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PubMed2005

A stainless steel bracket for orthodontic application.

Aesthetics has become an essential element when choosing orthodontic fixed appliances. Most metallic brackets used in orthodontic therapy are made from stainless steel (SS) with the appropriate physical properties and good corrosion resistance, and are available as types 304, 316 and 17-4 PH SS. However, localized corrosion of these materials can frequently occur in the oral environment. This study was undertaken to evaluate the accuracy of sizing, microstructure, hardness, corrosion resistance, frictional resistance and cytotoxicity of commercially available Mini-diamond (S17400), Archist (S30403) and experimentally manufactured SR-50A (S32050) brackets. The size accuracy of Mini-diamond was the highest at all locations except for the external horizontal width of the tie wing (P < 0.05). Micrographs of the Mini-diamond and Archist showed precipitates in the grains and around their boundaries. SR-50A showed the only austenitic phase and the highest polarization resistance of the tested samples. SR-50A also had the highest corrosion resistance [SR-50A, Mini-diamond and Archist were 0.9 x 10(-3), 3.7 x 10(-3), and 7.4 x 10(-3) mm per year (mpy), respectively], in the artificial saliva. The frictional force of SR-50A decreased over time, but that of Mini-diamond and Archist increased. Therefore, SR-50A is believed to have better frictional properties to orthodontic wire than Mini-diamond and Archist. Cytotoxic results showed that the response index of SR-50A was 0/1 (mild), Mini-diamond 1/1 (mild+), and Archist 1/2 (mild+). SR-50A showed greater biocompatibility than either Mini-diamond or Archist. It is concluded that the SR-50A bracket has good frictional property, corrosion resistance and biocompatibility with a lower probability of allergic reaction, compared with conventionally used SS brackets.

Biocompatible Materials↗
PubMedDate not supplied

Innovations in the design and performance of underpads for patients with burns.

The purpose of this study was to determine the biomechanical performance of commercially available underpads and bed linens to reduce the development of pressure sores in patients with burns who are at high risk. The three biomechanical performance parameters examined were coefficient of friction, absorbent capacity, and rewet. Because wetting either cotton or cotton/polyester bedsheets markedly increases their coefficients of friction, underpads should be used routinely to protect the skin against frictional forces. One disposable underpad is ideally suited to protect the skin of the patient at high risk. It has a polyolefin backing with a low coefficient of friction that serves as an effective barrier to moisture transmission while still shifting easily with the patient's movement. In addition, it is the only underpad studied that contains a superabsorbent polymer that provides a far superior absorbent capacity and minimizes rewet. Wet-back is further inhibited by its thick intermediate tissue layer and its spunbonded polypropylene coverstock.

Absorption↗
PubMed2004

Ontogenesis of the attachment ability in the bug Coreus marginatus (Heteroptera, Insecta).

Each tarsus of Coreus marginatus L. (Coreidae) bears a pair of smooth flexible pulvilli adapted for attachment to relatively smooth surfaces, such as their host plant Rumex crispus L. (Polygonaceae). This account quantifies insect attachment abilities on smooth surfaces at various stages of ontogenesis. Friction (shear) force (FF) of adults and juvenile insects was measured by the use of a computer controlled centrifugal force tester equipped with a fibre optical sensor. Pad area, body size and body mass were determined individually for each experimental insect. Light microscopy revealed no difference in pulvilli area between different leg pairs. Pulvilli area demonstrated a stronger increase with increasing linear dimensions, as predicted by scaling laws. Since friction coefficient (relationship between FF and body weight) (FC) was always higher than 1, it was concluded that adhesion has strongly contributed to the measured friction. The frictional properties of pulvilli do not change during ontogenesis. Thus, only the growth of pulvilli and, therefore, the increased contact area, contribute to the increasing attachment ability in insects at later larval stages. Due to different scaling of the body mass and area of attachment organs, smaller insects attach relatively more strongly. Both FF and FC were higher in experiments in which higher angular acceleration (AC) was applied. Lateral tenacity determined individually for experimental insects and pooled for all animals and accelerations is 0.097 N m(-2). These data led us to suggest that viscosity of the pad secretion and/or visco-elastic properties of the foam-like material of pulvilli play an important role in the attachment ability of insects.

