and mechanical properties including ultrahigh reversible extensibility and well-defined yielding behavior, despite their high clay contents. Thus, the serious disadvantages (intractability, mechanical fragility, optical turbidity, poor processing ability, low stimulus sensitivity and so on) associated with the conventional, chemically crosslinked polymeric materials were overcome in NC gels and M-NCs.Keywords: clay; hydrogel; mechanical properties; nanocomposite; network
A new class of polymer hydrogels, nanocomposite hydrogels (NC gels), consisting of a unique organic (polymer)/inorganic (clay)
network structure, was synthesized by in situ free-radical polymerization in the presence of exfoliated clay nanoparticles in an aqueous system. The resulting NC gels
overcame most of the disadvantages associated with chemically cross-linked hydrogels, such as mechanical fragility, structural
heterogeneity, and slow de-swelling rate. By using thermo-sensitive poly(N-isopropylacrylamide) (PNIPA) as a constituent polymer, NC gels with remarkable mechanical, optical, and swelling properties
as well as thermo-sensitivity were obtained. The various properties of NC gels, such as transparency, gel volume, cell culturing,
and surface friction changed significantly in response to the temperature and surrounding conditions. All the excellent properties
and new stimuli-responsive characteristics of NC gels are attributed to the unique PNIPA/clay network structure. The thermo-sensitivities
and the transition temperature can largely be controlled by varying the clay content and by the addition of solutes.
KeywordsStimulus responsive–Hydrogel–Nanocomposite–Poly(N-isopropylacrylamide)–Clay–Organic/inorganic–Network
Hydrogels, which consist of three-dimensional polymer networks and large amounts of water, have long been believed to be interesting but mechanically fragile materials limited to specific uses. Recently, important breakthroughs have been made as a result of the creation of nanocomposite hydrogels (NC gels), and most of the traditional limitations of hydrogels have been overcome. NC gels are prepared by in situ free-radical polymerization at high yield under mild conditions (near ambient temperature, without stirring), and various shapes and surface forms are readily obtained. Because of their unique organic (polymer)/inorganic (clay) network structure, high toughness and excellent optical properties and stimulus-sensitivity are simultaneously realized in NC gels. Furthermore, NC gels exhibit a number of interesting new characteristics. In this paper, the fundamental and recent developments related to NC gels are reviewed.
Nanocomposite type hydrogels (DMAA-NC gels) consisting of organic (polymer)/inorganic (clay) networks were prepared by in-situ free-radical polymerization of N,N-dimethylacrylamide (DMAA) in the presence of inorganic clay in aqueous solution. The composition of the NC gels could be controlled directly by altering the composition of the initial reaction mixture. The resulting DMAA-NC gels were mostly uniform and transparent, irrespective of their clay and polymer contents. From DSC, X-ray, TEM, and tensile mechanical measurements, the network structure was established. Contrary to conventional chemically cross-linked hydrogels (DMAA-OR gels) prepared by chemical cross-linking with a difunctional monomer, DMAA-NC gels exhibit superb mechanical properties with astonishingly large elongations at break, near to or greater than 1500%. The effects of the composition, such as the amounts of clay, polymer, and water content in DMAA-NC gels, on the tensile mechanical properties were investigated in detail. With increasing clay content from Cclay = 1 to 7 (Cclay is proportional to the weight of clay per unit volume of water, and a value of 1 corresponds to 0.762 g clay per 100 mL of water), the modulus and the ultimate tensile strength increased almost proportionally to the clay content, but the elongation at break decreased slightly. By altering the polymer content over 2 orders of magnitude, it was observed that the nature of the mechanical properties of DMAA-NC gels changed markedly. Above a lower critical polymer content (Cp ≈ 0.13), the elongation at break increased rapidly from near zero to more than 1000%. Thereafter, the hydrogels became very tough NC gels. With further increases in polymer content, the strength and the elongation at break showed maxima at certain polymer contents, whereas the modulus increased monotonically. The mechanical properties were also changed by altering the water content. The cross-link density was estimated for DMAA-NC gels with different clay contents. The effects of clay and polymer contents on the mechanical properties, as described above, were discussed on the basis of the network structure model for NC gel.
