Pinecone-Inspired Humidity-Responsive Paper Actuators with Bilayer Structure.

Many plant materials in nature have the ability to change their shape to respond to external stimuli, such as humidity or moisture, to ensure their survival or safe seed release. A well-known example for this phenomenon is the pinecone, which is able to open its scales at low humidity due to the specific bilayer structures of the scale. Inspired by this, we developed a novel humidity-driven actuator based on paper. This was realized by the lamination of untreated paper made from eucalyptus fibers to a paper-carboxymethyl cellulose (CMC) composite. As observed, the hygroexpansion of the composite can be easily controlled by the amount of CMC in the impregnated paper sheet, which, thus, controls the morphologic deformation of the paper bilayer. For a more detailed understanding of these novel paper soft robots, we also studied the dynamic water vapor adsorption, polymer distribution and hygroexpansion of the paper-polymer composites. Finally, we applied a geometrically nonlinear finite element model to predict the bending behavior of paper bilayers and compared the results to experimental data. From this, we conclude that due to the complexity of structure of the paper composite, a universal prediction of the hygromorphic behavior is not a trivial matter.

Single-fibre coating and additive manufacturing of multifunctional papers.

Paper-based materials with precisely designed wettabilities show great potential for fluid transport control, separation, and sensing. To tune the wettability of paper, paper sheets are usually modified after the paper manufacturing process. This limits the complexity of the local wettability design. We combined the wettability design of the individual fibres with subsequent paper sheet fabrication through either fibre deposition or fibre printing. Using silica-based cellulose fibre functionalization, the wettability of the paper sheets, containing only one specific fibre type, could be gradually tuned from highly hydrophilic to highly hydrophobic, resulting in water exclusion. The development of a silica-functionalized fibre library containing mesoporous or dense silica coatings, as well as silica with varying precursor compositions, further enabled the variation of the paper wettability and fluid flow. By combining this fibre library with the paper fabrication process by (i) fibre deposition or (ii) fibre printing, the paper wettability architecture and thus the local fibre composition were adjusted without any further processing steps. This enabled the fabrication of papers with wettability integration, such as a wettability pattern or a Janus paper design, containing wettability gradients along the paper sheet cross section. This asymmetric wettability along all three spatial dimensions enabled side-selective oil-water separation.

Controlling Fluorescent Readout in Paper-based Analytical Devices.

Paper is an ideal candidate for the development of new disposable diagnostic devices because it is a low-cost material, allows transport of the liquid on the device by capillary action, and is environmentally friendly. Today, colorimetric analysis is most often used as a detection method for rapid tests (test strips or lateral flow devices) but usually gives only qualitative results and is limited by a relatively high detection threshold. Here, we describe studies using fluorescence as a readout tool for paper-based diagnostics. We study how the optical readout is affected by light transmission, scattering, and fluorescence as a function of paper characteristics such as thickness (grammage), water content, autofluorescence, and paper type/composition. We show that paper-based fluorescence analysis allows better optical readout compared to that of nitrocellulose, which is currently the material of choice in colorimetric assays. To reduce the loss of analyte molecules (e.g., proteins) due to adsorption to the paper surface, we coat the paper fibers with a protein-repellent hydrogel. For this purpose, we use hydrophilic copolymers consisting of N,N-dimethyl acrylamide and a benzophenone-based cross-linker, which are photochemically transformed into a fiber-attached polymer hydrogel on the paper fiber surfaces in situ. We show that the combination of fluorescence detection and the use of a protein-repellent coating enables sensitive paper-based analysis. Finally, the success of the strategy is demonstrated by using a simple LFD application as an example.

Cross-Linking of Oxidized Hydroxypropyl Cellulose in Paper: Influence of Molecular Weight and Polymer Distribution on Paper Wet Strength Development.