Animals↗
PubMed2001

Investigation of hip joint prosthesis damage.

Following total hip arthroplasty the durability of a prosthetic implant depends on many factors but is generally limited by two types of damage: wearing of frictional surfaces of the acetabular and femoral component as well as the loosening and migration of the "rosthesis-cement-bone" system. Since it is possible to establish a cause-and-effect relationship between these two damage types, the aim of this research is to investigate the phenomena related to the contact surface and their influence on the prosthesis-bone relationship in general for various sizes and conditions of loading of the hip joint. The experimental study was conducted using a Timken testing device modified in such a way that simulation of relative movements of the joint elements was achieved using a mechanism that produces conditions similar to those in the human hip joint. The analysis of obtained measurements showed that actual radii of friction of the hip prosthesis did not correspond to the theoretical ones and that only a small portion of the total friction surface is realized. Variations of the radius of friction for the examined prosthesis type were very large, which changes the hypotheses on the ability of the prosthetic head and the "prosthesis-cement-bone" system to bear a certain amount of load. The construction and design of prosthetic implant components has a significant influence on both the amount of wear at the site of contact and the loosening of prosthesis.

Arthroplasty, Replacement, Hip↗
PubMed2003

Surface characterization of poly(ethylene glycol) coated human red blood cells by particle electrophoresis.

Recent studies have shown that the covalent attachment of poly(ethylene glycol), abbreviated as PEG, to the surface of human red blood cells (RBC) leads to masking of membrane antigenic sites and inhibition of RBC aggregation. The effects of PEG coating on the regions near the RBC glycocalyx were thus explored using cell micro-electrophoresis. Both linear (3.35, 18.5, 35.0) and an 8-arm 35.9 kDa reactive PEG were used; in one series, thick cross-linked coats were obtained using a branched PEG amine as a cross-linker. The results indicate marked decreases of RBC mobility (up to 90%) which were affected by polymer molecular mass and geometry. Since PEG is neutral and its covalent attachment is predominantly to primary amine groups, such decreases of mobility most likely reflect structural changes near and within the RBC glycocalyx rather than decreased surface charge density. Experimental data were analyzed using a theoretical approach which allows calculation of the thickness and friction of the polymer layers: (1) for linear PEGs, thickness increased and friction decreased with polymer mass; (2) compared to linear PEGs of similar molecular mass, thickness was less and friction was greater for the branched PEG; (3) cross-linked PEG coatings were more than 50 nm thick and were insensitive to changes of ionic strength. These observations are consistent with the aggregation behavior of PEG-coated RBC and indicate the usefulness of micro-electrophoresis methods for studies of covalently-attached polymers: the resulting calculated thickness and friction factors should be of value in achieving desired cellular surface characteristics or levels of cell-cell interaction.

Adult↗
PubMed2003

[Improve wear resistance of UHMWPE by O+ ion implanted].

Ultra high molecular weight polyethylene (UHMWPE) was implanted with 450 keV and 100 keV O+ ions at dosage of 1 x 10(15)/cm2, 5 x 10(15)/cm2, 3 x 10(14)/cm2, respectively. Its wear behaviors were studied under dry friction condition and lubrication by means of distilled water using a pin-on-disk tribometer with a Si3N4 ceramic ball as a counterface. The wear surfaces were examined with SEM. The experimental results showed that the wear rate of implanted UHMWPE is lower than that of un-implanted UHMWPE under both dry and distilled friction conditions, especially for 450 keV energy and 5 x 10(15)/cm2 dose implantation. The friction coefficient of O+ ions implanted UHMWPE is higher than that of un-implanted UHMWPE under both dry and distilled friction conditions. The adhesive, plow and plastic deformation are the wearing mechanism for un-implanted UHMWPE; the fatigue and abrasive wear are that for implanted UHMWPE.