For two different types of poly(N-isopropylacrylamide) (PNIPA) hydrogels, i.e., nanocomposite type PNIPA hydrogels (NC gel) and conventional chemically cross-linked PNIPA hydrogels (OR gel), the effects of cross-linker contents on various physical properties were investigated. In NC gels composed of a unique organic (PNIPA)/inorganic (clay) network, the inorganic clay acts as a multifunctional cross-linker in place of an organic cross-linker (BIS) as used in OR gels. In NC gels, which generally exhibit extraordinary mechanical toughness, the tensile moduli and tensile strengths are almost proportional to the clay content (Cclay), while the elongation at break tends to decrease slightly with increasing Cclay. On the other hand, in OR gels, which always exhibit weak and brittle natures, there was no detectable change in properties on altering the concentration of BIS (CBIS). The deswelling rate was affected markedly by the cross-linker content in both gels though in opposite directions. On increasing cross-linker contents NC gels exhibit decreases and OR gels exhibit increases in rates of deswelling. In NC gels, high deswelling rates and high structural homogeneities (transparencies) were achieved simultaneously. Also, volume changes related to the phase transition of PNIPA at LCST were also inclined to decrease on increasing cross-linker contents in both gels, although swelling ratios at temperatures below LCST were generally larger in NC gels than those in OR gels. As for transparency changes at LCST, in OR gels changes in transparency decrease on increasing CBIS, because below the LCST the transmittances themselves decrease steeply with increasing CBIS. On the contrary, NC gels exhibit large transparency changes regardless of Cclay and show a tendency to increase their transmittances above LCST in the high Cclay region. All results obtained were consistent with the proposed model structure for NC gels. On the basis of the theory of rubber elasticity and using tensile mechanical data, the number of effective cross-links and the molecular weight between cross-linking points were evaluated for all NC gels.
Aqueous solutions of poly(N-isopropyl acrylamide) show a lower critical solution temperature. The thermodynamic properties of the system have been evaluated from the phase diagram and the heat absorbed during phase separation and the phenomenon is ascribed to be primarily due to an entropy effect. From viscosity, sedimentation, and light-scattering studies of solutions close to conditions of phase separation, it appears that aggregation due to formation of nonpolar and intermolecular hydrogen bonds is important. In addition, a weakening of the ordering effect of the water-amide hydrogen bonds as the temperature is raised contributes to the stability of the two-phase system.
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The behaviour of deswollen (D) and equilibrium swollen (S) non-ionic poly(N-isopropylacrylamide) (PNIPA) hydrogel samples in aqueous hydroxides and chlorides of Na and K are investigated. The samples respond in dramatically different styles to increasing concentrations of the additive in the external aqueous medium. Both samples exhibit volume phase transition at additive concentrations characteristic of the anion. Thereafter, if left undisturbed in the same medium, the D samples shrink with increasing concentrations of the additive, while the S samples in a critical concentration range initially shrink to an opaque mass, then develop a lump on the surface which eventually turns into a clear transparent bubble. The bubble wall is permeable and the size and shape stable for months implying the contents within are in osmotic equilibrium with the external fluid.
The salt effect on the phase transition of N-isopropylacrylamide (NIPA) gel was studied. The swelling behavior of the NIPA gel strongly depends on the salt concentration and is well described as a function of the chemical potential difference of water molecules in solution from that at the transition. From the analysis of the OH stretching, Raman spectra in water and in various aqueous solutions in terms of collective proton motions reveals that the presence of salts tends to disrupt or distort the water molecules in hydrophobic hydration shell around the NIPA gel. This leads to inducing the growth of the cluster shell around the salts, which leads to gel collapse. The volume phase transitions due to the different types of perturbation (temperature, salt) are induced by the same mechanism, hydrophobic hydration and dehydration, and therefore can be described in a unified manner in terms of the chemical potential and the collective proton motions of water molecules. © 2000 American Institute of Physics.