With the overarching aim for the development of sustainable, nontoxic wet strength agents for paper, a novel polymer gel system based on oxidized hydroxypropyl cellulose (keto-HPC) cross-linked with polyamines was investigated in detail to gain a deeper insight into the wet strength mechanism. When applied to paper, this wet strength system significantly increases the relative wet strength by using only low amounts of polymer, and it is therefore comparable with established wet strength agents based on fossil resources, such as polyamidoamine epichlorohydrin resins. With the help of ultrasonic treatment, keto-HPC was degraded with respect to its molecular weight and further cross-linked in paper using polymeric amine-reactive counterparts. The resulting polymer-cross-linked paper mechanical properties were analyzed with respect to the dry and wet tensile strength, respectively. In addition, we analyzed the polymer distribution using fluorescence confocal laser scanning microscopy (CLSM). If high-molecular-weight samples are being used for cross-linking, we do find accumulation of the polymer mainly on the surface of the fibers and at fiber crossing points, accompanied with enhancing strong effects on paper's wet tensile strength. In contrast, if low-molecular-weight (i.e., degraded) keto-HPC is being applied, the macromolecules are capable of entering the inner porous structure of the paper fibers, and almost no accumulation at the fiber crossing points is observed, which also results in a lowered wet paper tensile strength, respectively. This insight into wet strength mechanisms of the keto-HPC/polyamine system can thus lead to new opportunities for the development of alternative biobased wet strength agents where molecular weight dependence of the wet tensile properties allows for a fine tuning of mechanical properties in the wet state.

Chemical Gradients in Polymer-Modified Paper Sheets-Towards Single-Layer Biomimetic Soft Robots.

Biomimetic actuators are typically constructed as functional bi- or multilayers, where actuating and resistance layers together dictate bending responses upon triggering by environmental stimuli. Inspired by motile plant structures, like the stems of the false rose of Jericho (Selaginella lepidophylla), we introduce polymer-modified paper sheets that can act as soft robotic single-layer actuators capable of hygro-responsive bending reactions. A tailored gradient modification of the paper sheet along its thickness entails increased dry and wet tensile strength and allows at the same time for hygro-responsiveness. For the fabrication of such single-layer paper devices, the adsorption behavior of a cross-linkable polymer to cellulose fiber networks was first evaluated. By using different concentrations and drying procedures fine-tuned polymer gradients throughout the thickness can be achieved. Due to the covalent cross-linking of polymer with fibers, these paper samples possess significantly increased dry and wet tensile strength properties. We furthermore investigated these gradient papers with respect to a mechanical deflection during humidity cycling. The highest humidity sensitivity is achieved using eucalyptus paper with a grammage of 150 g m-2 modified with the polymer dissolved in IPA (~13 wt%) possessing a polymer gradient. Our study presents a straightforward approach for the design of novel hygroscopic, paper-based single-layer actuators, which have a high potential for diverse soft robotic and sensor applications.

A Solvent-Free Approach to Crosslinked Hydrophobic Polymeric Coatings on Paper Using Vegetable Oil.

Hydrophobic coatings are of utmost importance for many applications of paper-based materials. However, to date, most coating methods demand vast amounts of chemicals and solvents. Frequently, fossil-based coating materials are being used and multiple derivatization reactions are often required to obtain desired performances. In this work, we present a solvent-free paper-coating process, where olive oil as the main biogenic component is being used to obtain a hydrophobic barrier on paper. UV-induced thiol-ene photocrosslinking of olive oil was pursued in a solvent-free state at a wavelength of 254 nm without addition of photoinitiator. Optimum reaction conditions were determined in advance using oleic acid as a model compound. Paper coatings based on olive oil crosslinked by thiol-ene reaction reach water contact angles of up to 120°. By means of Fourier transform infrared spectroscopy and differential scanning calorimetry, a successful reaction and the formation of a polymer network within the coating can be proven. These results show that click-chemistry strategies can be used to achieve hydrophobic polymeric paper coatings while keeping the amount of non-biobased chemicals and reaction steps at a minimum.

Reducing Unspecific Protein Adsorption in Microfluidic Papers Using Fiber-Attached Polymer Hydrogels.

