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Nano Surface‐Heterogeneities of Particles Modulate the Macroscopic Properties of Hydrogels

Nano Surface‐Heterogeneities of Particles Modulate the Macroscopic Properties of Hydrogels IntroductionNanoparticle's evolution is inspired mainly by the complexity of nature and living things.[1] Plenty of viruses, bacteria, and cells exist in complicated shapes and sizes, including the latest discovery of spikey‐spherical shaped SARS‐CoV‐2 (COVID‐19) virus [2] to a star‐shaped bacteria, to name a few. All such viruses have definite functions to interact with surfaces, passive diffusion, and active motility, which eventually affect the immune response of a human body.[3]Over two decades, polymer nanocomposites, where polymers are in the continuous phase and nanofillers/nanoparticles are embedded as discontinuous phase, have emerged as the awe‐materials for biomedical, aerospace, electronics, sports equipment, transportation, and household applications. [4,5] Likewise, hydrogels, the water‐swollen polymer 3D networks that mimic features of the native extracellular matrix (ECM), have also been augmented after embedding diverse nanoparticles.[6,7] These composite gels are of utmost interest in biomedical fields since they permit the encapsulation of cells and can be engineered with cells regulating signals. Predominantly, different nanoparticles, by changing their sizes, surface charge/functional groups, and concentrations, were harnessed to influence the macroscopic properties of nanocomposite materials like hydrogels. However, the latest discovery showed the different shapes of a nanoparticle have striking impact/influence on the intrinsic mechanical and adhesive properties of a composite hydrogel.[8]On the contrary, the surface‐heterogeneities of nanoparticles have been merely exploited. The dense suspensions of rough‐surfaced colloidal particles have been studied for their pronounced non‐Newtonian behavior, such as discontinuous shear thickening and shear jamming.[9] These studies recognized that surface roughness is a crucial parameter and demands careful consideration for materials development.[10] We hypothesized that the particles with surface heterogeneities could significantly affect the adsorption, motion, and interfacial forces when interacting with polymeric themselves. In a mechanistic point of view, the colloidal particles can modulate mechanical properties of composite hydrogels by changing macromolecular network resistance to deformation and fluid–solid interactions.[11,12] Rationally, the polymer chains, with specific functionalities, should have a higher friction coefficient while interacting with rough‐surfaced particles than smooth‐surfaced particles. Similarly, suspended rough‐surfaced particles in the fluid phase of a composite hydrogel could influence the frictional drag force during fluid exudation from the polymeric network as well as fluid phase shear stress. Such interactions should modulate the macroscopic physicochemical properties of a composite hydrogel such as stiffness, dissipation, and adhesion.Herein, we endeavored to exploit the influence of surface‐roughness of nanoparticles on the macroscopic properties of composite hydrogels. Raspberry‐like nanoparticles were synthesized for this work. We found that these rough‐surfaced particles played a significant role in the mechanical and adhesive properties of the hydrogel. A comparative study was also performed using smooth‐surfaced nanoparticles for the proof of concept.Results and DiscussionSynthesis of Particles, and Composite Hydrogels FormulationThe rough‐surfaced colloidal nanoparticles were reproduced using a method reported ref. [13]. Briefly, styrene (St), methyl methacrylate (MMA), and a crosslinker divinyl benzene (DVB) were injected in a mixture of water and ethanol, containing an initiator, 2,2′‐azobis(2‐methylpropionamide) dihydrochloride (AIBA), at 70 °C. The ratio between water and ethanol is crucial in producing the monodispersed colloidal nanoparticles. Scanning electron microscopy (SEM) analysis confirmed that each particles has multiple and significantly large bumps on its surface, imitating raspberry, and have a diameter of about 310 nm, as shown in Figure 1a. Zetasizer found that the hydrodynamic diameter (Dh) of these particles is ≈330 nm, and polydispersity index (PDI) is 3.7% (Figure 1b), whereas the surface charge, a Zeta potential is +39.0 (±3.57) mV. The initiator, AIBA, is responsible for imparting the positive surface charge on the nanoparticles. The synthesized raspberry‐like colloidal (RC) particles, in a colloidal solution, were present at a 4.06% volume fraction (or 3.7 wt%).1Figurea) Scanning electron microscopy (SEM) image of rough‐surfaced, raspberry‐like colloidal (RC) nanoparticles. Scale bar is 400 nm, b) the number averaged dynamic light scattering (DLS) size analysis of RC particles. The hydrodynamic diameter (Dh) is 330±11 and polydispersity index (PDI) is 0.037, c) SEM image of smooth‐surfaced, PMMA colloidal (SC) nanoparticles. Scale bar is 400 nm, d) the number averaged DLS size analysis of SC particles. The Dh is 315±17, PDI is 0.058, e) schematic representation of HRCs and HSCs hydrogels formulation. Constant amount of Poly (ethylene glycol) dimethacrylate (PEGDMA = 5.5 wt%) and nanofibril cellulose (NFC = 0.5 wt%) was mixed with a different concentration of RC and SC nanoparticles and thermally polymerized to afford composite hydrogels, f,g) actual composition of HRC‐2 and HSC‐2 composite hydrogels, respectively. It shows how many approximate numbers of particles could be present/available in HRC‐2 and HSC‐2 hydrogels.Since RC particles have a positive surface charge, the obvious choice would be to have negatively charged precursors of hydrogels. The electrostatic interaction should promote the interfacial interactions between RC particles and the polymeric units of a hydrogel. Thus, as reported earlier from our group, we selected a mixture of Poly(ethylene glycol) dimethacrylate (PEGDMA) and nanofibril cellulose (NFC) as the hydrogel's precursor.[14,15] We discovered that when 20 kDa PEGDAM (5.5 wt%) and NFC (0.5 wt%) are mixed, the zeta potential of the solution is about –36.0 (±6.43) mV. Subsequently, different concentrations of RC nanoparticles were mixed to a fixed concentration of PEGDMA+ NFC solution and thermally polymerized to afford various composite hydrogels called HRCs, as depicted in Figure 1e. Briefly, 200 µL RC particles (4.06 vol%) were mixed with PEGDMA + NFC and polymerized at 60 °C to afford a composite hydrogel. It is called HRC‐2. Please note, “2” in HRC‐2 refers to 200 µL of RC nanoparticles. We also calculated an approximate number of particles that could be present in respective hydrogels. Considering the volume fraction of RC particles and the density of styrene (a major part of RC colloids), we calculated that 1 mL of the colloidal solution has approx. ≈2.16 × 1012 RC‐nanoparticles. This indicates that if 200 µL RC colloids are used, there should be approx. ≈4.2 × 1011 nanoparticles in HRC‐2, as mentioned in Figure 1f. In the end, the water content in the hydrogels should be anywhere, between 93 and 94 wt% depending upon the particle's concentration.The hydrogels (HRCs) developed herein are multi‐network hydrogels where PEGDMA responsible for covalent networks, NFCs, and RC are oppositely charged nanofillers with different aspect ratios and responsible in creating various physical as well as electrostatic interactions within the hydrogel polymeric network. As mentioned above, for the synthesis of RC nanoparticles, cationic initiator AIBA has been used. These initiators are primarily responsible for producing positive surface charge on their surface. Moreover, NFCs carry negative surface charge.