Impact of Aqueous Grafting of Polystyrene through Methacrylate-Modified Cellulose Nanofibrils on Emulsion Stabilization and Drying Behavior.

Cellulose nanofibrils (CNFs) are abundant materials limited in application by their hydrophilic nature and fibrillar collapse during drying. Herein, hydrophobic CNFs (PS-MetCNFs) were produced via the grafting of polystyrene through a methacrylate handle on modified CNFs. This modification prevented fibrillar collapse of the CNFs upon drying with as low as 3.5 wt % polystyrene. System characterization through kinetics studies and controls revealed that a surfactant-free emulsion polymerization ran parallel to the grafting-through polymerization. Polystyrene on the PS-MetCNFs was both covalently bound and noncovalently bound. This noncovalently bound polymer was due to polymerization in the monomer-swollen polymer particles on the PS-MetCNF surface. The polystyrene modification interfered with CNF Pickering emulsion behavior, instead stabilizing monomer-swollen polymer particles entangled in the CNF network. Composites of PS-MetCNFs and poly(lactic acid) showed higher maximum tensile stress and modulus at 5 wt % loading relative to composites made with unmodified spray-dried CNFs, demonstrating a route to composite reinforcement with CNFs without energy-intensive spray drying.

Aqueous Polymer Modification of Cellulose Nanofibrils by Grafting-Through a Reactive Methacrylate Group.

Modifying the surface of cellulose nanofibrils (CNFs) produced by mechanical refinement with a variety of polymer functional groups in an entirely water-based system is challenging because only surface hydroxyl groups are accessible. To address this limitation, an entirely water-based, polymer modification scheme is developed. CNFs are functionalized with a reactive methacrylate functional group followed by subsequent grafting-through polymerization. This modification worked with a variety of water-soluble and water-insoluble (meth)acrylates and (meth)acrylamides, grafting up to 45 wt% polymer on to the CNFs. The reaction conditions introducing the methacrylate functional group are adjusted to vary the degree of functionality. Soxhlet extraction of modified samples demonstrates that the reactive methacrylate group is necessary to facilitate polymer grafting. The degree of functionalization of the polymers is studied via quantitative transmission IR spectroscopy and the morphology of the resulting cellulose nanofibrils is studied via a combination of optical, scanning electron, and atomic force microscopy. High levels of polymer modification do not significantly affect the micrometer-scale fibril morphology.

Fluorescent Dye Adsorption in Aqueous Suspension to Produce Tagged Cellulose Nanofibers for Visualization on Paper.

Cellulose nanofibers (CNFs) have great potential to be a layer in packaging materials because of their good barrier properties. When paper is coated with CNFs, they are difficult to distinguish from the base sheet. This issue creates challenges when trying to determine where CNFs migrate relative to the paper fibers during coating and drying. A three- dimensional analysis is possible by using confocal laser scanning microscopy (CLSM) if CNFs can be tagged with fluorescently active groups. In this study, CNFs were fluorescently tagged through adsorption of fluorescent dyes such as fluorescein isothiocyanate (FITC) and thioflavin by mixing with CNFs in their native suspension followed by purification. The adsorbed dye remained attached during typical coating procedures, low pH values, and high ionic strengths, but not for high pH and in contact with acetone. CNFs were also covalently tagged with FITC following methods reported in the literature as a comparison to already established methods for tagging cellulose nanocrystals (CNCs). Images of never dried samples indicated that covalently tagging CNFs altered the state of the fines dispersion, while dye adsorption did not. Coatings of the adsorbed dye tagged CNFs on paper were successfully imaged by CLSM since the concentration of dye in the water phase was low enough to provide a good contrast between regions of CNFs and paper. With this method, the location and potential migration of CNFs coated on paper were successfully determined for the first time to the best of our knowledge. CNF based coatings with solids larger than 2.8% were found to have a distinct layer of CNFs at the paper surface with little CNFs penetrating into the paper structure, but lower solids result in significant penetration into the paper.

Transdermal gelation of methacrylated macromers with near-infrared light and gold nanorods.

