Adhesion force mapping on wood by atomic force microscopy: influence of surface roughness and tip geometry

Ultrasoft and High-Adhesion Block Copolymers for Neuromorphic Computing

The "von Neumann bottleneck" is a formidable challenge in conventional computing, driving exploration into artificial synapses. Organic semiconductor materials show promise but are hindered by issues such as poor adhesion and a high elastic modulus. Here, we combine polyisoindigo-bithiophene (PIID-2T) with grafted poly(dimethylsiloxane) (PDMS) to synthesize the triblock-conjugated polymer (PIID-2T-PDMS). The polymer exhibited substantial enhancements in adhesion (4.8-68.8 nN) and reductions in elastic modulus (1.6-0.58 GPa) while maintaining the electrical characteristics of PIID-2T. The three-terminal organic synaptic transistor (three-terminal p-type organic artificial synapse (TPOAS)), constructed using PIID-2T-PDMS, exhibits an unprecedented analog switching range of 276×, surpassing previous records, and a remarkable memory on-off ratio of 106. Moreover, the device displays outstanding operational stability, retaining 99.6% of its original current after 1600 write-read events in the air. Notably, TPOAS replicates key biological synaptic behaviors, including paired-pulse facilitation (PPF), short-term plasticity (STP), and long-term plasticity (LTP). Simulations using handwritten digital data sets reveal an impressive recognition accuracy of 91.7%. This study presents a polyisoindigo-bithiophene-based block copolymer that offers enhanced adhesion, reduced elastic modulus, and high-performance artificial synapses, paving the way for the next generation of neuromorphic computing systems.

Exploring the Impact of Nanoparticle Stealth Coatings in Cancer Models: From PEGylation to Cell Membrane-Coating Nanotechnology

Nanotechnological platforms offer advantages over conventional therapeutic and diagnostic modalities. However, the efficient biointerfacing of nanomaterials for biomedical applications remains challenging. In recent years, nanoparticles (NPs) with different coatings have been developed to reduce nonspecific interactions, prolong circulation time, and improve therapeutic outcomes. This study aims to compare various NP coatings to enhance surface engineering for more effective nanomedicines. We prepared and characterized polystyrene NPs with different coatings of poly(ethylene glycol), bovine serum albumin, chitosan, and cell membranes from a human breast cancer cell line. The coating was found to affect the colloidal stability, adhesion, and elastic modulus of NPs. Protein corona formation and cellular uptake of NPs were also investigated, and a 3D tumor model was employed to provide a more realistic representation of the tumor microenvironment. The prepared NPs were found to reduce protein adsorption, and cell-membrane-coated NPs showed significantly higher cellular uptake. The secretion of proinflammatory cytokines in human monocytes after incubation with the prepared NPs was evaluated. Overall, the study demonstrates the importance of coatings in affecting the behavior and interaction of nanosystems with biological entities. The findings provide insight into bionano interactions and are important for the effective implementation of stealth surface engineering designs.

Effects of Thermal Aging on the Adhesion Forces of Biopolymers of Wood Cell Walls

Wood is the most important, industrially used renewable resource on the planet, but the aging mechanism of biopolymers on cell walls is poorly understood. Adhesion properties are of critical importance for wood and many other lignocellulosic materials. We used atomic force microscopy and defined the jump-off force ratio in the retract force-displacement curve to study the adhesion force phenomenon and the effects of heat treatments. Here, we identified two sigmoidal curves describing the shift of the adhesion force and the jump-off force ratio: the first curve was attributed to the movement of extractives and the second to the degradation of the hemicellulose-lignin matrix. We confirmed the hypothesis formulated in this paper by the cell wall surface topography, the analysis of the treated samples by Fourier transform infrared spectroscopy, and the analysis of volatiles during heating by headspace gas chromatography-mass spectrometry.

A biological nanofoam: The wall of coniferous bisaccate pollen

The outer layer of the pollen grain, the exine, plays a key role in the survival of terrestrial plant life. However, the exine structure in different groups of plants remains enigmatic. Here, modern and fossil coniferous bisaccate pollen were examined to investigate the detailed three-dimensional structure and properties of the pollen wall. X-ray nanotomography and volume electron microscopy are used to provide high-resolution imagery, revealing a solid nanofoam structure. Atomic force microscopy measurements were used to compare the pollen wall with other natural and synthetic foams and to demonstrate that the mechanical properties of the wall in this type of pollen are retained for millions of years in fossil specimens. The microscopic structure of this robust biological material has potential applications in materials sciences and also contributes to our understanding of the evolutionary success of conifers and other plants over geological time.