[16] Thus, positive surface charged particles (RC) create strong electrostatic interaction with negatively charged NFC+PEGDMA solution.ResultsThe roughness on the particle surface is known to dissipate frictional energy when they encounter smooth surfaces, depending on the load and the area of contact. Herein, as stated above, having significant roughness on the RC particles embedded in a hydrogel, should dissipate more energy under stress than a non‐RC hydrogel. Thus, as expected, once 50 µL RC particles added, the obtained HRC‐0.5 composite hydrogel showed an improvement in energy dissipation (Figure 2a) as well as compressive modulus (C‐modulus, Figure 2b) when compared to a non‐RC hydrogel aka cntr‐H. Further improvement was achieved when more RC particles were incorporated. We found that with 200 µL of RC particles, the energy dissipation, of HRC‐2 (2.40 ± 0.3 kJ m−3), was increased by ≈75% whereas C‐modulus (60.83 ± 6.5 kPa) was improved twofold. However, with 300 µL of RC, the energy dissipation of HRC‐3 hydrogel started to decline, but the C‐modulus still increased. As the amount of PEGDMA and NFC was remained constant, such improvement was solely coming after embedding the RC particles to the hydrogels. Nonetheless, it was not clear to emphasize whether nano surface‐heterogeneities present on the RC particles contributing to the dissipation and C‐modulus properties of the hydrogels or not. To this end, we thus considered to synthesize smooth‐surfaced nanoparticles.2FigureMechanical properties of composite hydrogels. a) Energy dissipation of HRCs and control hydrogels, i.e., cntr‐H. It shows that the energy dissipation was increased from HRC‐0.5 to reach to highest for HRC‐2 and started to decline from HRC‐3, b) compressive stiffness (C‐stiffness) of HRCs and cntr‐H hydrogels. Similarly, a subtle increased could be seen up to HRC‐2, and nearly saturated for HRC‐3, c,d) show comparative energy dissipation and C‐stiffness values of cntr‐H, HRC‐2, and HSC‐2, respectively, e) frequency sweep rheology data of HRC‐2, HSC‐2 and cntr‐H. All the hydrogels show an excellent viscoelastic characteristic. However, the elastic(storage) modulus (G') of HRC‐2 is nearly two‐fold and four‐fold higher than HSC‐2 and cntr‐H hydrogels, respectively, f) shear adhesion strength of three hydrogels. The difference was obvious, and as dissipation was increased from cntr‐H to HSC‐2 to HRC‐2 so does the adhesion strength. HRC‐2 shows highest adhesive properties.Previously, it was confirmed that polymerized MMA (PMMA), which is used with styrene and DVB during RC particles synthesis, contributes mainly to the surface roughness (bumps) present on these particles.[13] To mimic a similar surface interaction with the hydrogel network (NFCs+ PEGDMA), we further synthesized only PMMA‐based colloidal nanoparticles having a smooth surface (SC) in water using a method reported earlier.[17] SEM analysis confirmed that PMMA particles are smooth‐surfaced colloids (SC) of ≈290 nm diameter, as shown in Figure 1c. Their hydrodynamic diameter (Dh) is ≈315 nm (Figure 1d), PDI is 5.8% (Figure 1d), and the Zeta potential is +38.0 (±4.81) mV. PMMA or SC nanoparticles, in a colloidal solution, were present at a 4.01% volume fraction (or 3.8 wt%). Moreover, we calculated an approximate number of SC particles in 1 mL solution that is ≈2.44 × 1012.Subsequently, we synthesized two SC nanoparticles‐based composite hydrogels; one with 100 µL, called HSC‐1, and second with 200 µL called HSC‐2, keeping PEGDMA and NFC concentration as same as HRCs, as depicted in Figure 1e,g. We next examined the dissipation, and C‐modulus of both HSC‐1 and HSC‐2 hydrogels, see Figure 2c,d. We found that the difference was not significant, and they (HCS‐1 and HSC‐2) showed similar mechanical properties. Thus, we decided, as the number of particles in HSC‐2 (Figure 2) is comparable to HRC‐2 and total wt% of hydrogel's precursors would be the same, to continue with HSC‐2 hydrogel for a comparative study.When comparing, we discovered that the energy dissipation of HRC‐2 hydrogel (2.40 ± 0.3 kJ m−3) was significantly higher than cntr‐H (1.46 ± 0.22 kJ m−3) and HSC‐2 (1.64 ± 0.16 kJ m−3) hydrogels, as shown in Figure 2c. This confirms that the essential dissipative property of HRC hydrogels is driven by the roughness present on the surface of RC particles. We further examined and compared the C‐modulus and viscoelastic properties of the hydrogels. The HRC‐2 exhibited much higher C‐modulus (60.83 ± 6.5 kPa) than cntr‐H (35.01 ± 7.57 kPa) and HSC‐2 (42.86 ± 2.3 kPa) hydrogels, as represented in Figure 2d. The rheological analysis was performed to understand the viscoelastic behavior of the hydrogels. To this end, we first acquired the oscillatory amplitude sweep at constant frequency to identify the linear viscoelastic region (LVR) of all developed/synthesized hydrogels. Subsequently, the oscillatory frequency sweep was executed within (LVR) to determine the shear storage (elastic) modulus (G') of hydrogels. At 1 rad s−1, HSC‐2 displayed an astounding G’ value, i.e., ≈4500 Pa. It was nearly fourfold, and twofold higher than cntr‐H (≈1200 Pa) and HSC‐2 (≈2050 Pa) hydrogels, respectively. Moreover, HSC hydrogels have superior mechanical properties than the cntr‐H hydrogel. This implies that the embedded smooth‐surfaced particles, expectedly, contribute to the bulk properties of composite hydrogels, like previous findings. Nonetheless, this contribution is not as substantial as having surface‐roughness on the (RC) particles. We further analyzed the swelling ratio and optical transmittance properties of the hydrogels. We found that the swelling ratio of HRC‐2 hydrogel is slightly lower than that of HSC‐2 and cntr‐H hydrogels, see Figure S1 (Supporting Information). Additionally, the swelling ratio in the presence of NFC was almost reduced by half in the studied groups of hydrogels. The optical transmission curve (Figure S2, Supporting Information) of the studied hydrogels was significantly changed in the visible region (380–780 nm). We observed that the transmittance of cntr‐H, HSC‐2, and HRC‐2 is ≈80%, 25%, and 15%, respectively. This indicates that the optical transmission properties of hydrogels can be tuned by changing their composition. Considering these findings, we can readily claim that the nano‐surface heterogeneity of RC nanoparticles dictates the macroscopic properties of composite hydrogels (HRCs).Moreover, the importance of NFC in hydrogel network was also examined by analyzing the C‐modulus of three hydrogels without NFC. We found that without NFC, the C‐modulus of three hydrogels was almost the same, see Figure S3 (Supporting Information). This suggests that the presence of NFC is crucial for forming an efficient hydrogel network and observing such differences in the mechanical properties of HRC and HSC hydrogels.We further examined the adhesive characteristic of composite hydrogels. The adhesive property of hydrogels depends mainly on two different but synergetic aspects. The first is the intrinsic work of adhesion, i.e., the interfacial interactions between the surfaces, and the second is the dissipation mechanism of hydrogels.