The small angle neutron scattering experiments were conducted on N‐isopropyl acrylamide (NIPA) gels in D2O and on the corresponding NIPA solutions. The NIPA gels underwent a sharp, but a continuous volume phase transition at 34.6 °C from a swollen state to a shrunken state with increasing temperature. In the case of the gels, an excess scattering due to the presence of crosslinks was observed at low q region (q≤0.02 Å−1), where q is the magnitude of the scattering vector. The scattered intensity function for the gel was well described with a combination of Gauss and Lorentz‐type functions, i.e., I(q)=IG(0)exp[−Ξ2q2]+[IL(0)/(1+ξ2q2)] as proposed by Geissler et al. IG(0) and IL(0) are the intensities at q=0 for the contributions of Gaussian and Lorentzian functions, respectively. The Gaussian part results from solidlike inhomogeneity, having a characteristic size of Ξ, which is due to the introduction of crosslinks into the system. The Lorentzian part is originated from the liquid nature of the local concentration fluctuations of the gel characterized with a thermal blob of dimension ξ. Ξ decreases systematically with polymer volume fraction, ϕ, indicating the nature of Ξ being the solidlike inhomogeneity. On the other hand, the intensity function for solutions was well fitted with the so‐called Ornstein–Zernike (OZ) equation (a Lorentzian function), i.e., I(q)=[IL(0)/(1+ξ2q2)].
The surfaces of polymeric nanocomposite (NC) hydrogels consisting of hydrophilic constituents such as poly(N-isopropylacrylamide) (PNIPA), hydrophilic clay, and water, were found to exhibit high contact angles. The PMIPA-NC gels were prepared by free-radical polymerization of NIPA monomer in the presence of exfoilated clay uniformly dispersed in aqueous medium. The sessil-drop contact angle measured at the surfaces of N-NC6 gels shows a distinctly hydrophobic surface for the N-NC6 gel despite the hydrophilic nature of all the constituents. A gradual decrease of contact angle observed for all samples is due to the evaporation of water from droplets under the same conditions of a constant solid-liquid contact area. The results also show that the rough surface of NC hydrogels are effective in producing ultrahigh values of contact angle and the high contact angles on the surfaces of N-NC gels are derived from the combined effects of amphiphilicity of PNIPA and network structure.
The mechanical properties and structures of nanocomposite gels (NC gels), consisting of poly(N-isopropylacrylamide) (PNIPA) and inorganic clay (hectorite), prepared using a wide range of clay concentration (25 mol % against water) were investigated. All NC gels were uniform and transparent, almost independent of the clay content, Cclay. The tensile modulus (E) and the strength (σ) were controlled without sacrificing extensibility by changing Cclay. The E, σ, and fracture energy observed for as-prepared NC gels attained 1.1 MPa, 453 kPa, and 3300 times that of a conventional chemically cross-linked gel, respectively, and σ increased to 3.0 MPa for a once-elongated NC25 gel. From the tensile and compression properties, in addition to optical transparency, it was concluded that a unique organic/inorganic network structure was retained regardless of Cclay. The effects of Cclay on the tensile mechanical properties on the first and second cycles, the time-dependent recovery from the first large elongation and the optical anisotropy of NC gels, and also the disappearance of the glass transition and the formation of clay−polymer intercalation in the dried NC gel were revealed. Thus, it became clear that the properties and the structure changed dramatically for an NC gel with a critical clay content ( ≈ NC10) or above. The structural models for NC gels with low and high Cclay, exhibiting different clay orientation and residual strain, were depicted.