Microfluidic paper combines pump-free water transport at low cost with a high degree of sustainability, as well as good availability of the paper-forming cellulosic material, thus making it an attractive candidate for point-of-care (POC) analytics and diagnostics. Although a number of interesting demonstrators for such paper devices have been reported to date, a number of challenges still exist, which limit a successful transfer into marketable applications. A strong limitation in this respect is the (unspecific) adsorption of protein analytes to the paper fibers during the lateral flow assay. This interaction may significantly reduce the amount of analyte that reaches the detection zone of the microfluidic paper-based analytical device (µPAD), thereby reducing its overall sensitivity. Here, we introduce a novel approach on reducing the nonspecific adsorption of proteins to lab-made paper sheets for the use in µPADs. To this, cotton linter fibers in lab-formed additive-free paper sheets are modified with a surrounding thin hydrogel layer generated from photo-crosslinked, benzophenone functionalized copolymers based on poly-(oligo-ethylene glycol methacrylate) (POEGMA) and poly-dimethyl acrylamide (PDMAA). This, as we show in tests similar to lateral flow assays, significantly reduces unspecific binding of model proteins. Furthermore, by evaporating the transport fluid during the microfluidic run at the end of the paper strip through local heating, model proteins can almost quantitatively be accumulated in that zone. The possibility of complete, almost quantitative protein transport in a µPAD opens up new opportunities to significantly improve the signal-to-noise (S/N) ratio of paper-based lateral flow assays.

Porosity Centrifuge: Determination of Pore Sizes of Swellable Porous Materials under Hypergravity.

Porous materials are ubiquitous and essential for many processes in nature as well as in industry, and the need to produce them from renewable materials will definitely increase. A prominent example for such a fully recyclable and biogenic porous material is paper, a material that contains macropores formed in between the fibers as well as a large distribution of much finer pores on and within the fiber walls. While the determination of pore sizes is of central importance for the characterization of such materials, their determination is usually only possible with complex methodologies. The determination of pore sizes in the context of water has remained largely unsolved to date, in particular, if water-swellable materials are considered. Here, we introduce a completely new way of determining pore sizes of materials even under swelling conditions. Using a centrifugal device and studying the imbibition of water into paper at various centrifugal forces that oppose the capillary forces, we can access the mean pore size of different paper materials in an experimentally simple fashion. In addition, we can show that the pore size values obtained with our "centrifugal porosimetry" are consistent with the values obtained using other methods, usually much more involved methods. For this purpose, we measure well-characterized translucent macroporous materials using water, ranging from simple glass capillaries to standard filters and nitrocellulose membranes.

Toward Fabrication of Bioactive Papers: Covalent Immobilization of Peptides and Proteins.

Herein, we report a novel two-step method for the covalent, site-directed, and efficient immobilization of proteins on lab-made paper sheets. First, paper fibers were modified with a peptidic anchor comprising enzyme recognition motifs. Four different conjugation strategies for peptide immobilization were evaluated with respect to reproducibility and fiber loading efficiency. After manufacturing of the peptide-preconditioned paper, oriented conjugation of the model protein tGFP containing a C-terminal recognition sequence for either sortase A or microbial transglutaminase was assessed semiquantitatively by fluorescence measurement and inspected by confocal laser scanning microscopy (CLSM). The two enzymes utilized for protein conjugation used the same oligoglycine peptide anchor, and both proved to be suitable for controlled oriented linkage of substrate proteins at physiological conditions.

Diazo-Based Copolymers for the Wet Strength Improvement of Paper Based on Thermally Induced CH-Insertion Cross-Linking.

We present an alternative to commonly used, but from an environmental point of view, problematic wet strength agents, which are usually added to paper to prevent a loss of mechanical stability and finally disintegrate when they get into contact with water. To this end, diazoester-containing copolymers are generated, which are coated onto paper and by heating to 110-160 °C for short periods of time become activated and form carbene intermediates, which undergo a CH-insertion cross-linking reaction. The process leads to a simultaneous cross-linking of the polymer and its attachment to the cellulose substrate. The immobilization process of copolymers consisting of a hydrophilic matrix based on N,N-dimethylacrylamide and a diazoester-based comonomer to a cellulose model surface and to laboratory-engineered, fibrous paper substrates is investigated as a function of time, temperature, and cross-linker composition. The distribution of the polymer in the fiber network is studied using confocal fluorescence microscopy. Finally, the tensile properties of modified wet and dry eucalyptus sulfate papers are measured to demonstrate the strong effect of the thermally cross-linked copolymers on the wet strength of paper substrates. Initial experiments show that the tensile indices of the modified and wetted paper samples are up to 50 times higher compared to the values measured for unmodified samples. When dry and wet papers coated with the above-described wetting agents are compared, relative wet strengths of over 30% are observed.