[14,18] With sufficient interfacial interactions, higher the dissipation, the greater the adhesion.[19] Expectedly, as shown in Figure 2f, HRC‐2 exhibited the highest (6.57 ± 1.62 kPa), HSC‐2 median (5.29 ± 0.78 kPa), and the cntr‐H showed the lowest adhesive property (3.79±0.68 kPa). This finding is consistent concerning their dissipation energy, as shown in Figure 2c.To understand the hydrogels’ network, we next analyzed the freeze‐dried hydrogels by the scanning electron microscopy (SEM). In HRC‐2 hydrogel, the RC nanoparticles were seen throughout the porous network, not aggregated rather dispersed, Figure 3a. Importantly, as depicted in Figure 3b, the particles seem well integrated into the hydrogel's framework and polymer + NFCs are quite involved with RC particles. It appears that there is a strong interaction between the particles and the polymeric network. Conversely, the SC particles, in HSC‐2 hydrogel, seem more like embedded, like usual composite system, Figure 3c,d. They do not seem interacting as good as RC particles do with polymer structures in their respective gels. The SEM of freeze‐dried cntr‐H hydrogel shows a usual porous and crosslinked structure in Figure 3e,f.3FigureScanning electron microscopy (SEM) analysis of composite hydrogels. a) Image shows porous network of HRC‐2 hydrogel where RC particles are seen everywhere evenly distributed, b) RC particles are seen integrated into the hydrogel frameworks. We do not see much of aggregation of particles, c) the porous network of HSC‐2 hydrogels, d) rather embedded SC particles into the hydrogel framework. We do not see as good interaction of SC particles with its hydrogel network, as RC particles have with HRC hydrogel, e,f) images show a usual hydrogel structure of cntr‐H hydrogel. Scale bar of (a,c,e) is 10 µm and (b,d,f) is 1 µm.DiscussionEnergy dissipation is a natural phenomenon where different kinds of energy are transmitted from systems to surroundings in form of heat, friction, etc. There are many innate body reactions that help us to dissipate impacted (pain) energy. For instance, stepping on a hard pointed object while walking/running, the natural reaction, apart from the physiological biomechanics, would be to squeeze/press the foot, which will create muscle bumps, so that the pain energy can be dissipated as quickly as possible. Considering a rather simple example of having two different balls with the same size, weight, and volume but the surface. One has a smooth surface like a tennis ball and another one has a dotted/spikey surface. If we drop them from the same height, we see that the bounce height of a smooth‐surfaced ball is higher than the spikey ball. Moreover, plenty of synthetic engineered materials have been developed to keep us safe from unforeseen accidents after dissipating an impacted energy. For example, the car tires with thread depths and patterns are better with grips and preventing skidding than the flat tires. Similarly, the mouth guards, various helmets, and patterned‐packaging foams, etc. are the most visible examples of such materials. All these materials have macroscopic patterns that help dissipate a major part of an impacted energy anisotropically with high friction coefficient at the interfaces.At molecular level, the mechanism of energy dissipation through the hydrogen‐bond or supramolecular bonds breakages has previously been observed.[11,12,20,21] Additionally, as learned recently, the embedded NFC in PEGDMA hydrogels dissipates more energy than the PEGDMA hydrogels themselves.[14,22] This means we can readily confirm, herein, that the presence of nanoparticles further elevates this property of composite HRC and HSC hydrogels. However, when RC particles with nano‐surface heterogeneities are present in HRC hydrogels, overall developments in the macroscopic properties were overwhelming. Initially, ionic interaction brings nanoparticles (positive zeta potential) and NFC + monomer mixture (negative zeta potential) together, and after the polymerization, the composite hydrogels formed. SEM images confirmed an interfacial interaction between nanoparticles, rather stronger with RC nanoparticles, and the polymers. Thus, as stated above, we can assume that the roughness on the surface of RC nanoparticles should encourage more friction during the dynamic movements of polymer chains and fibers than SC nanoparticles. When the dynamic movement of polymer unites and NFCs is affected by the roughness of RC nanoparticles, the overall relaxation behavior of HRC hydrogels under mechanical load should be different than the HSC hydrogel and cntr‐H hydrogel. To this end, we further examined the stress relaxation behavior of HRC‐2, HSR‐2, and cntr‐H hydrogels, (Figure 4).The modified Maxwellian model as expressed in Equation 1 with four viscoelastic branches was used to extrapolate the test data (acquired for 2000 s) until equilibrium state.[23] A custom‐written MATLAB code was employed to estimate parameters of the model that best fitted the measured data.1σr(t) =σeq +∑i = 1n = 4σie−t/τi\[\begin{array}{*{20}{c}}{{\sigma _{\rm{r}}}\left( t \right)\; = {\sigma _{{\rm{eq}}}}\; + \mathop \sum \limits_{i\; = \;1}^{{\rm{n}}\; = \;4} {\sigma _i}{e^{ - t/{\tau _i}}}}\end{array}\]where, σeq is the equilibrium stress after infinite time and σie−t/τi${\sigma _i}{e^{ - t/{\tau _i}}}$ represents transient decaying stress with time.The relaxation behavior of HRC‐2, HSR‐2, and cntr‐H hydrogels was then analyzed by comparing the required time to relax 90% of transient stress during constant strain load (Tr90%).The results obtained from the stress relaxation tests showed that the rough‐surfaced particles have a significant role in viscoelastic behavior of the composite HRCs hydrogels, as shown in Figure 4. Not only peak stress was higher for HRC‐2 compared to HSR‐2 and cntr‐H, but also a longer relaxation time was observed for this group. Various factors could contribute to the viscoelastic behavior of the composite hydrogel such as material composition, crosslinking nature, and morphological features of colloidal nanoparticles which is the focus of the present study.