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Fig. S5 † shows the overlayed force curves extracted from Fig. 4g–i. Similar to the corresponding Δ f curves ( Fig. 4g), the force curves in NaCl buffer overlap with one another, showing a slow increase to F = 3.5 nN at the interface with the onset of F increase at Δ h = 4 nm (Fig. S5a †). This is reasonable because the interface between hydrated polyelectrolyte brushes and the electrolyte is diffusive. 52,53 In the presence of an additional 1 mM CaCl 2, the onset of F increase was observed at a shallower position (Δ h = 2 nm), and F increased to F = 2 nN at the interface (Fig. S5b †). A minor deviation in the force curves indicated that the pAA-Cys5 brushes were uniformly compacted and therefore stiffened following the binding of Ca 2+ ions. Interestingly, the force curves in the presence of Cd 2+ ions showed a much more pronounced deviation (Fig. S5c †). Both the onset of F increase and the F value at Δ h = 0 nm exhibited wide distributions, implying that pAA-Cys5 brushes formed stiff domains on the surface of the phospholipid monolayer. The formation of collapsed domains in the presence of Cd 2+ ions suggested that the compaction-induced dehydration of pAA-Cys5 brushes altered the affinity of the solvent (water) to monomers. Further investigations, including in situ observations of changes in the viscoelasticity of pAA-Cys5 using 3D-SFM or other techniques ( e.g., QCM-D) would provide further insights into the ion-specific, dynamic modulation of polyelectrolyte brushes. Conclusions In this study, we investigated the modulation of topography and mechanical properties of supported lipid monolayers incorporating lipids functionalized with linear poly(acrylic acid) chains with cysteine side chains (DOPE-pAA-Cys5) induced by different metal ions. We found that the topographical profile of pAA-Cys5 brushes obtained by 2D-FM-AFM in the electrolyte containing no divalent cation (100 mM NaCl, 10 mM Tris–HCl, pH = 7.4) was very homogeneous, whose RMS 0 = 0.23 nm ( Fig. 3a). In fact, autocorrelation analysis of Δ h showed no clear feature ( Fig. 3d). The mechanical landscape, characterized by the 3D Δ f map ( Fig. 4a) and its xz cross-section ( Fig. 4d), was also highly uniform, suggesting that the hydrated pAA-Cys5 brushes are stretched into the bulk electrolyte. The slow increase in the Δ f–Δ z curves ( Fig. 4g) and a very low RMS(Δ f) = 0.24 kHz at Δ z = 0 nm ( Fig. 4j) implied a smooth transition from the polymer to the bulk electrolyte. Introduction Stimuli-responsive polymers have attracted interest for use in various drug delivery and microencapsulation applications because these substances can protect and/or release materials in response to the surrounding environment. 1 In particular, polyelectrolyte brushes have been grafted on surfaces to give the materials adaptable functionalities. In these cases, the physical properties of the surface can be modulated based on the electrostatic properties of the external media ( e.g., pH and salt concentrations). 2–4 The structures and mechanical properties of polymer brushes have been measured experimentally using X-ray/neutron reflectivity, 5,6 quartz crystal microbalance with dissipation (QCM-D), 7,8 tribology, 9,10 and microinterferometry. 11,12 For the 3D-SFM measurements, the cantilever tip scanned vertically following a rapid sinusoidal curve, while slowly scanning in the lateral direction. During this tip scan, the Δ f induced by the force variation was recorded to produce a 3D Δ f image; the tip-sample distance was regulated continuously such that the average Δ f was equal to a setpoint value Δ f sp. This allowed us to simultaneously obtain a 3D Δ f image and a 2D height image. Sections of 100 × 100 nm in area with a height of 6 nm (for NaCl and NaCl + CaCl 2 solutions) or 8 nm (for NaCl + CdCl 2 solutions) were scanned (128 × 128 × 256 pixels) with a constant setpoint to compare the forces measured in all the buffer solutions. In this study, we carefully optimized the setpoint at 3.9 kHz, at which we could gain the highest contrast. Data analysis AFM data were processed with the open-source software, Gwyddion, 35 to analyze the topography images and a self-written program in LabVIEW (National Instruments, Austin, TX, USA) to analyze the 3D-SFM images. Based on current evidence, no strong conclusions can be drawn concerning any specific oral hygiene devices as adjuncts to toothbrushing for patient self-care in periodontal maintenance [footnote 5][