Injectable hydrogels provide locally controlled tissue bulking and a means to deliver drugs and cells to the body. The formation of hydrogels in vivo may involve the delivery of two solutions that spontaneously crosslink when mixed, with pH or temperature changes, or with light (e.g., visible or ultraviolet). With these approaches, control over the kinetics of gelation, introduction of the initiation trigger (e.g., limited penetration of ultraviolet light through tissues), or alteration of the material physical properties (e.g., mechanics) may be difficult to achieve. To overcome these limitations, we used the interaction of near-infrared (NIR) light with gold nanorods (AuNRs) to generate heat through the photothermal effect. NIR light penetrates tissues to a greater extent than other wavelengths and provides a means to indirectly initiate radical polymerization. Specifically, this heating coupled with a thermal initiator (VA-044) produced radicals that polymerized methacrylated hyaluronic acid (MeHA) and generated hydrogels. A range of VA-044 concentrations changed the gelation time, yielding a system stable at 37 ° C for 22 min that gels quickly (~3 min) when heated to 55 ° C. With a constant irradiation time (10 min) and laser power (0.3 W), different VA-044 and AuNR concentrations tuned the compressive modulus of the hydrogel. By changing the NIR irradiation time we attained a wide range of moduli at a set solution composition. In vivo mouse studies confirmed that NIR laser irradiation through tissue could gel an injected precursor solution transdermally.

Tunable, thiol-ene, interpenetrating network hydrogels of norbornene-modified carboxymethyl cellulose and cellulose nanofibrils.

Carboxymethyl cellulose modified with norbornene groups (NorCMC) and cellulose nanofibrils (CNFs) produced through mechanical refining without chemical pretreatment formed interpenetrating network hydrogels through a UV-light initiated thiol-ene reaction. The molar ratio of thiols in crosslinkers to norbornene groups off the NorCMC (T:N), total polymer weight percent in the hydrogel, and weight percent of CNFs of the total polymer content of the hydrogels were varied to control hydrogel properties. This method enabled orders of magnitude changes to behavior. Swelling in aqueous environments could be significant (>150 %) without CNFs to minimal (<15 %) with the use of 50 % CNFs. NorCMC and CNF networks interacted synergistically to create hydrogels with compression modulus values spanning 1 to 150 kPa - the values of most biological tissues. T:N and total polymer weight percent could be varied to create hydrogels with different CNF content, but the same compression modulus, targeting 10 and 100 kPa hydrogels and providing a system that can independently vary fibrillar content and bulk modulus. Analysis of the effective crosslinks, thiol-ene network mesh size, and burst release of the polymer indicated synergistic interactions of the NorCMC thiol-ene and CNFs networks. These interactions enhanced modulus and degradation control of the network under physiological conditions.

Optimizing lignocellulosic nanofibril dimensions and morphology by mechanical refining for enhanced adhesion.

Using lignocellulosic nanofibrils as adhesive binders in structural composites is a growing field of interest attributable to their renewability, recyclability, and strength. A fundamental understanding of their adhesion mechanisms is crucial to tailor performance and optimize production costs. These mechanisms were elucidated by studying the morphology dependent adhesion in a model system composed of cellulose nanofibrils (CNFs) at different degrees of refinement and porous paper substrates. CNFs and lignin containing cellulose nanofibrils (LCNF) were characterized at different length scales using optical, atomic force, and scanning electron microscopy, revealing a complex distribution of sizes, spanning the macroscale to the nanoscale, which are modified unequally by refinement. Strong adhesion was correlated to a decrease in fiber size on the largest length scale and with an increase in relative fibril surface area. Flocculation hampered effective LCNF adhesion, but adding suspension stabilizers improved adhesion to levels comparable to CNF.

The influence of versatile thiol-norbornene modifications to cellulose nanofibers on rheology and film properties.

Cellulose nanofibrils (CNF) can form impressive barrier layers but difficult rheological properties, brittleness, and sensitivity to moisture limit their use. To overcome these challenges, esterification reactions were performed in water without volatile organic solvents to create carbic-functionalized CNFs (cCNFs) that enabled versatile, thiol-norbornene secondary modifications. Chemical analysis determined that on average 5% anhydroglucose repeat units were functionalized with norbornene groups. Thiol-norbornene reactions added molecules with varying polar surface areas to the CNFs. Modifications did not change the film properties to a large extent. All CNF films were excellent grease barriers. The modifications significantly changed the rheology of CNF suspensions as the complex viscosity of the modified CNF was 27 times lower than unmodified CNFs. Modification also reduced the filtration rate by a factor of four. Surface modifications appeared to alter the colloidal forces between fibers in suspension that influence the flow and drainage properties.

Thiol-norbornene reactions to improve natural rubber dispersion in cellulose nanofiber coatings.