[23–25] The increased peak stress for HRC‐2 indicates superior resistance of the composite network to deformation owing to stronger interactions of rough‐surfaced RC particles with polymeric chains and NFC. In parallel, the longer relaxation time could be associated with enhanced frictional drag of interstitial fluid due to particles roughness and/or more stable engagement of solid network components that are dynamically linked by rough RC particles. This further confirmed that the rough‐surfaced nanoparticles (RC) modulate the macroscopic properties of HRC hydrogels as explained.4FigureThe role of nanoparticles roughness in stress relaxation behavior of composite hydrogels. a) Incorporation of rough surfaced nanoparticles in composite hydrogels almost doubled relaxation time of HRC‐2 group, b) representative transient decaying stress of fabricated hydrogels until equilibrium state under constant strain load obtained by fitting a modified Maxwellian model (dashed lines) on measured data (solid lines).ConclusionRough‐surfaced raspberry‐like colloidal (RC) nanoparticles were synthesized to afford the HRC composite hydrogels. Furthermore, smooth‐surfaced PMMA colloidal (SC) nanoparticles were chosen to develop the HSC composite hydrogels, and also for a comparative study. The comparison proved that HRC‐2 hydrogel displayed higher and robust dissipative (≈75%), compressive (≈twofold), viscoelastic (≈fourfold), and adhesive properties (≈50%) than HSC‐2 and cntr‐H hydrogels. The relaxation behavior of HRC‐2, HSR‐2, and cntr‐H hydrogels was then analyzed by comparing the required time to relax 90% of transient stress during constant strain load (Tr90%). Considering the relaxation behavior of HRC‐2 hydrogel, we readily claim that the nano‐surface heterogeneities of RC particles are majorly responsible to control the dynamic and molecular interactions of the polymer unites of HRC gels that enhance the frictional drag at the interfaces and eventually temper the macroscopic properties of composite hydrogels. As there is no roughness on SC particles, we did not observe such improvements in the intrinsic properties of HSC hydrogels. We believe that such nanoparticles with surface heterogeneities can be used to manifest various daily life‐use materials for safety, load‐bearing, and mechanosensitive biomedical applications, to name a few.Experimental SectionAll starting chemicals were purchased from Merck's Sigma Aldrich and used as received unless stated otherwise. PEGDMA was purchased from Polysciences with a molecular weight of 20 kDa. The biodurable nanofibrillated cellulose (NFC) was provided by EMPA (Swiss Federal Laboratories for Materials Science and Technology, Dübendorf, Switzerland). The aqueous NFC had a negatively charged surface due to the abundant hydroxyl groups present in the cellulose polymer.[26] All aqueous solutions were made in doubly Milli‐Q water with a resistivity of 18.2 MΩ cm−1 at 25 °C.The rough‐surfaced raspberry‐like colloidal (RC),[13] and smooth‐surfaced PMMA colloidal (SC) nanoparticles[17] were synthesized as previously reported.Dynamic Light ScatteringDynamic light scattering (DLS) size, and zeta potential (ZP) surface charge measurements were performed on Malvern Zetasizer NS90 instrument. All the measurements were repeated at least five times.Scanning Electron Microscopy (SEM)Scanning electron microscopy (SEM) images were recorded by using InLens detector of GEMINI SEM 300 (Carl Zeiss Germany) at high voltage (HV). The samples were placed on a dual‐sided carbon tape stuck to a holder and coated by 10 nm Iridium thin film before the analysis.Compression and Dissipation MeasurementCompression tests of the developed hydrogels were carried out by preparing cylindrical shape samples with a diameter of 8 mm and a height of 4 mm. The compressive loading was applied at a displacement rate of 0.1 mm. s−1 and the load‐displacement was recorded. The samples were loaded to 50% of the compressive strain. The compressive modulus was calculated from the slope of the best linear fit to the stress–strain curve. The bulk energy dissipation of the specimens was measured from the hysteresis loop area in the stress‐strain curve during the loading‐unloading process (n ≥ 3) The mechanical testing was performed using an Instron E3000 linear mechanical testing machine (Norwood, MA, USA) with a 50 N load cell.Lap‐Shear Adhesion MeasurementThe adhesive strength of the nanoparticle‐reinforced hydrogel systems was evaluated using a shear test setup. Briefly, 100 µL of the hydrogel precursors was applied and cured between the overlap area of two gelatin‐coated glass slides with a contact area of 20 mm × 25 mm, so that a thin layer of the hydrogel formed attached to the slides. Subsequently, the glass slides were gripped into the mechanical setup. The adhesion measurements were conducted under shear loading at a constant loading rate of 1 mm.s−1 using an Instron mechanical machine, as described previously. The adhesion strength of the samples was measured by dividing the maximum load by the contact area (n ≥ 3).Relaxation Behavior TestingUnconfined stress relaxation experiments were performed on the developed hydrogels by application of 40% compressive strain with a uniaxial testing machine (Instron E3000, Norwood, Massachusetts, USA). The force data was collected over a period of 2000 s and then converted to engineering stress values based on initial cross section area of the samples. After curve‐fitting with the generalized Maxwell viscoelastic model, the stress relaxation time was determined.[22]Rheology MeasurementsThe rheological behavior of hydrogels was evaluated using a rheometer (BohlinX) in a parallel disc configuration with the disc diameter of 8 mm. To know the linear viscoelastic region (LVER), the oscillatory amplitude sweep was recorded at 10 rad s−1 frequencies from 1% to 100% strain. The evolution in frequency sweep was recorded in oscillatory mode at strain oscillation of 1 strain (%). The evolution of the storage modulus (G') and viscous (loss) modulus(G'') were measured from frequency sweep.Fabrication of Composite HydrogelsThe nanoparticles and NFC‐reinforced hydrogels were synthesized by dissolving 5.5 wt% of poly (ethylene glycol) dimethacrylate (PEGDMA) in distilled water and mixing with Nanofibril cellulose (NFC, 0.5 vol.%) and ammonium persulfate (0.1 wt%). The rough‐surfaced RC or smooth‐surfaced PMMA SC colloidal particles were subsequently added to the solution at different concentrations (50 µL, 100 µL, 200 µL and 300 µL of respective volume fraction). The mixture was then poured into specific molds and covered with microscope slides. The control hydrogel without colloidal particles was synthesized in the same manner. The polymerization was performed at 60 °C for 60 min.Swelling Ratio MeasurementTo measure the swelling, the polymerized hydrogels were immersed into distilled water to reach the equilibrium state. The swelling ratio was measured by the following equation:2Swelling ratio (%)=(Ws−W0)/W0×100\[\begin{array}{*{20}{c}}{{\rm{Swelling}}\:{\rm{ratio}}\;\left( \\end{array}\]where Ws is the weight of the swollen hydrogel in equilibrium state and W0 is the initial weight.Optical Transmittance MeasurementThe optical transmittance of the hydrogels was measured using the ultraviolet‐visible spectrophotometer (Cary 100 Bio, Varian) within 300–800 nm spectral range with a scan speed of 600 nm min−1 and sampling interval of 1 nm.AcknowledgementsV.K.R. and P.K. contributed equally to this work. The financial support to this work was given by the Swiss National Science Foundation (grant CRSII5_189913).Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available from the corresponding author upon reasonable request.B. A. Grzybowski, W. T. Huck, Nat. Nanotechnol. 2016, 11, 585.A. Mittal, K. Manjunath, R. K. Ranjan, S. Kaushik, S. Kumar, V. Verma, PLoS Pathog. 2020, 16, e1008762.C. Kinnear, T. L. Moore, L. Rodriguez‐Lorenzo, B. Rothen‐Rutishauser, A. Petri‐Fink, Chem. Rev. 2017, 117, 11476.C. Harito, D. V. Bavykin, B. Yuliarto, H. K. Dipojono, F. C. Walsh, Nanoscale 2019, 11, 4653.P. Karami, V. K. Rana, Q. Zhang, A. Boniface, Y. Guo, C. Moser, D. P. Pioletti, Biomacromolecules 2022, 23, 5007.V. K. Rana, A. Tabet, J. A. Vigil, C. J. Balzer, A. Narkevicius, J. Finlay, C. Hallou, D. H. Rowitch, H. Bulstrode, O. A. Scherman, ACS Macro Lett. 2019, 8, 1629.S. Utech, A. R. Boccaccini, J. Mater. 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Nano Surface‐Heterogeneities of Particles Modulate the Macroscopic Properties of Hydrogels