The nanocomposite (NC) gels made of poly(N-isopropylacrylamide) (PNIPA) and synthetic clay show extraordinarily high mechanical properties, e.g., high extensibility and high strength at break. The structure and deformation mechanism of the NC gels have been investigated by small-angle neutron scattering (SANS). Two-dimensional SANS intensity patterns were obtained for deformed NC4 gels with the stretching ratio up to 6 times, where 4 denotes the clay concentration being 0.04 mol/L (≈3.05 wt %). A so-called abnormal-butterfly pattern was observed in the low q region, i.e., q < 0.02 Å-1, while a normal-butterfly pattern was obtained in the larger q (≥0.02 Å-1). Here q is the magnitude of the scattering vector. However, contrast-matched SANS experiments disclosed that the abnormal-butterfly pattern is not originated from cross-link inhomogeneities observed in conventional gels but from oriented-clay platelets by deformation. These results indicate that clay platelets, 300 Å in diameter and 10 Å thick embedded in the PNIPA network, are highly aligned with their surface normal parallel to the stretching direction, and the PNIPA chains are elongated parallel to the stretching direction. It is concluded that the high mechanical properties of NC gels are ascribed to “plane cross-linking” with long PNIPA chains between platelets compared with those of conventional chemical gels having “point cross-linking”.
The structure and dynamics of poly(N-isopropylacrylamide) (PNIPA)−clay nanocomposite (NC) gels have been investigated by small-angle neutron scattering and dynamic light scattering in order to account for their high strength and high deformability. The structure of clay dispersion in water was an assembly of clay platelets 300 Å in diameter and 10 Å thick. The structure factors of the clay dispersions were reduced to a master curve by normalizing with the clay concentration, Cclay, even across its sol−gel transition. On the other hand, the structure factors of the NC gels cannot be represented with a simple addition of the structure factors of the components, but with a sum of Lorentz (L) and squared-Lorentz (SL) functions. It was found that the SL term, indicating the gel inhomogeneities, becomes dominant by increasing Cclay. The chain dynamics of the gels was found to be similar to the conventional PNIPA gels chemically cross-linked with bis-acrylamide.
The process of forming the unique organic/inorganic network structure of nanocomposite hydrogels (NC gels) was studied through changes in viscosity, optical transparency, X-ray diffraction, and mechanical properties. It was concluded that, during the preparation of the initial reaction solutions, a specific solution structure was formed from monomer (NIPA) and clay, where NIPA prevents gel formation of clay itself, and initiator (KPS) is located near the clay surface through ionic interactions. In subsequent in-situ free-radical polymerization, it was observed that the viscosity increased markedly during NC gel syntheses and in a manner similar to that in OR gel syntheses. Also, NC gels with different polymer contents exhibit characteristic two-step changes in the stress−strain curves, which correspond to the primary network formation and subsequent increase of cross-link density. These are because the polymerization proceeds on the clay particles which are relatively immobile, and clay platelets act as effective multifunctional cross-linking agents (plane cross-link). Then, it was proposed that clay−brush particles, consisting of exfoliated clay platelets with numbers of polymer chains grafted to their surfaces, were formed in the very early stage of polymerization, at around 7% of monomer conversion. Novel decreases in transparency were observed corresponding to the formation of clay−brush particles, but transparency recovered on further polymerization. Clay−brush particle formation was confirmed by XRD measurements on dried NC gels prepared using small amounts of monomer. Thus, a mechanism for forming the unique organic/inorganic network structure, including the formation of clay−brush particles in the synthetic pathway, is proposed. Furthermore, it was found that NC gels with excellent mechanical properties and structural homogeneity could not be prepared using other methods such as mixing clay and polymer solutions or by in-situ polymerization in the presence of the other inorganic nanoparticles instead of clay. These results indicate that the formation of organic/inorganic network structures in NC gels is highly specific and only realized by in-situ free-radical polymerization in the presence of clay.