Cellulose nanofibrils (CNF) coatings are excellent grease barriers for biodegradable packaging. However, barrier properties are moisture sensitive, so hydrophobic components such as latexes or polymers are needed to impart moisture resistance, but incompatibility leads to poor dispersion. In this work, CNF modified with norbornenes was reacted with natural rubber (NR) latex in water to improve dispersion, coating formation, and coating moisture resistance by creating hybrid particles. Mixtures and reacted samples were coated through filtration onto paper. The hybrid particles improved NR retention and drainage rate as compared to the CNF/latex mixture. Hybrid particle coatings showed more uniform NR dispersion as compared to the mixtures, which formed NR and CNF layers. NR addition to coatings increased the water contact angle, reduced water absorption, and decreased the water vapor transmission rate. All coatings passed a 12-kit grease test before folding and the mixtures formed a crack-resistant CNF layer on the surface.

Dose and Timing of N-Cadherin Mimetic Peptides Regulate MSC Chondrogenesis within Hydrogels.

The transmembrane glycoprotein N-cadherin (NCad) mediates cell-cell interactions found during mesenchymal condensation and chondrogenesis. Here, NCad-derived peptides (i.e., HAV) are incorporated into hyaluronic acid (HA) hydrogels with encapsulated mesenchymal stem cells (MSCs). Since the dose and timing of NCad signaling are dynamic, HAV peptide presentation is tuned via alterations in peptide concentration and incorporation of an ADAM10-cleavable domain between the hydrogel and the HAV motif, respectively. HA hydrogels functionalized with HAV result in dose-dependent increases in early chondrogenesis of encapsulated MSCs and resultant cartilage matrix production. For example, type II collagen and glycosaminoglycan production increase ≈9- and 2-fold with the highest dose of HAV (i.e., 2 × 10-3 m), respectively, when compared to unmodified hydrogels, while incorporation of an efficient ADAM10-cleavable domain between the HAV peptide and hydrogel abolishes increases in chondrogenesis and matrix production. Treatment with a small-molecule ADAM10 inhibitor restores the functional effect of the HAV peptide, indicating that timing and duration of HAV peptide presentation is crucial for robust chondrogenesis. This study demonstrates a nuanced approach to the biofunctionalization of hydrogels to better emulate the complex cell microenvironment during embryogenesis toward stem-cell-based cartilage production.

Synthesis and Spatiotemporal Modification of Biocompatible and Stimuli-Responsive Carboxymethyl Cellulose Hydrogels Using Thiol-Norbornene Chemistry.

Carboxymethyl cellulose (CMC) is functionalized with norbornene groups to undergo thiol-norbornene cross-linking reactions. Hydrogels synthesized from a single norbornene-modified carboxymethyl cellulose (NorCMC) via a light-initiated thiol-ene cross-linking reaction with a variety of dithiol cross-linkers yield hydrogels with a tunable compression modulus ranging from 1.7 to 103 kPa. Additionally, thermoresponsiveness is spatiotemporally imparted to NorCMC hydrogels by photopatterning a dithiol-terminated poly(N-isopropyl acrylamide) cross-linker, enabling swelling and topological control of the hydrogels as a function of incubation temperature. NorCMC hydrogels are cytocompatible as the viability of encapsulated human mesenchymal stem cells (hMSCs) is greater than 85% after 21 d while using a variety of cross-linkers. Moreover, hMSCs can remodel, adhere, and spread in the NorCMC matrix cross-linked with a matrix metalloproteinase-degradable peptide, further demonstrating the utility of these materials as a tunable biomaterial.

Dry-Spun Neat Cellulose Nanofibril Filaments: Influence of Drying Temperature and Nanofibril Structure on Filament Properties.

Cellulose nanofibrils (CNF) were spun into filaments directly from suspension without the aid of solvents. The influence of starting material properties and drying temperature on the properties of filaments produced from three different CNF suspensions was studied. Refiner-produced CNF was ground using a microgrinder at grinding times of 50 and 100 minutes. Filament spinning was performed using a syringe pump-heat gun setting at three drying temperatures of 210 °C, 320 °C and 430 °C. The structure of starting CNF materials was first evaluated using a combination of optical and atomic force microscopy (AFM) techniques. Surface free energy analysis and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR⁻FTIR) were used to study changes in hydrophobicity due to grinding. Morphology of the filaments was studied using SEM micrographs. The influence of different drying temperatures and grinding times on mechanical properties of the CNF filaments were further investigated through tensile tests and results were compared using statistical analysis .It was observed that drying temperature did not significantly influence the tensile properties of the filaments while cellulose nanofiber suspension type (grinding time) had a significant influence and improved mechanical properties. FTIR results confirmed an increase in crystallinity index and decrease in hydroxyl group availability due to grinding.

Spatiotemporal Modification of Stimuli-Responsive Hyaluronic Acid/Poly(N-isopropylacrylamide) Hydrogels.