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10.1002/admi.202202248
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Abstract

IntroductionNanoparticle's evolution is inspired mainly by the complexity of nature and living things.[1] Plenty of viruses, bacteria, and cells exist in complicated shapes and sizes, including the latest discovery of spikey‐spherical shaped SARS‐CoV‐2 (COVID‐19) virus [2] to a star‐shaped bacteria, to name a few. All such viruses have definite functions to interact with surfaces, passive diffusion, and active motility, which eventually affect the immune response of a human body.[3]Over two decades, polymer nanocomposites, where polymers are in the continuous phase and nanofillers/nanoparticles are embedded as discontinuous phase, have emerged as the awe‐materials for biomedical, aerospace, electronics, sports equipment, transportation, and household applications. [4,5] Likewise, hydrogels, the water‐swollen polymer 3D networks that mimic features of the native extracellular matrix (ECM), have also been augmented after embedding diverse nanoparticles.[6,7] These composite gels are of utmost interest in biomedical fields since they permit the encapsulation of cells and can be engineered with cells regulating signals. Predominantly, different nanoparticles, by changing their sizes, surface charge/functional groups, and concentrations, were harnessed to influence the macroscopic properties of nanocomposite materials like hydrogels. However, the latest discovery showed the different shapes of a nanoparticle have striking impact/influence on the intrinsic mechanical and adhesive properties of a composite hydrogel.[8]On the contrary, the surface‐heterogeneities of nanoparticles have been merely exploited. The dense suspensions of rough‐surfaced colloidal particles have been studied for their pronounced non‐Newtonian behavior, such as discontinuous shear thickening and shear jamming.[9] These studies recognized that surface roughness is a crucial parameter and demands careful consideration for materials development.[10] We hypothesized that the particles with surface heterogeneities could significantly affect the adsorption, motion, and interfacial forces when interacting with polymeric themselves. In a mechanistic point of view, the colloidal particles can modulate mechanical properties of composite hydrogels by changing macromolecular network resistance to deformation and fluid–solid interactions.[11,12] Rationally, the polymer chains, with specific functionalities, should have a higher friction coefficient while interacting with rough‐surfaced particles than smooth‐surfaced particles. Similarly, suspended rough‐surfaced particles in the fluid phase of a composite hydrogel could influence the frictional drag force during fluid exudation from the polymeric network as well as fluid phase shear stress. Such interactions should modulate the macroscopic physicochemical properties of a composite hydrogel such as stiffness, dissipation, and adhesion.Herein, we endeavored to exploit the influence of surface‐roughness of nanoparticles on the macroscopic properties of composite hydrogels. Raspberry‐like nanoparticles were synthesized for this work. We found that these rough‐surfaced particles played a significant role in the mechanical and adhesive properties of the hydrogel. A comparative study was also performed using smooth‐surfaced nanoparticles for the proof of concept.Results and DiscussionSynthesis of Particles, and Composite Hydrogels FormulationThe rough‐surfaced colloidal nanoparticles were reproduced using a method reported ref. [13]. Briefly, styrene (St), methyl methacrylate (MMA), and a crosslinker divinyl benzene (DVB) were injected in a mixture of water and ethanol, containing an initiator, 2,2′‐azobis(2‐methylpropionamide) dihydrochloride (AIBA), at 70 °C. The ratio between water and ethanol is crucial in producing the monodispersed colloidal nanoparticles. Scanning electron microscopy (SEM) analysis confirmed that each particles has multiple and significantly large bumps on its surface, imitating raspberry, and have a diameter of about 310 nm, as shown in Figure 1a. Zetasizer found that the hydrodynamic diameter (Dh) of these particles is ≈330 nm, and polydispersity index (PDI) is 3.7% (Figure 1b), whereas the surface charge, a Zeta potential is +39.0 (±3.57) mV. The initiator, AIBA, is responsible for imparting the positive surface charge on the nanoparticles. The synthesized raspberry‐like colloidal (RC) particles, in a colloidal solution, were present at a 4.06% volume fraction (or 3.7 wt%).1Figurea) Scanning electron microscopy (SEM) image of rough‐surfaced, raspberry‐like colloidal (RC) nanoparticles. Scale bar is 400 nm, b) the number averaged dynamic light scattering (DLS) size analysis of RC particles. The hydrodynamic diameter (Dh) is 330±11 and polydispersity index (PDI) is 0.037, c) SEM image of smooth‐surfaced, PMMA colloidal (SC) nanoparticles. Scale bar is 400 nm, d) the number averaged DLS size analysis of SC particles. The Dh is 315±17, PDI is 0.058, e) schematic representation of HRCs and HSCs hydrogels formulation. Constant amount of Poly (ethylene glycol) dimethacrylate (PEGDMA = 5.5 wt%) and nanofibril cellulose (NFC = 0.5 wt%) was mixed with a different concentration of RC and SC nanoparticles and thermally polymerized to afford composite hydrogels, f,g) actual composition of HRC‐2 and HSC‐2 composite hydrogels, respectively. It shows how many approximate numbers of particles could be present/available in HRC‐2 and HSC‐2 hydrogels.Since RC particles have a positive surface charge, the obvious choice would be to have negatively charged precursors of hydrogels. The electrostatic interaction should promote the interfacial interactions between RC particles and the polymeric units of a hydrogel. Thus, as reported earlier from our group, we selected a mixture of Poly(ethylene glycol) dimethacrylate (PEGDMA) and nanofibril cellulose (NFC) as the hydrogel's precursor.[14,15] We discovered that when 20 kDa PEGDAM (5.5 wt%) and NFC (0.5 wt%) are mixed, the zeta potential of the solution is about –36.0 (±6.43) mV. Subsequently, different concentrations of RC nanoparticles were mixed to a fixed concentration of PEGDMA+ NFC solution and thermally polymerized to afford various composite hydrogels called HRCs, as depicted in Figure 1e. Briefly, 200 µL RC particles (4.06 vol%) were mixed with PEGDMA + NFC and polymerized at 60 °C to afford a composite hydrogel. It is called HRC‐2. Please note, “2” in HRC‐2 refers to 200 µL of RC nanoparticles. We also calculated an approximate number of particles that could be present in respective hydrogels. Considering the volume fraction of RC particles and the density of styrene (a major part of RC colloids), we calculated that 1 mL of the colloidal solution has approx. ≈2.16 × 1012 RC‐nanoparticles. This indicates that if 200 µL RC colloids are used, there should be approx. ≈4.2 × 1011 nanoparticles in HRC‐2, as mentioned in Figure 1f. In the end, the water content in the hydrogels should be anywhere, between 93 and 94 wt% depending upon the particle's concentration.The hydrogels (HRCs) developed herein are multi‐network hydrogels where PEGDMA responsible for covalent networks, NFCs, and RC are oppositely charged nanofillers with different aspect ratios and responsible in creating various physical as well as electrostatic interactions within the hydrogel polymeric network. As mentioned above, for the synthesis of RC nanoparticles, cationic initiator AIBA has been used. These initiators are primarily responsible for producing positive surface charge on their surface. Moreover, NFCs carry negative surface charge.