We attempted to perform quantitative measurements of the adsorption of surfactant molecules by NIPA gels and to explain the effect of surfactant binding on the phase transitions of the gels. It was found that NaDBS binds to the networks in the near vicinity of the surface but not within the body of NIPA gel through hydrophobic interaction, converting the gel into an ionic gel. The hydrophobic interaction was fairly strong; as a result, the micelles of NaDBS formed at concentrations above the critical micelle concentration were destroyed during the process of the binding of the surfactant to the gel. Such surfactant binding brought about a local increase in the charge density of the gel. Thus, NaDBS-bound ionic NIPA gels exhibited an increase in phase transition temperature accompanied by an increase in gel volume, the aspects of which are different from usual polyelectrolyte gels with a similar charge density, such as a copolymer gel consisting of NIPA and sodium acrylate. Such ionic gels will find technological applications due to the heterogeneously ionized layer(s) of networks which exist within them.
The swelling equilibrium of N-isopropylacrylamide (NIPA) gel was studied as a function of temperature in aqueous solutions of surfactants: sodium dodecyl sulfate (SDS, anionic), dodecyltrimethylammonium chloride (DTAC, cationic), and nonaoxyethylene dodecyl ether (NODE, nonionic). In pure water the gel showed a discontinuous volume phase transition at 33.6-degrees-C with a volume change of V(swollen)/V(collapsed) = 8. With the addition of SDS, the transition temperature as well as the volume change at the transition increased. At 3.47 X 10(-2) M of SDS concentration, the transition temperature became 88-degrees-C and the volume ratio increased to V(swollen)/V(collapsed) = 32. A similar but less drastic change was observed when DTAC was used. In contrast, NODE induced little change in both the temperature and temperature phase transition for the concentration range studied (<6.94 X 10(-2) M). In order to understand these effects of ionic surfactants, the association and dissociation of SDS with the gel or homopolymer of NIPA were investigated in an aqueous system. The amount of free SDS ions was measured at different temperatures by a conductometric method. An increase in the transition temperature was brought about by SDS binding to the gel or polymer whereupon the bound SDS was completely dissociated by cooling. The association and dissociation of SDS were reversible and, respectively, occurred in the temperature ranges above and below 30-degrees-C. Thus, the ionic surfactant molecules convert the gel into an ionic gel due to binding through the hydrophobic interaction. The obtained experimental results have been qualitatively discussed in terms of the classical Flory-Huggins theory with modifications to account for the free energy of association of surfactant molecules with the gel network or polymer and surfactant self-association.
Effects of salts on volume phase transition of N-isopropylacrylamide gels and cloud points of poly-(N-isopropylacrylamide) solution were measured. The experiments were performed with inorganic salts and tetraalkylammonium salts. The transition temperature for inorganic salts was dependent on anions rather than cations, and the change in transition temperature was linearly correlated with the viscosity B coefficient of anions. On the other hand, the transition temperature for tetraalkylammonium bromides strongly depended on alkyl chain length, which was suspected to result from an adsorption of the salts to the polymer segments.
(Chemical Equation Presented) Novel porous nanocomposites consisting of polyN-isopropylacrylamide) and inorganic clay, prepared from nanocomposite hydrogels, spontaneously formed characteristic layered morphologies, with the most typical being a concentric three-layer morphology consisting of, successively, a fine-porous layer/a dense layer/a coarse-porous layer with polyhedral pores.
Sliding frictional behavior on the surface of novel nanocomposite hydrogels (NC gels), composed of poly( N ‐isopropylacrylamide) and exfoliated clay, was investigated and compared with that of conventional chemically‐crosslinked hydrogels (OR gels). Since OR gels are so weak, the sliding test, in general, could not be carried out on their surfaces except when wet. On the contrary, NC gels can withstand sliding friction under high loading and it was found that the frictional forces are sensitive to the environmental surroundings (wet or in‐air), the gel composition (water, clay contents), and loading. In air, NC gels exhibit a characteristic force profile with a maximum static frictional force (max‐SFF) and subsequent constant dynamic frictional force. In contrast, NC gels exhibit very low frictional forces under wet conditions. The change in frictional force with surroundings was very distinctive for NC gels, particularly for those with low clay contents. Max‐SFF for NC1 gel decreased more than 100‐fold by changing the environment from in‐air to wet. The characteristic sliding frictional behaviors are explained on the basis of a unique organic/inorganic network structure of NC gels.