Current methods to spatiotemporally modify stimuli response in hydrogels are typically subtractive and lead to a decrease in response. To increase the breadth of hydrogel applications and biomedical systems, new formulations are needed that can introduce and increase stimuli response spatiotemporally in hydrogels. In this work, the light-induced thiol-norbornene click chemistry reaction was used to modify the stimuli response of robust hyaluronic acid hydrogels through an additive process in spatiotemporal fashion, overcoming this limitation. These stimuli-responsive hydrogels were made from norbornene-functionalized hyaluronic acid (NorHA) cross-linked with thermoresponsive dithiol-terminated poly(N-isopropylacrylamide) (DTPN). Variation of the cross-linker molecular weight and gelation conditions led to a range of compression modulus (5 to 54 kPa) and mass loss (9 to 33%) upon heating to 37 °C while retaining a majority of NorHA in the hydrogel. The thermoresponse of these hydrogels could be controlled not only by the cross-link density but also by heating to 55 °C to increase the dewatering of the hydrogels. The stimuli response of the hydrogels was temporally increased by introducing additional DTPN and UV initiator to an original hydrogel with subsequent irradiation. This modification was extended to spatiotemporally changing the stimuli response by photopatterning DTPN into a NorHA hydrogel, yielding a hydrogel that changed shape and topology through heating. Furthermore, human mesenchymal stem cells could adhere and proliferate on the DTPN-patterned surface, demonstrating that the materials could be used for studies where cells are present.

Nanofibrous hydrogels with spatially patterned biochemical signals to control cell behavior.

The ability to spatially pattern biochemical signals into nanofibrous materials using thiol-ene reactions of thiolated molecules to presented norbornene groups is demonstrated. This approach is used to pattern three molecules independently within one scaffold, to pattern molecules through the depth of a scaffold, and to spatially control cell adhesion and morphology.

Nanometer-scale self-assembly of amphiphilic copolymers to control and prevent biofouling.

Bacterial infections occur on nearly 4% of all implanted medical devices, leading to a loss in patient quality of life, higher medical costs, and in some cases permanent disability. These infections typically form biofilms that limit the effectiveness of antibiotics and may require removal of the device. Since the infections are difficult to cure once established, methods to prevent the initial bacterial infection have been investigated. Biocidal surfaces can be effective in preventing bacterial colonization, but they do not prevent the non-specific adhesion of biomacromolecules that are the precursor to bacterial attachment. Self-assembled monolayers can prevent biomacromolecule adsorption, but their effectiveness diminishes over time due to monolayer desorption. Robust amphiphilic copolymers that self-assemble into distinct phases on the nanometer-scale can prevent biomacromolecule adsorption, and subsequent organism adhesion and biofilm formation. These coatings phase separate on the length scale of biomacromolecules and disrupt their adhesion mechanism. In this review, the development of amphiphilic polymer architectures that phase separate on the nanometer-scale is discussed with a focus on the different amphiphilic copolymer architectures used and their prevention of biofouling. Though a nascent technique in this field, phase separated amphiphilic copolymer coatings have significant potential to prevent bacterial infections on implanted medical devices.

Synthesis and orthogonal photopatterning of hyaluronic acid hydrogels with thiol-norbornene chemistry.

The patterning of chemical and mechanical signals within hydrogels permits added complexity towards their use as cell microenvironments for biomedical applications. Specifically, photopatterning is emerging to introduce heterogeneity in hydrogel properties; however, currently employed systems are limited in the range of properties that can be obtained, as well as in decoupling mechanical properties from changes in chemical signals. Here, we present an orthogonal photopatterning system that utilizes thiol-norbornene chemistry and permits extensive hydrogel modification, including with multiple signals, due to the number of reactive handles accessible for secondary reaction. Hyaluronic acid was functionalized with norbornene groups (NorHA) and reacted with di-thiols to create non-toxic hydrogels with a wide range of mechanical properties. For example, for 4 wt% NorHA at 20% modification, hydrogel mechanics from ≈ 1 kPa up to ≈ 70 kPa could be obtained by simply changing the amount of crosslinker. By limiting the initial extent of crosslinking, NorHA gels were synthesized with remaining pendent norbornene groups that could be reacted with thiol containing molecules in the presence of light and an initiator, including with spatial control. Secondary reactions with a di-thiol crosslinker changed mechanical properties, whereas reaction with mono-thiol peptides had no influence on the gel elastic modulus. This orthogonal chemistry was used sequentially to pattern multiple peptides into a single hydrogel, demonstrating the robustness of this system for the formation of complex hydrogels.