[16] Thus, positive surface charged particles (RC) create strong electrostatic interaction with negatively charged NFC+PEGDMA solution.ResultsThe roughness on the particle surface is known to dissipate frictional energy when they encounter smooth surfaces, depending on the load and the area of contact. Herein, as stated above, having significant roughness on the RC particles embedded in a hydrogel, should dissipate more energy under stress than a non‐RC hydrogel. Thus, as expected, once 50 µL RC particles added, the obtained HRC‐0.5 composite hydrogel showed an improvement in energy dissipation (Figure 2a) as well as compressive modulus (C‐modulus, Figure 2b) when compared to a non‐RC hydrogel aka cntr‐H. Further improvement was achieved when more RC particles were incorporated. We found that with 200 µL of RC particles, the energy dissipation, of HRC‐2 (2.40 ± 0.3 kJ m−3), was increased by ≈75% whereas C‐modulus (60.83 ± 6.5 kPa) was improved twofold. However, with 300 µL of RC, the energy dissipation of HRC‐3 hydrogel started to decline, but the C‐modulus still increased. As the amount of PEGDMA and NFC was remained constant, such improvement was solely coming after embedding the RC particles to the hydrogels. Nonetheless, it was not clear to emphasize whether nano surface‐heterogeneities present on the RC particles contributing to the dissipation and C‐modulus properties of the hydrogels or not. To this end, we thus considered to synthesize smooth‐surfaced nanoparticles.2FigureMechanical properties of composite hydrogels. a) Energy dissipation of HRCs and control hydrogels, i.e., cntr‐H. It shows that the energy dissipation was increased from HRC‐0.5 to reach to highest for HRC‐2 and started to decline from HRC‐3, b) compressive stiffness (C‐stiffness) of HRCs and cntr‐H hydrogels. Similarly, a subtle increased could be seen up to HRC‐2, and nearly saturated for HRC‐3, c,d) show comparative energy dissipation and C‐stiffness values of cntr‐H, HRC‐2, and HSC‐2, respectively, e) frequency sweep rheology data of HRC‐2, HSC‐2 and cntr‐H. All the hydrogels show an excellent viscoelastic characteristic. However, the elastic(storage) modulus (G') of HRC‐2 is nearly two‐fold and four‐fold higher than HSC‐2 and cntr‐H hydrogels, respectively, f) shear adhesion strength of three hydrogels. The difference was obvious, and as dissipation was increased from cntr‐H to HSC‐2 to HRC‐2 so does the adhesion strength. HRC‐2 shows highest adhesive properties.Previously, it was confirmed that polymerized MMA (PMMA), which is used with styrene and DVB during RC particles synthesis, contributes mainly to the surface roughness (bumps) present on these particles.[13] To mimic a similar surface interaction with the hydrogel network (NFCs+ PEGDMA), we further synthesized only PMMA‐based colloidal nanoparticles having a smooth surface (SC) in water using a method reported earlier.[17] SEM analysis confirmed that PMMA particles are smooth‐surfaced colloids (SC) of ≈290 nm diameter, as shown in Figure 1c. Their hydrodynamic diameter (Dh) is ≈315 nm (Figure 1d), PDI is 5.8% (Figure 1d), and the Zeta potential is +38.0 (±4.81) mV. PMMA or SC nanoparticles, in a colloidal solution, were present at a 4.01% volume fraction (or 3.8 wt%). Moreover, we calculated an approximate number of SC particles in 1 mL solution that is ≈2.44 × 1012.Subsequently, we synthesized two SC nanoparticles‐based composite hydrogels; one with 100 µL, called HSC‐1, and second with 200 µL called HSC‐2, keeping PEGDMA and NFC concentration as same as HRCs, as depicted in Figure 1e,g. We next examined the dissipation, and C‐modulus of both HSC‐1 and HSC‐2 hydrogels, see Figure 2c,d. We found that the difference was not significant, and they (HCS‐1 and HSC‐2) showed similar mechanical properties. Thus, we decided, as the number of particles in HSC‐2 (Figure 2) is comparable to HRC‐2 and total wt% of hydrogel's precursors would be the same, to continue with HSC‐2 hydrogel for a comparative study.When comparing, we discovered that the energy dissipation of HRC‐2 hydrogel (2.40 ± 0.3 kJ m−3) was significantly higher than cntr‐H (1.46 ± 0.22 kJ m−3) and HSC‐2 (1.64 ± 0.16 kJ m−3) hydrogels, as shown in Figure 2c. This confirms that the essential dissipative property of HRC hydrogels is driven by the roughness present on the surface of RC particles. We further examined and compared the C‐modulus and viscoelastic properties of the hydrogels. The HRC‐2 exhibited much higher C‐modulus (60.83 ± 6.5 kPa) than cntr‐H (35.01 ± 7.57 kPa) and HSC‐2 (42.86 ± 2.3 kPa) hydrogels, as represented in Figure 2d. The rheological analysis was performed to understand the viscoelastic behavior of the hydrogels. To this end, we first acquired the oscillatory amplitude sweep at constant frequency to identify the linear viscoelastic region (LVR) of all developed/synthesized hydrogels. Subsequently, the oscillatory frequency sweep was executed within (LVR) to determine the shear storage (elastic) modulus (G') of hydrogels. At 1 rad s−1, HSC‐2 displayed an astounding G’ value, i.e., ≈4500 Pa. It was nearly fourfold, and twofold higher than cntr‐H (≈1200 Pa) and HSC‐2 (≈2050 Pa) hydrogels, respectively. Moreover, HSC hydrogels have superior mechanical properties than the cntr‐H hydrogel. This implies that the embedded smooth‐surfaced particles, expectedly, contribute to the bulk properties of composite hydrogels, like previous findings. Nonetheless, this contribution is not as substantial as having surface‐roughness on the (RC) particles. We further analyzed the swelling ratio and optical transmittance properties of the hydrogels. We found that the swelling ratio of HRC‐2 hydrogel is slightly lower than that of HSC‐2 and cntr‐H hydrogels, see Figure S1 (Supporting Information). Additionally, the swelling ratio in the presence of NFC was almost reduced by half in the studied groups of hydrogels. The optical transmission curve (Figure S2, Supporting Information) of the studied hydrogels was significantly changed in the visible region (380–780 nm). We observed that the transmittance of cntr‐H, HSC‐2, and HRC‐2 is ≈80%, 25%, and 15%, respectively. This indicates that the optical transmission properties of hydrogels can be tuned by changing their composition. Considering these findings, we can readily claim that the nano‐surface heterogeneity of RC nanoparticles dictates the macroscopic properties of composite hydrogels (HRCs).Moreover, the importance of NFC in hydrogel network was also examined by analyzing the C‐modulus of three hydrogels without NFC. We found that without NFC, the C‐modulus of three hydrogels was almost the same, see Figure S3 (Supporting Information). This suggests that the presence of NFC is crucial for forming an efficient hydrogel network and observing such differences in the mechanical properties of HRC and HSC hydrogels.We further examined the adhesive characteristic of composite hydrogels. The adhesive property of hydrogels depends mainly on two different but synergetic aspects. The first is the intrinsic work of adhesion, i.e., the interfacial interactions between the surfaces, and the second is the dissipation mechanism of hydrogels.