Biocompatible Materials↗
PubMed2000

From small charged molecules to oligomers: a semiempirical approach to the modeling of actual mobility in free solution.

According to Stokes' treatment, the ionic mobility of particles, which are small with respect to Debye length, is usually considered to be proportional to the nominal charge and inversely proportional to the hydrodynamic radius. Experimentally, it is well known, however, that the ionic mobility of a small multicharged molecule does not depend linearly on its nominal charge in a wide range. This behavior can be accounted for by a condensation of the charge or a modification of the friction coefficient with the charge. This paper presents a semiempirical modeling of the actual mobility based on the assumption of additivity of frictional contributions pertaining to the uncharged molecular backbone and to each charged or uncharged moiety. Condensation of the charge was not considered. The model first appeared to be suitable for multicharged analytes having a characteristic dimension smaller than the Debye length, such as benzene polycarboxylic acids and polysulfated disaccharides. This approach was then adapted to account for the actual mobilities of singly and evenly charged oligomers (N-mers) having a dimension smaller than or similar to the Debye length. Rather good experimental agreement was obtained for polyalanines and polyglycines (N < or = 6), fatty acid homologs, fully sulfonated polystyrene oligomers (N < or = 13), and polycytidines (N < or = 10). Especially the influence of the polymerization degree on the mobility of oligomers having identical charge densities was clarified. It is also shown that the electrophoretic contribution to the overall friction coefficient increases linearly with the nominal charge but hardly depends on the chemical nature of the charged moieties. This model should be of interest to evaluate the role of various physicochemical phenomena (hydrodynamic and electrophoretic frictions, hydrodynamic coupling, charge condensation) involved in the migration of charged oligomers.

Benzene↗
PubMed2002

Electrophoretic mobilities of large organic ions in nonaqueous solvents: determination by capillary electrophoresis in propylene carbonate, N,N-dimethylformamide, N,N,-dimethylacetamide, acetonitrile and methanol.

The mobilities of the monocharged permanent tertraphenylphosphonium cation and tetraphenylborate anion are determined by capillary zone electrophoresis in different organic solvents as a function of the ionic strength, I, of the background electrolyte. The nonaqueous solvents are propylene carbonate (PC), N,N-dimethylformamide (DMF), N,N,-dimethylacetamide (DMA), acetonitrile (MeCN) and methanol (MeOH). The ionic strength is between 5 and 50 mmol/L. The mobility as a function of I is in good agreement with the theory of Debye, Hückel and Onsager (DHO), extended by the ion size parameter as introduced by Falkenhagen and Pitts. The values of the limiting DHO slopes of the mobility vs. I curves (the slopes express the influence of the solvent on the reduction of the mobility with increase of I) decrease in the order MeCN > MeOH > DMF > DMA > PC. Absolute mobilities (obtained by extrapolation to I = 0) of a particular ion differ by a factor of about 7 between the solvents. However, constancy within 10% is observed for their Walden products (the absolute mobility multiplied with the solvent's macroviscosity). The role of dielectric friction on the mobility of the present monocharged, large analyte ions is discussed according to the theory of Hubbard and Onsager. Based on the radii of the ions, the static permittivity of the solvent and its permittivity at infinite frequency, and the relaxation time of polarization, an equal contribution of dielectric and hydrodynamic friction is predicted in MeOH as solvent. Experimental data are in contrast to this prediction, indicating the overestimation of dielectric friction, and the dominance of hydrodynamic friction on the migration of the analyte ions in all solvents under consideration.

Acetamides↗
PubMed2003

Influence of methanol as a buffer additive on the mobilities of organic cations in capillary electrophoresis.