Local force titration of wood surfaces by chemical force microscopy

Chemical force microcopy, a variation of atomic force microscopy, opened the door to visualize chemical nano-properties of various materials in their natural state. The key function of this method is given by translating adhesion forces between a functionalized tip and the sample to chemical surface behavior. In force titration, these adhesion forces are studied in different pH buffers, which allows estimating the pK _a value of the analyzed surface. Herein, we report the use of this method to study natural and chemically treated wood surfaces, which are of interest in sustainable material design. First, we show varying adhesion phenomena of OH- and COOH-functionalized tips on native spruce wood cells. Then, we demonstrate how peak force tapping with chemically functionalized tips can be used to estimate the pK _a value of gold substrates (pK _a ≈ 5.2) and different wood cell wall layers with high spatial resolution. Additionally, the swelling behavior of wood samples is analyzed in varying pH buffers. With the applied method, chemical surface properties of complex natural substrates can be analyzed.

Beyond What Meets the Eye: Imaging and Imagining Wood Mechanical-Structural Properties

Wood presents a hierarchical structure, containing features at all length scales: from the tracheids or vessels that make up its cellular structure, through to the microfibrils within the cell walls, down to the molecular architecture of the cellulose, lignin, and hemicelluloses that comprise its chemical makeup. This structure renders it with high mechanical (e.g., modulus and strength) and interesting physical (e.g., optical) properties. A better understanding of this structure, and how it plays a role in governing mechanical and other physical parameters, will help to better exploit this sustainable resource. Here, recent developments on the use of advanced imaging techniques for studying the structural properties of wood in relation to its mechanical properties are explored. The focus is on synchrotron nuclear magnetic resonance spectroscopy, X-ray diffraction, X-ray tomographical imaging, Raman and infrared spectroscopies, confocal microscopy, electron microscopy, and atomic force microscopy. Critical discussion on the role of imaging techniques and how fields are developing rapidly to incorporate both spatial and temporal ranges of analysis is presented.

Influence of force volume indentation parameters and processing method in wood cell walls nanomechanical studies

Since the established correlations between mechanical properties of a piece of wood at the macroscopic scale and those of the cell wall at the submicron scale, techniques based on atomic force microscopy (AFM) have become widespread. In particular Peak Force tapping, allowing the differentiation of various layers, has become the new standard for wood cell wall's nanomechanical characterization. However, its use requires fully elastic indentation, a good knowledge of stiffness of the probe and assumes a perfect tip shape of known radius (sphere) or angle (cone). Those strong hypotheses can result in large approximations in the extracted parameters for complex, nanostructured, and stiff and viscous materials such as wood. In this work, we propose a reliable and complementary alternative based on AFM force-volume indentation by refining the Oliver and Pharr nanoindentation processing and calibration procedure for AFM cantilever and tip. The introduced area-function calibration (AFC) method allows to considerably reduce these approximations and provides semi-quantitative measurements. No prior knowledge of the tip shape and cantilever stiffness are required and viscoplasticity is investigated through a qualitative index. Indentation parameters variations are shown to impact the resulting measurements, i.e., indentation modulus, viscoplasticity index, adhesion force and energy. AFC method, applied to map regions of tension wood, provides very stable mechanical parameters characteristic of each region, which makes this method of high interest for plant cell wall studies.

Nanoscale Chemical Features of the Natural Fibrous Material Wood

Peak force infrared (PFIR) microscopy is a recently developed approach to acquire multiple chemical and physical material properties simultaneously and with nanometer resolution: topographical features, infrared (IR)-sensitive maps, adhesion, stiffness, and locally resolved IR spectra. This multifunctional mapping is enabled by the ability of an atomic force microscope tip in the peak force tapping mode to detect photothermal expansion of the sample. We report the use of the PFIR to characterize the chemical modification of bio-based native and intact wooden matrices, which has evolved into an increasingly active research field. The distribution of functional groups of wood cellulose aggregates, either in native or carboxylated states, was detected with a remarkable spatial resolution of 16 nm. Furthermore, mechanical and chemical maps of the distinct cell wall layers were obtained on polymerized wooden matrices to localize the exact position of the modified regions. These findings shall support the development and understanding of functionalized wood materials.