Functional paper-based materials for diagnostics.

Functional papers are the subject of extensive research efforts and have already become an irreplaceable part of our modern society. Among other issues, they enable fast and inexpensive detection of a plethora of analytes and simplify laboratory work, for example in medical tests. This article focuses on the molecular and structural fundamentals of paper and the possibilities of functionalization, commercially available assays and their production, as well as on current and future challenges in research in this field.

Cross-Linking Strategies for Fluorine-Containing Polymer Coatings for Durable Resistant Water- and Oil-Repellency.

Functional coatings for application on surfaces are of growing interest. Especially in the textile industry, durable water and oil repellent finishes are of special demand for implementation in the outdoor sector, but also as safety-protection clothes against oil or chemicals. Such oil and chemical repellent textiles can be achieved by coating surfaces with fluoropolymers. As many concerns exist regarding (per)fluorinated polymers due to their high persistence and accumulation capacity in the environment, a durable and resistant coating is essential also during the washing processes of textiles. Within the present study, different strategies are examined for a durable resistant cross-linking of a novel fluoropolymer on the surface of fibers. The monomer 2-((1,1,2-trifluoro-2-(perfluoropropoxy)ethyl)thio)ethyl acrylate, whose fluorinated side-chain is degradable by treatment with ozone, was used for this purpose. The polymers were synthesized via free radical polymerization in emulsion, and different amounts of cross-linking reagents were copolymerized. The final polymer dispersions were applied to cellulose fibers and the cross-linking was induced thermally or by irradiation with UV-light. In order to investigate the cross-linking efficiency, tensile elongation studies were carried out. In addition, multiple washing processes of the fibers was performed and the polymer loss during washing, as well as the effects on oil and water repellency were investigated. The cross-linking strategy paves the way to a durable fluoropolymer-based functional coating and the polymers are expected to provide a promising and sustainable alternative to functional coatings.

Tailored oxidation of hydroxypropyl cellulose under mild conditions for the generation of wet strength agents for paper.

Secondary hydroxyl groups of hydroxypropyl cellulose (HPC) are transformed into reactive carbonyl groups selectively via TEMPO-mediated oxidation in the presence of sodium hypochlorite. By using this oxidation protocol, we introduced carbonyl functions in HPC under mild conditions, with a controlled degree of oxidation (DOx) up to 2.5 and a low degradation of the polysaccharide. The effect of the concentration of sodium hypochlorite on the resulting oxidized alcohol groups has been investigated in detail. Oxidized HPC crosslinks spontaneous at room temperature and mild pH-values with a variety of amines to form water stable hydrogels. If applied on lab-made paper sheet, thermally cross-linking this polymer with amines significantly increased the wet tensile strength. The utilization of such wet strength agents could lead to new approaches in terms of recyclability and biodegradability of wet strength agents interesting for a large number of different paper grades.

Wettability-defined droplet imbibition in ceramic mesopores.

Wettability-defined liquid infiltration into porous materials in nature and several industrial applications is of fundamental interest. Direct observation of wetting-controlled imbibition in mesopores is anticipated to deliver important insights into the interplay between nanoconfined liquid movement and nanoscale wettability. We present a systematic study of water imbibition into mesoporous silica thin films with wetting properties precisely adjusted through chemical functionalization. We observe the liquid infiltration, resulting in an imbibition ring around the water droplet, by top-view imaging using a camera with collimated coaxial illumination. With decreasing hydrophilicity, the maximum imbibition area around the droplet decreases, accompanied by a simultaneous change in the imbibition kinetics and imbibition mechanism. Initially, the imbibition kinetics follow a modified Lucas-Washburn law that considers a strong influence of evaporation. However, with increasing imbibition time after reaching constant imbibition ring dimensions, the imbibition area starts to increase again, causing a deviation from the applied model. This observation is ascribed to water-mediated surface activation at the imbibition front, leading to a slightly increased wettability, which is also confirmed by water adsorption measurements. Furthermore, recently described spontaneous condensation-evaporation imbalances that cause oscillations of the imbibition front could be verified and were studied with regard to changing wetting properties. By increasing the contact angle of the material and therefore the partial pressure needed for capillary condensation, the amplitude of the imbibition front oscillations decreases. These results provide insights into the wettability-defined complex movement of water in mesoporous structures, which has practical implications, e.g., for nano/microfluidic devices and water purification or harvesting.