The mobilities of a series of aromatic ammonium ions, ranging in charge from +1 to + 3, were investigated by capillary electrophoresis using buffers consisting of 0-75% v/v methanol. This is an extension of our previous studies involving anion mobility in methanol-water media [1]. Absolute mobilities were determined by extrapolation of the effective mobilities to zero ionic strength according to the Pitts' equation. For all of the buffer compositions studied, the ionic strength effect increased with increasing cation charge, and varied as a function of solvent 1/eta epsilon (1/2) as predicted by the electrophoretic term within the Pitts' equation. In the presence of methanol, the ionic strength effects became more dramatic. The absolute mobilities of the cations were altered by the addition of methanol to the electrophoretic media. For example, at 75% MeOH, a migration order reversal was observed between the + 2 and + 3 ammonium ions. These solvent-induced selectivity changes are attributed to dielectric friction. As predicted by the Hubbard-Onsager dielectric friction model, dielectric friction increased with increasing methanol content and with increasing analyte charge. Further, the changes in cation mobility correlated to the changes in solvent relaxation time (tau), epsilon and eta. Although not predicted by the Hubbard-Onsager theory, the + 3 ammonium ion experienced more dielectric friction than the - 3 sulfonate and - 3 carboxylate investigated previously [1]. This apparent failure of the Hubbard-Onsager model results from its continuum nature, whereby ion-solvent interactions are not taken into account.

Buffers↗
PubMed1976

Wear of polymeric bearing materials: the effects of in vivo implantation.

The gross biocompatibility of the materials implanted was generally satisfactory. Weight gain during implantation due to net sorption of fluid was negligible for PPO and UHMWPE. Delrin AF specimens gained less than 0.5%, while SP-1 and SP-21 gained up to 2% in weight. Amoco-AI specimens gained up to 7% in weight. The friction coefficient of implanted samples of UHMWPE was lower by 20 to 40% than for the virgin material. The friction coefficient of the other implanted materials was usually somewhat higher than the friction coefficient of the corresponding virgin material. The largest increase in the friction coefficient was 38% for Delrin AF. Wear characteristics of UHMWPE were not significantly changed by implantation for periods of three years. UHMWPE was the only acceptable bearing material tested. Significantly higher wear rates were observed for implanted blocks of the SP-21, a Delrin AF, and an Amoco-AI material. Because of excellent creep resistance, virgin wear rates, and other favorable properties of the polyimides, further testings are in progress to indicate potential application of these polymers.

Acetals↗
PubMed2003

Control of flexor motoneuron activity during single leg walking of the stick insect on an electronically controlled treadwheel.

In the present study, motoneurons innervating the flexor tibiae muscle of the stick insect (Cuniculina impigra) middle leg were recorded intracellularly while the single leg performed walking-like movements on a treadwheel. Different levels of belt friction (equivalent to a change in load) were used to study the control of activity of flexor motoneurons. During slow leg movements no fast motoneurons were active, but a recruitment of these neurons could be observed during faster leg movements. The firing rate of slow and fast motoneurons increased with incremented belt friction. Also, the force applied to the treadwheel at different frictional levels was adapted closely to the friction of the treadwheel to be overcome. The motoneurons innervating the flexor tibiae were recruited progressively during the stance phase, with the slow motoneurons being active earlier than the fast (half-maximal spike frequency after 10-15% and 50-60% of the stance phase, respectively). The resting membrane potential was more hyperpolarized in fast motoneurons (64.6 +/- 6.5 mV) than in slow motoneurons (-52.9 +/- 5.4 mV). However, the threshold for the initiation of action potentials was not statistically significantly different in both types of flexor motoneurons. Therefore, action potentials were generated in fast motoneurons after a longer period of depolarization and thus later during the stance phase than in slow motoneurons. We show that motoneurons of the flexor tibiae receive substantial common excitatory inputs during the stance phase and that the difference in resting membrane potential between slow and fast motoneurons is likely to play a crucial role in their consecutive recruitment.

Action Potentials↗
PubMed1987

Signals in tactile afferents from the fingers eliciting adaptive motor responses during precision grip.