[14,18] With sufficient interfacial interactions, higher the dissipation, the greater the adhesion.[19] Expectedly, as shown in Figure 2f, HRC‐2 exhibited the highest (6.57 ± 1.62 kPa), HSC‐2 median (5.29 ± 0.78 kPa), and the cntr‐H showed the lowest adhesive property (3.79±0.68 kPa). This finding is consistent concerning their dissipation energy, as shown in Figure 2c.To understand the hydrogels’ network, we next analyzed the freeze‐dried hydrogels by the scanning electron microscopy (SEM). In HRC‐2 hydrogel, the RC nanoparticles were seen throughout the porous network, not aggregated rather dispersed, Figure 3a. Importantly, as depicted in Figure 3b, the particles seem well integrated into the hydrogel's framework and polymer + NFCs are quite involved with RC particles. It appears that there is a strong interaction between the particles and the polymeric network. Conversely, the SC particles, in HSC‐2 hydrogel, seem more like embedded, like usual composite system, Figure 3c,d. They do not seem interacting as good as RC particles do with polymer structures in their respective gels. The SEM of freeze‐dried cntr‐H hydrogel shows a usual porous and crosslinked structure in Figure 3e,f.3FigureScanning electron microscopy (SEM) analysis of composite hydrogels. a) Image shows porous network of HRC‐2 hydrogel where RC particles are seen everywhere evenly distributed, b) RC particles are seen integrated into the hydrogel frameworks. We do not see much of aggregation of particles, c) the porous network of HSC‐2 hydrogels, d) rather embedded SC particles into the hydrogel framework. We do not see as good interaction of SC particles with its hydrogel network, as RC particles have with HRC hydrogel, e,f) images show a usual hydrogel structure of cntr‐H hydrogel. Scale bar of (a,c,e) is 10 µm and (b,d,f) is 1 µm.DiscussionEnergy dissipation is a natural phenomenon where different kinds of energy are transmitted from systems to surroundings in form of heat, friction, etc. There are many innate body reactions that help us to dissipate impacted (pain) energy. For instance, stepping on a hard pointed object while walking/running, the natural reaction, apart from the physiological biomechanics, would be to squeeze/press the foot, which will create muscle bumps, so that the pain energy can be dissipated as quickly as possible. Considering a rather simple example of having two different balls with the same size, weight, and volume but the surface. One has a smooth surface like a tennis ball and another one has a dotted/spikey surface. If we drop them from the same height, we see that the bounce height of a smooth‐surfaced ball is higher than the spikey ball. Moreover, plenty of synthetic engineered materials have been developed to keep us safe from unforeseen accidents after dissipating an impacted energy. For example, the car tires with thread depths and patterns are better with grips and preventing skidding than the flat tires. Similarly, the mouth guards, various helmets, and patterned‐packaging foams, etc. are the most visible examples of such materials. All these materials have macroscopic patterns that help dissipate a major part of an impacted energy anisotropically with high friction coefficient at the interfaces.At molecular level, the mechanism of energy dissipation through the hydrogen‐bond or supramolecular bonds breakages has previously been observed.[11,12,20,21] Additionally, as learned recently, the embedded NFC in PEGDMA hydrogels dissipates more energy than the PEGDMA hydrogels themselves.[14,22] This means we can readily confirm, herein, that the presence of nanoparticles further elevates this property of composite HRC and HSC hydrogels. However, when RC particles with nano‐surface heterogeneities are present in HRC hydrogels, overall developments in the macroscopic properties were overwhelming. Initially, ionic interaction brings nanoparticles (positive zeta potential) and NFC + monomer mixture (negative zeta potential) together, and after the polymerization, the composite hydrogels formed. SEM images confirmed an interfacial interaction between nanoparticles, rather stronger with RC nanoparticles, and the polymers. Thus, as stated above, we can assume that the roughness on the surface of RC nanoparticles should encourage more friction during the dynamic movements of polymer chains and fibers than SC nanoparticles. When the dynamic movement of polymer unites and NFCs is affected by the roughness of RC nanoparticles, the overall relaxation behavior of HRC hydrogels under mechanical load should be different than the HSC hydrogel and cntr‐H hydrogel. To this end, we further examined the stress relaxation behavior of HRC‐2, HSR‐2, and cntr‐H hydrogels, (Figure 4).The modified Maxwellian model as expressed in Equation 1 with four viscoelastic branches was used to extrapolate the test data (acquired for 2000 s) until equilibrium state.[23] A custom‐written MATLAB code was employed to estimate parameters of the model that best fitted the measured data.1σr(t) =σeq +∑i = 1n = 4σie−t/τi\[\begin{array}{*{20}{c}}{{\sigma _{\rm{r}}}\left( t \right)\; = {\sigma _{{\rm{eq}}}}\; + \mathop \sum \limits_{i\; = \;1}^{{\rm{n}}\; = \;4} {\sigma _i}{e^{ - t/{\tau _i}}}}\end{array}\]where, σeq is the equilibrium stress after infinite time and σie−t/τi${\sigma _i}{e^{ - t/{\tau _i}}}$ represents transient decaying stress with time.The relaxation behavior of HRC‐2, HSR‐2, and cntr‐H hydrogels was then analyzed by comparing the required time to relax 90% of transient stress during constant strain load (Tr90%).The results obtained from the stress relaxation tests showed that the rough‐surfaced particles have a significant role in viscoelastic behavior of the composite HRCs hydrogels, as shown in Figure 4. Not only peak stress was higher for HRC‐2 compared to HSR‐2 and cntr‐H, but also a longer relaxation time was observed for this group. Various factors could contribute to the viscoelastic behavior of the composite hydrogel such as material composition, crosslinking nature, and morphological features of colloidal nanoparticles which is the focus of the present study.