Entrapment of Hydrophobic Biocides into Cellulose Acetate Nanoparticles by Nanoprecipitation.

This contribution reports an efficient method for the production and use of biocide-loaded cellulose acetate nanoparticles. As well-known model biocides 4-Hexylresorcinol and Triclosan were used for in situ nanoparticle loading during a nanoprecipitation process. We show that the nanoparticle size can be well-controlled by variation of the cellulose acetate concentration during nanoprecipitation. Apart from strong evidence suggesting cellulose acetate particle formation according to a nucleation-aggregation mechanism, we further show that the biocide loading of the particles occurs by a diffusion process and not via co-precipitation. The quantity of particle loading was analyzed by 1H-NMR spectroscopy of re-dissolved nanoparticles, and it was observed that a decisive factor for high packaging efficiency is the use of a biocide with low water solubility and high hydrophobicity. SEM studies showed no influence on the particle morphology or size by both biocides 4-Hexylresorcinol and Triclosan. Finally, an aqueous nanoparticle dispersion can be coated onto model paper sheets to yield pronounced antimicrobial surface-properties. Nanoparticles loaded with the biocide Triclosan showed a high antimicrobial activity against Bacillus subtilis, a cellulase producing bacteria, if applied to model paper substrates, even at extremely low coating weights of 1-5 g/m2, respectively. Additional long-term efficacy renders these nanoparticles ideal for various applications.

Electrical Sensing of Phosphonates by Functional Coupling of Phosphonate Binding Protein PhnD to Solid-State Nanopores.

Combining the stability of solid-state nanopores with the unique sensing properties of biological components in a miniaturized electrical hybrid nanopore device is a challenging approach to advance the sensitivity and selectivity of small-molecule detection in healthcare and environment analytics. Here, we demonstrate a simple method to design an electrical hybrid nanosensor comprising a bacterial binding protein tethered to a solid-state nanopore allowing high-affinity detection of phosphonates. The diverse family of bacterial substrate-binding proteins (SBPs) binds specifically and efficiently to various substances and has been implicated as an ideal biorecognition element for analyte detection in the design of hybrid bionanosensors. Here, we demonstrate that the coupling of the purified phosphonate binding protein PhnD via primary amines to the reactive NHS groups of P(DMAA-co-NMAS) polymers inside a single track-etched nanopore in poly(ethylene terephthalate) (PET) foils results in ligand-specific and concentration-dependent changes in the nanopore current. Application of the phosphonate 2-aminoethylphosphonate (2AEP) or ethylphosphonate (EP) induces a large conformational rearrangement in PnhD around the hinge in a venus flytrap mechanism resulting in a concentration depended on increase of the single pore current with binding affinities of 27 and 373 nM, respectively. Thus, the specificity and stability of this simple hybrid sensor concept combine the advantages of both, the diversity of ligand-specific substrate-binding proteins and solid-state nanopores encouraging further options to produce robust devices amenable to medical or environmental high-throughput-based applications in nanotechnology.

Analysis of free chlorine in aqueous solution at very low concentration with lateral flow tests.

Test strips are convenient tools for rapid, semi-quantitative analysis of a variety of parameters by dipping them for a few seconds in a sample solution followed by a simple colorimetric read-out. Their sensitivity is mainly determined by the reactivity of the test dyes on the reaction zone and is not sufficient for some applications. The detection limit of commercially available free chlorine test strips, for example, is at present not low enough to confirm the absence of this analyte as disinfectant in rinsing solutions after disinfection or to control required residual amounts of chlorine in drinking water. Therefore, we developed a user-friendly lateral flow test which is capable to detect very low amounts of free chlorine. The latter relies on a larger sample volume passing the reaction zone as compared to simple dip test strips. An amount of as low as 0.05 ppm chlorine can, however, only be detected if oxidation stable flow test substrates are used. The eventually developed flow test reaches a 10x higher sensitivity than a commercial dip test. The result is obtained within 4-5 min flow time, whereby no action is required by the user during this analysis time.

Paper-based microfluidic aluminum-air batteries: toward next-generation miniaturized power supply.