While human subjects lift small objects using the precision grip between the tips of the fingers and thumb the ratio between the grip force and the load force (i.e. the vertical lifting force) is adapted to the friction between the object and the skin. The present report provides direct evidence that signals in tactile afferent units are utilized in this adaptation. Tactile afferent units were readily excited by small but distinct slips between the object and the skin revealed as vibrations in the object. Following such afferent slip responses the force ratio was upgraded to a higher, stable value which provided a safety margin to prevent further slips. The latency between the onset of the a slip and the appearance of the ratio change (74 +/- 9 ms) was about half the minimum latency for intended grip force changes triggered by cutaneous stimulation of the fingers. This indicated that the motor responses were automatically initiated. If the subjects were asked to very slowly separate their thumb and the opposing finger while the object was held in air, grip force reflexes originating from afferent slip responses appeared to counteract the voluntary command, but the maintained upgrading of the force ratio was suppressed. In experiments with weak electrical cutaneous stimulation delivered through the surfaces of the object it was established that tactile input alone could trigger the upgrading of the force ratio. Although, varying in responsiveness, each of the three types of tactile units which exhibit a pronounced dynamic sensitivity (FA I, FA II and SA I units) could reliably signal these slips. Similar but generally weaker afferent responses, sometimes followed by small force ratio changes, also occurred in the FA I and the SA I units in the absence of detectable vibrations events. In contrast to the responses associated with clear vibratory events, the weaker afferent responses were probably caused by localized frictional slips, i.e. slips limited to small fractions of the skin area in contact with the object. Indications were found that the early adjustment to a new frictional condition, which may appear soon (ca. 0.1-0.2 s) after the object is initially gripped, might depend on the vigorous responses in the FA I units during the initial phase of the lifts (see Westling and Johansson 1987). The role of the tactile input in the adaptation of the force coordination to the frictional condition is discussed.

Adaptation, Physiological↗
PubMed1993

Slipperiness of the shoe-floor interface: comparison of objective and subjective assessments.

Tribollogically and biomechanically valid slip-resistance measurement methods are needed urgently to facilitate new developments for improving the safety of footwear and floor coverings. Objective coefficient-of-friction measurements and subjective walking experiments were compared to scrutinize and validate anti-slip assessments of the shoe-floor interface. Four shoe types were tested on a smooth stainless steel floor contaminated with viscous glycerol. The friction utilization ratio during one step, the sliding distance, and the subjective rating of slipperiness were evaluated by seven test subjects in 420 walking trials. All subjects assessed the footwear in the same rank of slipperiness. The friction utilization ratios during the walking trial were a poor indicator for anti-slip assessments. The apparatus-based kinetic friction values mu(k1), however, were more valid and, in particular, more reproducible. The safe level of mu(k1) was approximately 0.22, which is in accordance with previous slip-resistance clasification of shoes and floors. Futher trials in other shoe-contaminant-floor conditions and walking tasks are needed to verify the findings in this study.

Journal Article↗
PubMedDate not supplied

Theoretical analysis of the experimental artifact in trabecular bone compressive modulus.

A theoretical analysis was performed to characterize potential experimental artifacts in conventional compression testing of trabecular bone, where strains are based on the relative displacements of the two loading platens. We assumed that the total experimental artifact for modulus was the sum of a damage and friction artifact and derived equations to describe these artifacts. The two unknown constants in these equations were found using a combination of data derived from linear finite element analyses and in vitro uniaxial compression tests. Subsequent finite element analyses allowed estimation of the artifacts for a wide range of specimens (cube, 1:4-3:1 aspect ratio cylinders). If friction is completely eliminated at the specimen-platen interface, the Young's modulus of a 5 mm sized (1:1 aspect ratio dimension) specimen which has a damage artifact due to machining may be underestimated by at least 45% regardless of specimen geometry; otherwise, the platens modulus may vary from less than 30 to over 175% of the Young's modulus, depending upon the specimen geometry and Poisson's ratio of the bone. Increasing the specimen size reduces the artifact only slightly. Since Poisson's ratio can be large for trabecular bone and is rarely known a priori, the precision of the conventional compression test will, therefore, be poor unless friction is completely eliminated at the specimen-platen interface. However, without friction at the interface, the platens modulus will always underestimate Young's modulus, thereby reducing the accuracy of this test. There was also evidence that the strength may be affected by these artifacts.(ABSTRACT TRUNCATED AT 250 WORDS)

Artifacts↗
PubMed1996

Estimation of increased flow resistance in a narrow catheterized artery--a theoretical model.