Fast preparation and recycling method for colloidal probe cantilevers in hydrophobic mapping applications

Probe contamination of atomic force microscope (AFM) tips with colloidal probes is limiting the lifetime of the probe and the reproducibility in force interaction measurements, rendering cantilevers useless. Earlier proposed cleaning methods like mechanical scrubbing, UV, plasma and solvent cleaning procedures have limitations especially for inorganic particulate contaminations. In this paper we demonstrate a fast procedure to recycle contaminated colloidal probe cantilevers and reequip them with pristine colloids without affecting the mechanical and structural properties of the cantilever. The proposed procedure reduces the total time for probe preparation and allows extended experimental test work with singular cantilevers reducing the deviations by cantilever calibration.

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fast preparation

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recyclable cantilevers

A close-up view of the wood cell wall ultrastructure and its mechanics at different cutting angles by atomic force microscopy

AFM measurements on spruce sample cross-sections reveal that the structural appearance of the S₂ layer changes from a network structure to a concentric lamellar texture depending on the cutting angle. The structural assembly of wood constituents within the secondary cell wall has been subject of numerous studies over the last decades, which has resulted in contradicting models on the spatial arrangement and orientation of the wood macromolecules. Here, we use multichannel atomic force microscopy by means of quantitative imaging, to gain new insights into the macromolecular assembly. Cross-sections of spruce wood, which had been cut at different angles ranging from 0° to 30° were investigated. Strikingly, depending on the cutting angle, the structural appearance of the S₂ layer changed from a network-like structure to a distinct concentric lamellar texture. This makes us conclude that the often visualized lamellar organization of the secondary cell wall is not the consequence of a continuous inherent ring pattern, but rather a result of the specific surface cross-section appearance of cellulose aggregates at larger cutting angles. By analyzing the recorded force distance curves in every pixel, a nano-mechanical characterization of the secondary cell wall was conducted. Substantially lower indentation modulus values were obtained compared to nanoindentation values reported in the literature. This is potentially due to a smaller interaction volume of the probe with a by far less deep indentation.

Nano-mechanical characterization of the wood cell wall by AFM studies: comparison between AC- and QI™ mode

BACKGROUND

Understanding the arrangement and mechanical properties of wood polymers within the plant cell wall is the basis for unravelling its underlying structure-property relationships. As state of the art Atomic Force Microscopy (AFM) has been used to visualize cell wall layers in contact resonance- and amplitude controlled mode (AC) on embedded samples. Most of the studies have focused on the structural arrangement of the S₂ layer and its lamellar structure.

RESULTS

In this work, a protocol for AFM is proposed to characterize the entire cell wall mechanically by quantitative imaging (QI™) at the nanometer level, without embedding the samples. It is shown that the applied protocol allows for distinguishing between the cell wall layers of the compound middle lamella, S₁, and S₂ of spruce wood based on their Young's Moduli. In the transition zone, S₁₂, a stiffness gradient is measured.

CONCLUSIONS

The QI™ mode pushes the limit of resolution for mechanical characterization of the plant cell wall to the nanometer range. Comparing QI™- against AC images reveals that the mode of operation strongly influences the visualization of the cell wall.

Adhesion force mapping on wood by atomic force microscopy: influence of surface roughness and tip geometry

This study attempts to address the interpretation of atomic force microscopy (AFM) adhesion force measurements conducted on the heterogeneous rough surface of wood and natural fibre materials. The influences of wood surface roughness, tip geometry and wear on the adhesion force distribution are examined by cyclic measurements conducted on wood surface under dry inert conditions. It was found that both the variation of tip and surface roughness of wood can widen the distribution of adhesion forces, which are essential for data interpretation. When a common Si AFM tip with nanometre size is used, the influence of tip wear can be significant. Therefore, control experiments should take the sequence of measurements into consideration, e.g. repeated experiments with used tip. In comparison, colloidal tips provide highly reproducible results. Similar average values but different distributions are shown for the adhesion measured on two major components of wood surface (cell wall and lumen). Evidence supports the hypothesis that the difference of the adhesion force distribution on these two locations was mainly induced by their surface roughness.