The widespread misuse of drugs has increased the number of multiresistant bacteria1, and this means that tools that can rapidly detect and characterize bacterial response to antibiotics are much needed in the management of infections. Various techniques, such as the resazurin-reduction assays2, the mycobacterial growth indicator tube3 or polymerase chain reaction-based methods4, have been used to investigate bacterial metabolism and its response to drugs. However, many are relatively expensive or unable to distinguish between living and dead bacteria. Here we show that the fluctuations of highly sensitive atomic force microscope cantilevers can be used to detect low concentrations of bacteria, characterize their metabolism and quantitatively screen (within minutes) their response to antibiotics. We applied this methodology to Escherichia coli and Staphylococcus aureus, showing that live bacteria produced larger cantilever fluctuations than bacteria exposed to antibiotics. Our preliminary experiments suggest that the fluctuation is associated with bacterial metabolism. Highly sensitive cantilever sensors can now be used to quantitatively characterize a bacterial response to drugs before they even replicate.

Rapid detection of bacterial resistance to antibiotics using AFM cantilevers as nanomechanical sensors

Analysis of load-relaxation in compressed segments of lumbar spine.

Stress autocorrelation functions (SAF) of entangled polymer melts are calculated by coarse-grained molecular dynamics (MD) simulations. We show that time-averaged stresses reduce the strong noise in SAFs while still capturing most relevant relaxations of the chains. Plateau values of the SAF compare well with plateau values predicted from the entanglement length evaluated via primitive path analysis (PPA) and from experiment (where available). Three types of polymer models are studied: a flexible bead spring model, a semiflexible bead spring model, and a coarse grained bisphenol A-polycarbonate (BPAPC) model. This approach provides a straightforward way to analyze rheological properties of polymer melts on the fly during a simulation.

Entangled Polymer Melts: Relation between Plateau Modulus and Stress Autocorrelation Function

An analysis is presented of the discrete Fourier transform of a monotonically decaying function, represented by a sum of exponentials with negative exponents. The results are compared with those of a previous analysis based on the analytical Fourier transform, which proposed a method for extending the frequency range of the Fourier transform of experimental data by combining transforms performed over different time periods. The principle of the method is confirmed but comparison shows that results derived for the analytical transform cannot always be applied directly to the discrete transform. Modifications are therefore proposed which improve the accuracy and mitigate the aliasing effects evident in short-time transforms while keeping the computing time to a minimum. These results are particularly applicable to stress relaxation and creep in viscoelastic materials and an example from articular cartilage shows the compliance modulus over a frequency range from 10-3 Hz to 230 Hz from one creep experiment of duration 18.7 min.

https://iopscience.iop.org/article/10.1088/0022-3727/24/6/002/pdf

Aliasing effects in Fourier transforms of monotonically decaying functions and the calculation of viscoelastic moduli by combining transforms over different time periods

The major Hot-Mix Asphalt (HMA) input for mechanistic-empirical (M-E) flexible pavement design is the dynamic complex modulus obtained from either the uniaxial or triaxial compressive dynamic modulus test. Furthermore, as part of the performancebased mix design process, the triaxial dynamic modulus has been selected to predict rutting and fatigue cracking, and the Indirect Tension (IDT) creep compliance test to predict low-temperature thermal cracking. The creep compliance and dynamic modulus are measured responses (viscoelastic functions) of viscoelastic materials under transient and cyclic loading, respectively. Under the assumptions of linearity, linear viscoelastic functions are equivalent. Moreover, these properties should be the same whether they are obtained from a uniaxial compressive or IDT test. In this dissertation, the applicability of linear viscoelastic (LVE) theory to HMA mixes was tested. Furthermore, a number of studies suggested that HMA properties obtained under tension are different from properties obtained under compression. Therefore, the need to model HMA as a bimodular material to interpret IDT test results was also investigated. A testing program was developed to experimentally measure the uniaxial compression, and IDT creep compliance, and the uniaxial compression dynamic modulus for different HMA mixes. The uniaxial compressive creep compliance and dynamic modulus master curves are constructed and the shift factors obtained from each test are compared. Interconversion between the creep compliance and dynamic modulus experimental results confirm the applicability of LVE theory for the HMA mixes investigated. Based on the applicability of LVE theory, a methodology to determine HMA LVE properties from the combined creep compliance and dynamic modulus test results was developed. As a practical application that is relevant to the M-E flexible pavement design procedure, LVE theory was used and compared to proposed approximate methods to perform the conversion of testing frequency to loading time. Specifically, dynamic modulus results were converted to relaxation modulus, creep compliance, and resilient modulus. Finally, the HMA IDT creep compliance test results at low and intermediate temperature (<20C) were successfully analyzed using a HMA bimodular material model based on the Ambartsumyan model. The difference between the compressive modulus and the modulus calculated from the IDT test using Hondros’ stress distribution is calculated. In addition, a method to determine the compressive-to-tensile modulus ratio using uniaxial compressive and IDT test results is illustrated for one of the tested HMA mixes.

Analysis of Hot Mix Asphalt (HMA) Linear Viscoelastic and Bimodular Properties Using Uniaxial Compression and Indirect Tension (IDT) Tests

A powerful new experimental method of determining the dynamic mechanical behaviour of viscoelastic materials is described. The method, Fourier transform mechanical spectroscopy (FTMS), consists of superposing and deconvoluting discrete Fourier transforms obtained from a single stress relaxation experiment using a novel sampling scheme. The theory of the sampling scheme and the superposition of the transforms is described elsewhere. In this paper the authors present the results of using the new method on Nylon over the frequency range 5*10-4 to 20 Hz and temperatures from 25 to 110 degrees C. There is good agreement between the values of storage and loss moduli, obtained by means of the transform method, and those obtained by conventional means. An activation energy of 143 kcal mol-1 is obtained for the relaxation in this temperature range. The new method has considerable potential for the rapid determination of the mechanical properties of novel polymers and composites.

http://iopscience.iop.org/article/10.1088/0022-3727/19/6/001/pdf

P J Barham

Papers

Fourier transform mechanical spectroscopy