The changed flow pattern in a narrow catheterized artery is studied and an estimate of the increased flow resistance is made. The anomalous behaviour of blood in small blood vessels has been taken into account by modelling blood as a Casson fluid possessing some finite yield stress. Both the cases of steady and pulsatile flow situations are studied. The pulsatile flow is analysed by considering the pressure gradient as a periodic function of time with small inertial effects. The resulting quasi-steady non-linear coupled implicit system of differential equations governing the flow are solved using a perturbation analysis, where it is assumed that the Womersley frequently parameter is small (alpha < 1) which is reasonable for physiological situations in small blood vessels as well as in coronary arteries. The effect of pulsatility, catheter radius and yield stress of the fluid on the yield plane locations, velocity distribution, flow rate, shear stress and frictional resistance are investigated. Because of the yield stress theta, two yield surfaces are found to be located in the flow field. Depending on the ration kappa (catheter size/vessel size) ranging from 0.3 to 0.7 (which is widely used in coronary angioplasty procedures), the frictional resistance to flow in large blood vessels, where the effect of yield stress can be neglected (i.e. theta = 0), increases by a factory ranging from 3 to 33. In small blood vessels with the same range of catheter size and an unit pressure gradient, frictional resistance increase was by a factor of 7-21 when theta = 0.05 and 11-294 when theta = 0.1. For small values of kappa and theta, the frictional resistance increased to several hundred times thus implying that the combined effect of increased catheter radius and yield stress is to obstruct the fluid movement considerably.

Animals↗
PubMed2004

Flow and injection characteristics of pharmaceutical parenteral formulations using a micro-capillary rheometer.

A micro-capillary rheometer consisted of a fine needle with an internal diameter of 347 microm attached to a 1 ml removable-needle syringe within an Instron device that operated in compression mode to provide various crosshead speeds ranging from 150 to 950 mm min(-1) covering typical clinical injection rates, and that determined the resulting force on the plunger. The crosshead speed and the resulting force were used to calculate the shear rate and the shear stress respectively. These were used in standard capillary flow expressions together with an independent measurement of the wall frictional force and allowed the viscosity of parenteral Newtonian solutions and non-Newtonian suspensions to be measured quantitatively and their rheological behaviour in needles of clinical dimensions to be established. Commercial pharmaceutical parenteral formulations consisting of three oil-based solutions and three aqueous suspensions were chosen for this study. The net injection forces were also obtained and it was shown that both the oil-based solutions and the aqueous suspensions covered similar ranges. The viscosities for the parenteral solutions were determined from the slope of the linear regression (R(2)>0.97) between shear stress and shear rate and ranged between 0.029 and 0.060 Pas. For the aqueous suspensions examined, viscosities decreased from low shear rate to high shear rate, following a power-law model and indicating a pseudo plastic behaviour. Standardisation of the micro-capillary rheometer with Newtonian silicone oils calibrated with a Rheometrics Fluids Spectrometer showed viscosity values consistent between the rotational flow measurements and capillary flow measurements which were within 5% and showed very high degrees of reproducibility between replicate samples. This degree of reproducibility allowed differences in the contribution of the wall frictional force to the required plunger force for both the oil-based and aqueous parenteral formulations to be determined reliably. The wall frictional force values for all formulations were similar (0.6-1.6 N) but the frictional forces of aqueous systems were found to decline significantly with plunger speed. The micro-capillary rheometer has been used to evaluate the impact of concentration changes due to sedimentation on the injectability of one of the aqueous suspensions, where it was shown that not only the viscosity increased but the shear thinning behaviour ceased at higher shear rates. The micro-capillary rheometer which was able to operate in clinical shear rate ranges has been shown to detect deteriorations in the injectable rheology of suspensions, which in the case here was due to pre-injection sedimentation.