Self-care is vitally important to prevent and manage plaque-induced periodontitis [footnote 5] ( Chapter 2: Table 2). For people with periodontal diseases this becomes vitally important throughout the rest of life and good oral hygiene may take longer than the recommended 2 minutes. The patient’s existing method of brushing may need to be modified to clean all tooth surfaces systematically, maximise plaque removal and to brush the gum line carefully [footnote 6]. No particular technique has been shown to be better than another [footnote 7]. Disclosing tablets can help to indicate areas that are being missed. For people with extensive inflammation, it is good practice to start with toothbrushing advice, followed by interdental plaque control [footnote 8]. Whilst this guidance seeks to ensure a consistent UK-wide approach to prevention of oral diseases, some differences in operational delivery and organisational responsibilities may apply in Wales, Northern Ireland and England. In Scotland the guidance will be used to inform oral health improvement policy.T. Ikarashi, T. Yoshino, N. Nakajima, K. Miyata, K. Miyazawa, Y. Morais Jaques, A. S. Foster, M. Uno, C. Takatoh and T. Fukuma, ACS Appl. Nano Mater., 2020, 4, 71–83 CrossRef. where δ ind is the indentation depth, μ is Poisson's ratio, and R is the tip radius. However, it is well established that AFM indentation data for a thin film on a stiff substrate cannot be treated with a conventional Hertz model because the film deformation is limited under a high load. 46,47 If the film is softer than the underlying solid substrate, one can analytically calculate Young's modulus of very thin, soft films using a transition function that linearly connects the influence of two elastic layers, i.e., the film and the substrate 48,49 as shown in eqn (6), Oral hygiene practices include toothbrushing and the use of other aids for cleaning teeth. Toothbrushing is widely practiced across the UK. In a 2009 national survey of adults [footnote 1], more women reported cleaning their teeth at least twice a day than men (82% compared with 67%). Three quarters (76%) of dentate adults reported using toothpaste with 1,350 to 1,500 parts per million level of fluoride and a further 18 per cent used a brand with a medium (1,000 to 1,350 parts per million) fluoride level. Most dentate adults (58%) used additional products as well as a standard toothpaste and brush, the most common of which were mouthwash (31%), powered toothbrushes (26%) and dental floss (21%). Women and middle-aged adults were more likely to use additional products than men, younger and very old adults (85 years and over). Nanoscale Adv., 2022, 4, 5027-5036 Ion-specific nanoscale compaction of cysteine-modified poly(acrylic acid) brushes revealed by 3D scanning force microscopy with frequency modulation detection † In this study, we functionalized the surface of planar lipid membranes ( i.e., supported membranes) 22,23 with poly(acrylic acid) brushes bearing cysteine side chain functional groups (pAA-Cys) by incorporating the lipids covalently coupled with pAA-Cys ( i.e., DOPE-pAA-Cys5) into the matrix lipids ( Fig. 1). In these materials, pAA-Cys5 moieties interact with divalent Cd 2+ ions similar to naturally occurring proteins 24 because pAA-Cys has both –SH and –COOH groups. We investigated how pAA-Cys brushes adapt their structure and mechanical landscape near the interface following the addition of Cd 2+ ions. We used lipids and lipopolymers with identical hydrocarbon chains to prevent phase separation. 25 The lateral average distance between lipopolymer molecules 〈 d〉 can be precisely controlled by taking advantage of the self-assembling nature of lipids and lipopolymers and tuning the molar fraction of lipopolymers χ lipo, such that