[23–25] The increased peak stress for HRC‐2 indicates superior resistance of the composite network to deformation owing to stronger interactions of rough‐surfaced RC particles with polymeric chains and NFC. In parallel, the longer relaxation time could be associated with enhanced frictional drag of interstitial fluid due to particles roughness and/or more stable engagement of solid network components that are dynamically linked by rough RC particles. This further confirmed that the rough‐surfaced nanoparticles (RC) modulate the macroscopic properties of HRC hydrogels as explained.4FigureThe role of nanoparticles roughness in stress relaxation behavior of composite hydrogels. a) Incorporation of rough surfaced nanoparticles in composite hydrogels almost doubled relaxation time of HRC‐2 group, b) representative transient decaying stress of fabricated hydrogels until equilibrium state under constant strain load obtained by fitting a modified Maxwellian model (dashed lines) on measured data (solid lines).ConclusionRough‐surfaced raspberry‐like colloidal (RC) nanoparticles were synthesized to afford the HRC composite hydrogels. Furthermore, smooth‐surfaced PMMA colloidal (SC) nanoparticles were chosen to develop the HSC composite hydrogels, and also for a comparative study. The comparison proved that HRC‐2 hydrogel displayed higher and robust dissipative (≈75%), compressive (≈twofold), viscoelastic (≈fourfold), and adhesive properties (≈50%) than HSC‐2 and cntr‐H hydrogels. The relaxation behavior of HRC‐2, HSR‐2, and cntr‐H hydrogels was then analyzed by comparing the required time to relax 90% of transient stress during constant strain load (Tr90%). Considering the relaxation behavior of HRC‐2 hydrogel, we readily claim that the nano‐surface heterogeneities of RC particles are majorly responsible to control the dynamic and molecular interactions of the polymer unites of HRC gels that enhance the frictional drag at the interfaces and eventually temper the macroscopic properties of composite hydrogels. As there is no roughness on SC particles, we did not observe such improvements in the intrinsic properties of HSC hydrogels. We believe that such nanoparticles with surface heterogeneities can be used to manifest various daily life‐use materials for safety, load‐bearing, and mechanosensitive biomedical applications, to name a few.Experimental SectionAll starting chemicals were purchased from Merck's Sigma Aldrich and used as received unless stated otherwise. PEGDMA was purchased from Polysciences with a molecular weight of 20 kDa. The biodurable nanofibrillated cellulose (NFC) was provided by EMPA (Swiss Federal Laboratories for Materials Science and Technology, Dübendorf, Switzerland). The aqueous NFC had a negatively charged surface due to the abundant hydroxyl groups present in the cellulose polymer.[26] All aqueous solutions were made in doubly Milli‐Q water with a resistivity of 18.2 MΩ cm−1 at 25 °C.The rough‐surfaced raspberry‐like colloidal (RC),[13] and smooth‐surfaced PMMA colloidal (SC) nanoparticles[17] were synthesized as previously reported.Dynamic Light ScatteringDynamic light scattering (DLS) size, and zeta potential (ZP) surface charge measurements were performed on Malvern Zetasizer NS90 instrument. All the measurements were repeated at least five times.Scanning Electron Microscopy (SEM)Scanning electron microscopy (SEM) images were recorded by using InLens detector of GEMINI SEM 300 (Carl Zeiss Germany) at high voltage (HV). The samples were placed on a dual‐sided carbon tape stuck to a holder and coated by 10 nm Iridium thin film before the analysis.Compression and Dissipation MeasurementCompression tests of the developed hydrogels were carried out by preparing cylindrical shape samples with a diameter of 8 mm and a height of 4 mm. The compressive loading was applied at a displacement rate of 0.1 mm. s−1 and the load‐displacement was recorded. The samples were loaded to 50% of the compressive strain. The compressive modulus was calculated from the slope of the best linear fit to the stress–strain curve. The bulk energy dissipation of the specimens was measured from the hysteresis loop area in the stress‐strain curve during the loading‐unloading process (n ≥ 3) The mechanical testing was performed using an Instron E3000 linear mechanical testing machine (Norwood, MA, USA) with a 50 N load cell.Lap‐Shear Adhesion MeasurementThe adhesive strength of the nanoparticle‐reinforced hydrogel systems was evaluated using a shear test setup. Briefly, 100 µL of the hydrogel precursors was applied and cured between the overlap area of two gelatin‐coated glass slides with a contact area of 20 mm × 25 mm, so that a thin layer of the hydrogel formed attached to the slides. Subsequently, the glass slides were gripped into the mechanical setup. The adhesion measurements were conducted under shear loading at a constant loading rate of 1 mm.s−1 using an Instron mechanical machine, as described previously. The adhesion strength of the samples was measured by dividing the maximum load by the contact area (n ≥ 3).Relaxation Behavior TestingUnconfined stress relaxation experiments were performed on the developed hydrogels by application of 40% compressive strain with a uniaxial testing machine (Instron E3000, Norwood, Massachusetts, USA). The force data was collected over a period of 2000 s and then converted to engineering stress values based on initial cross section area of the samples. After curve‐fitting with the generalized Maxwell viscoelastic model, the stress relaxation time was determined.[22]Rheology MeasurementsThe rheological behavior of hydrogels was evaluated using a rheometer (BohlinX) in a parallel disc configuration with the disc diameter of 8 mm. To know the linear viscoelastic region (LVER), the oscillatory amplitude sweep was recorded at 10 rad s−1 frequencies from 1% to 100% strain. The evolution in frequency sweep was recorded in oscillatory mode at strain oscillation of 1 strain (%). The evolution of the storage modulus (G') and viscous (loss) modulus(G'') were measured from frequency sweep.Fabrication of Composite HydrogelsThe nanoparticles and NFC‐reinforced hydrogels were synthesized by dissolving 5.5 wt% of poly (ethylene glycol) dimethacrylate (PEGDMA) in distilled water and mixing with Nanofibril cellulose (NFC, 0.5 vol.%) and ammonium persulfate (0.1 wt%). The rough‐surfaced RC or smooth‐surfaced PMMA SC colloidal particles were subsequently added to the solution at different concentrations (50 µL, 100 µL, 200 µL and 300 µL of respective volume fraction). The mixture was then poured into specific molds and covered with microscope slides. The control hydrogel without colloidal particles was synthesized in the same manner. The polymerization was performed at 60 °C for 60 min.Swelling Ratio MeasurementTo measure the swelling, the polymerized hydrogels were immersed into distilled water to reach the equilibrium state. The swelling ratio was measured by the following equation:2Swelling ratio (%)=(Ws−W0)/W0×100\[\begin{array}{*{20}{c}}{{\rm{Swelling}}\:{\rm{ratio}}\;\left( \\end{array}\]where Ws is the weight of the swollen hydrogel in equilibrium state and W0 is the initial weight.Optical Transmittance MeasurementThe optical transmittance of the hydrogels was measured using the ultraviolet‐visible spectrophotometer (Cary 100 Bio, Varian) within 300–800 nm spectral range with a scan speed of 600 nm min−1 and sampling interval of 1 nm.AcknowledgementsV.K.R. and P.K. contributed equally to this work. 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Journal

Advanced Materials Interfaces – Wiley

Published: May 1, 2023

Keywords: adhesive properties; hydrogels; mechanical properties; raspberry nanoparticles; relaxation behavior; smooth‐surfaced particles; surface roughness

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