Capillary Action↗
PubMed1993

The nature of ion and water barrier crossings in a simulated ion channel.

Using a combination of techniques, including molecular dynamics, time-correlation analysis, stochastic dynamics, and fitting of continuum diffusion theory to electrophysiological data, a characterization is made of thermally driven sodium, water, and D2O motion within the gramicidin A channel. Since the channel contents are constrained to move in a single-file fashion, the motion that corresponds to experimentally measurable rates of permeation of the membrane is the motion of the center of mass of the channel contents. We therefore emphasize channel contents center-of-mass motion in our analysis of molecular dynamics computations. The usual free energy calculation techniques would be of questionable validity when applied to such motion. As an alternative to those techniques, we postulate a periodic sinusoidal free energy profile (related to the periodic structure of the helical channel) and deduce the fluid dynamic diffusion coefficient and the height and spacing of the free energy barriers from the form of the mean-square-deviation function, using stochastic computations. The fluid dynamic friction in each case appears similar to that for aqueous solution. However, the diffusive motions are modulated by a spatially periodic free energy profile with a periodicity characteristic of an L-D pair of amino acids in the gramicidin helix, approximately 1.7 A in the model we use. The barrier height depends on which substance is moving in the channel, but in each case is several times thermal energy. For barriers of this width and height, the motion is intermediate between the low-friction (transition-state) and high-friction (Brownian) limits. Thus, neither of these formalisms that have been used commonly to describe membrane permeation gives an accurate picture of the underlying physical process (although the Brownian description seems closer to correct). The non-Markovian Langevin equation must be solved to describe properly the statistics of the process. The "channel state of matter" characteristic of the channel contents appears to have some properties typical of the solid and some typical of the liquid state. The magnitude of the local friction and nature of the ion solvation are similar to the liquid state, but the periodicities of structure, free energy, and dynamics are somewhat solid-like. The alignment of water dipoles in the channel bears some resemblance to the orientational ordering of a nematic liquid crystal, but unlike a nematic liquid crystal, the waters have a degree of translational order as well. Thus, the "channel state" is not adequately described by analogy to either the solid or liquid states or to liquid crystals but must be dealt with as its own characteristic type of condensed matter.

Biological Transport, Active↗
PubMed1999

Discrimination of DNA hybridization using chemical force microscopy.

Atomic force microscopy (AFM) can be used to probe the mechanics of molecular recognition between surfaces. In the application known as "chemical force" microscopy (CFM), a chemically modified AFM tip probes a surface through chemical recognition. When modified with a biological ligand or receptor, the AFM tip can discriminate between its biological binding partner and other molecules on a heterogeneous substrate. The strength of the interaction between the modified tip and the substrate is governed by the molecular affinity. We have used CFM to probe the interactions between short segments of single-strand DNA (oligonucleotides). First, a latex microparticle was modified with the sequence 3'-CAGTTCTACGATGGCAAGTC and epoxied to a standard AFM cantilever. This DNA-modified probe was then used to scan substrates containing the complementary sequence 5'-GTCAAGATGCTACCGTTCAG. These substrates consisted of micron-scale, patterned arrays of one or more distinct oligonucleotides. A strong friction interaction was measured between the modified tip and both elements of surface-bound DNA. Complementary oligonucleotides exhibited a stronger friction than the noncomplementary sequences within the patterned array. The friction force correlated with the measured strength of adhesion (rupture force) for the tip- and array-bound oligonucleotides. This result is consistent with the formation of a greater number of hydrogen bonds for the complementary sequence, suggesting that the friction arises from a sequence-specific interaction (hybridization) of the tip and surface DNA.

Base Sequence↗