N. D. Spencer and W. T. Tysoe, The Cutting Edge of Tribology, World Scientific Publishing, 2015 Search PubMed. H. Wang, Z. Chen, L. Xin, J. Cui, S. Zhao and Y. Yan, J. Polym. Sci., Part A: Polym. Chem., 2015, 53, 2175–2185 CrossRef CAS. e Department of Macromolecular Science, Graduate School of Science, Osaka University, Osaka 560-0043, Japan The addition of 1 mM CaCl 2 caused a distinct change in both topographical and mechanical properties of the brush/electrolyte interface. In the presence of Ca 2+ ions, pAA-Cys5 brushes became smoother, which could be characterized by the very small roughness, RMS Ca = 0.15 nm ( Fig. 3b) with no characteristic feature ( Fig. 3e). The 3D Δ f map and its xz cross-section ( Fig. 4b and e) were smoother, and the increase in Δ f was much sharper ( Fig. 4h) than the corresponding values obtained in the absence of Ca 2+ ions, RMS(Δ f) = 0.37 kHz at Δ z = 0 nm ( Fig. 4j), indicating that pAA-Cys5 brushes are uniformly compacted by the presence of Ca 2+ ions. The most remarkable difference was observed when 1 mM CdCl 2 was added to the electrolyte. The surface topography became very rough (RMS Cd = 1.47 nm, Fig. 3c), displaying hemi-ellipsoidal features. The autocorrelation analysis suggested that these protrusions had a similar size and assembled randomly ( Fig. 3f). Accordingly, the 3D Δ f map became highly heterogeneous ( Fig. 4c), and the xz cross-section ( Fig. 4f) clearly indicates the coexistence of stiff protrusions and defects. The heterogeneity of the mechanical landscape was clearly visible from a broad distribution of the Δ f–Δ z curves, resulting in RMS(Δ f) = 1.90 kHz at Δ z = 0 nm ( Fig. 4j). These data suggest that pAA-Cys5 brushes “collapsed” in the presence of Cd 2+ ions, where water might not be a good solvent for pAA-Cys5. V. E. Podasca, T. Buruiana, C. D. Varganici and E. C. Buruiana, J. Polym. Res., 2017, 24, 103 CrossRef.J. L. Cuellar, I. Llarena, S. E. Moya and E. Donath, Macromolecules, 2013, 46, 2323–2330 CrossRef CAS. H. Rieger, H. Y. Yoshikawa, K. Quadt, M. A. Nielsen, C. P. Sanchez, A. Salanti, M. Tanaka and M. Lanzer, Blood, 2015, 125, 383–391 CrossRef CAS. A. Yamamoto, K. Hayashi, A. Sumiya, F. Weissenfeld, S. Hinatsu, W. Abuillan, M. Nakahata and M. Tanaka, Front. Soft Matter, 2022, 2, 959542 CrossRef. T. Fukuma, M. Kimura, K. Kobayashi, K. Matsushige and H. Yamada, Rev. Sci. Instrum., 2005, 76, 053704 CrossRef. Among the various techniques, atomic force microscopy (AFM) is commonly used to investigate the mechanical properties of polymer brushes via nanoindentation. 13,14 Recently, Fukuma et al., developed three-dimensional scanning force microscopy (3D-SFM) based on frequency modulation AFM (FM-AFM), 15 which has been utilized to construct force maps of material surfaces. During the measurement process, a cantilever tip scans in the vicinity of the interface both parallel and perpendicular to the surface, and the frequency shift of the oscillating cantilever is recorded in 3D space. This technique has been successful in visualizing the 3D force maps of the surfaces of various materials in water, e.g., minerals, 16,17 graphene, 18 graphite, 19 and supported phospholipid bilayers. 20 A key advantage of this technique is that it can be used to investigate the density distribution of water at the solid/water interface. By exploiting this unique functionality, 3D-SFM has been applied to observe the ammonia-mediated hydration of poly(vinyl alcohol) coated surfaces. 21 However, to our knowledge, no experimental studies have demonstrated the potential of 3D-SFM to detect dynamic modulations of hydrated polymer brushes driven by external chemical stimuli.
The risk of dental caries (Chapter 4) and periodontal diseases (Chapter 5) can both be reduced by the practice of regular careful oral hygiene involving toothbrushing with fluoride toothpaste. The particular benefit in preventing dental caries, relates to the fluoride in toothpaste (Chapter 9). Good oral hygiene reduces the risk of periodontal diseases; however, periodontal health also requires effective interproximal plaque removal. Oral hygiene advice for the population in general, and specific advice for those at higher risk, are presented below for each oral condition. Dental cariesH. Asakawa, S. Yoshioka, K.-i. Nishimura and T. Fukuma, ACS Nano, 2012, 6, 9013–9020 CrossRef CAS PubMed. C. Monzel, M. Veschgini, J. Madsen, A. L. Lewis, S. P. Armes and M. Tanaka, Langmuir, 2015, 31, 8689–8696 CrossRef CAS PubMed. Y. Higaki, B. Frohlich, A. Yamamoto, R. Murakami, M. Kaneko, A. Takahara and M. Tanaka, J. Phys. Chem. B, 2017, 121, 1396–1404 CrossRef CAS. X. Xu, D. Mastropietro, M. Ruths, M. Tirrell and J. Yu, Langmuir, 2019, 35, 15564–15572 CrossRef CAS. where Δ x and Δ y represent the lag from the corresponding x and y positions, respectively. However, this definition is computationally laborious. Therefore, the autocorrelation of an image was calculated using the Wiener–Khinchin theorem via fast Fourier transforms, which is defined as
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