The model of sliding process on NC gel and effects of clay content of NC gels on the sliding frictional behavior.
magnified image The model of sliding process on NC gel and effects of clay content of NC gels on the sliding frictional behavior.
The swelling of crosslinked poiy(N,N′-alkyl substituted acrylamides) in water was studied in relation to temperature changes. Conventional swelling theory and separation of the polymer solvent interaction parameter into enthalpic and entropic contributions were used to characterize the temperature dependence of swelling in water. The thermosensitivity of swelling can be attributed to the delicate hydrophilic/hydrophobic balance of polymer chains and is affected by the size, configuration, and mobility of alkyl side groups. A sharp swelling transition may occur at an optimum hydrophilic/hydrophobic balance but was found only in the N-isopropylacrylamide network among the networks tested. This swelling transition pattern was also reflected by the endothermic peak of the DSC thermogram of the swollen sample.
The phase behaviour of linear poly(N-isopropylacrylamide) (PNIPA) and chemically cross-linked PNIPA in water has been determined by calorimetric and optical techniques. Experiments for the linear polymer were conducted for three different molar masses and encompassed the whole accessible concentration range. In addition the binodal of the ‘monomer’ N-(isopropyl)propionamide (NIPPA) was determined by cloud point measurements. The phase behaviour was analysed in terms of the Flory–Huggins–Staverman theory amended with a strong concentration dependent interaction function. The analysis established that PNIPA/water is exemplar of Type II phase behaviour, indicating that the system does not fit the classic Flory–Huggins scheme. Distinctive of Type II behaviour is the existence of an off-zero liquid–liquid critical composition for a hypothetical polymer of infinite molar mass and the absence of the classic FH Θ-critical state. The Type II phase behaviour of the linear polymer is also found in the network and is shown to be responsible for the discontinuous volume response of the PNIPA gel. The peculiar swelling behaviour is a direct consequence of the invariant interference of the swelling curve with the liquid–liquid miscibility gap. Finally, the dependence of the enthalpy of melting of water in the polymer solution is not related to the formation of a strong complex between the polymer and the water molecules. Instead, the crystallisation and melting of water are impeded by the vitrification of the aqueous polymer solution.
(Figure Presented) Polymer clay for chemists: High strengths and high fracture energies of polymeric hydrogels have been demonstrated by incorporating a substantial amount of nanodispersed inorganic clay during the in situ free-radical polymerization of N-isopropylacrylamide. By varying the amount of clay used, mechanical properties as well as the swelling and transparency of the nanocomposite (NC) could be controlled (see picture; NC20 contains more clay than NC5).
Cell cultivation on the surface of a novel poly(N-isopropylacrylamide) (PNIPA) hydrogel (N-NC gel), consisting of a specific type of organic (PNIPA)/inorganic (clay) network, was studied using three cell types; HepG2 human hepatoma cells, human dermal fibroblasts, and human umbilical vein endothelial cells. For the first time, it was found that cells could be cultured to be confluent on the surfaces of PNIPA hydrogels using N-NC gels, regardless of gel thickness. Cell adhesion and proliferation on N-NC gels exhibit strong dependencies on clay concentration (C(clay)), and the numbers of cultured cells are maximum at about C(clay) = 6 x 10(-2) mol (45.72 g)/1 L of H(2)O. On the contrary, it was almost impossible to culture cells on conventional, chemically crosslinked PNIPA hydrogels, regardless of their cross-linker concentration. The reasons why cells cultured only on the surfaces of N-NC gels with their specific network structure and composition were discussed in terms of water content, protein adsorption, surface flatness, hydrophobicity of dehydrated PNIPA chains, and the anionic charge on exfoliated clay. Finally, it was found that cells cultured on the surfaces of N-NC gels could be detached in the forms of sheets of cells without trypsin treatment, but by just decreasing the temperature to 20 degrees C.