Paper-based microfluidics (lab on paper) emerges as an innovative platform for building small-scale devices for sensing, diagnosis, and energy storage/conversions due to the power-free fluidic transport capability of paper via capillary action. Herein, we report for the first time that paper-based microfluidic concept can be employed to fabricate high-performing aluminum-air batteries, which entails the use of a thin sheet of fibrous capillary paper sandwiched between an aluminum foil anode and a catalyst coated graphite foil cathode without using any costly air electrode or external pump device for fluid transport. The unique microfluidic configuration can help overcome the major drawbacks of conventional aluminum-air batteries including battery self-discharge, product-induced electrode passivation, and expensive and complex air electrodes which have long been considered as grand obstacles to aluminum-air batteries penetrating the market. The paper-based microfluidic aluminum-air batteries are not only miniaturized in size, easy to fabricate and cost-effective, but they are also capable of high electrochemical performance. With a specific capacity of 2750 A h kg-1 (@20 mA cm-2) and an energy density of 2900 W h kg-1, they are 8.3 and 12.6 times higher than those of the non-fluidic counterpart and significantly outperform many other miniaturized energy sources, respectively. The superior performance of microfluidic aluminum-air batteries originates from the remarkable efficiency of paper capillarity in transporting electrolyte along with O2 to electrodes.

Reactivity of Isocyanate-Functionalized Lignins: A Key Factor for the Preparation of Lignin-Based Polyurethanes.

Using isocyanate-functionalized Kraft lignin as a reactive macromonomer for the preparation of polyurethane foams by a prepolymer technique is a well-known strategy to incorporate the biomacromolecule into a higher value polymer material. However, as of today the mechanical properties of the resulting materials are still insufficient for a number of possible applications. One reason for this limitation is that the reaction pathway and the morphological arrangement of such foams is of uttermost complexity and depends on a large number of influencing material-intrinsic factors. One important parameter is the reactivity of the functionalized lignin, which has a great impact on the interphase reaction kinetics and thus, on the geometry and mechanical properties of the resulting polyurethane foams. The reactivity is implied, amongst others, by the electron affinity of the isocyanate moiety. Herein, we investigate the reactivity of Kraft lignin modified with different commercially used isocyanates in the reaction with conventional polyols. Therefore, differently reactive prepolymers were synthesized, characterized and polyurethane foams were prepared thereof by using these compounds and the foam formation kinetics, morphological as well as mechanical properties were investigated. Finally, the results were supported by quantum mechanical calculations of the electron affinities of representative model compounds for the lignin-based prepolymers. This work gives rise to a better understanding of the effect of the reactivity and isocyanate structure linked to Kraft lignin on the polyurethane formation and enables rational choice of the isocyanate for pre-functionalization of lignin to prepare materials with better mechanical performance.

Spatially Resolved Crosslinking of Hydroxypropyl Cellulose Esters for the Generation of Functional Surface-Attached Organogels.

Chemistry, geometric shape and swelling behavior are the key parameters that determine any successful use of man-made polymeric networks (gels). While understanding of the swelling behavior of both water-swellable hydrogels and organogels that swell in organic solvents can be considered well-advanced with respect to fossil fuel-based polymer networks, the understanding, in particular, of wood-derived polymers in such a network architecture is still lacking. In this work, we focus on organogels derived from hydroxypropyl cellulose (HPC) ester. The latter polymer was functionalized with saturated and unsaturated fatty acids, respectively. Due to their tailored chemical constitution, we demonstrated that such polysaccharide can be crosslinked and simultaneously surface-bound by using a photo-induced radical reaction using a photo-initiator. Based on the choice of fatty acid used in the design of the HPC ester, and by controlling the degree of substitution (DS) obtained during the esterification of the polysaccharide, modular manipulation of the physical properties (e.g., polarity) of the resulting gel is possible. Depending on the initiator employed, different wavelengths of light, from UV to visible, can be utilized for the crosslinking reaction, which facilitates the deployment of a range of light sources and different lithographic methods. Additionally, we showed that altering of the illumination time allows to tailor the netpoint density, and thus, the degree of linear deformation in equilibrium and the swelling kinetics. Finally, we performed a proof-of-principle experiment to demonstrate the application of our material for the generation of spatially resolved polymer patches to enrich organic molecules from a solution within a microfluidic channel.