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Ramsey Kraya

Experimental Conditions for Mapping Nanomechanical Properties of Live Cells with AFM

My question is in regards to the paper by Raman et al published in Nature Nanotechnology in 2011 (DOI: 10.1038/NANO.2011.186).

How does driving the cantilever in the repulsive or attractive regimes (instead of exactly at the resonant frequency) effect calculations of stiffness and damping?

Can the procedure outlined in the paper be applied to cells in media? Especially with a large concentration of serum?

Thank you in advance

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    Alexander Xavier Cartagena

    Dear Ramsey Kraya;

    About your first question, the proposed multi-harmonic AFM method outlined in the paper is for the repulsive regime specifically permanent contact. This method to be valid requires a large net sample indentation when compared to the oscillation amplitude to ensure that the AFM probe is in permanent contact with the soft live cell during the entire oscillation cycle. Also, this method requires that the cantilever should be directly excited (magnetic, Lorentz force, photothermal) with a driving frequency to be near the resonant frequency.

    Now about the second question, the outlined experimental and theoretical procedure should be able to be performed for cells in media. However, media has a lot of proteins and nutrients necessary for optimal cell growth and this can potentially contaminate your AFM tip rather quickly. Also, the larger the concentration of serum in your media the more viscous the liquid becomes, and the added mass (added mass hydrodynamic effects) on the cantilever becomes very large and can overdamped the response of the cantilever.

    I hope you find these responses useful. Alexander Cartagena

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      Ramsey Kraya

      Thank you for the reply. Also, when calculating the indentation is this simply the sum of the topography (Z) and the 0th harmonic amplitude (deflection)?

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        Alexander Xavier Cartagena

        Hi Ramsey; Mean indentation is not simply the sum of topography and cantilever DC mean deflection. Average tip indentation is given by delta=-(Z+A0)the sum of the Z-piezo displacement pushing on the sample and the cantilever mean deflection. In tapping mode AFM Z-piezo displacement is not recorded and we worked out a way to approximate/estimate the average indentation simultaneously with the material properties (stiffness and damping or storage and loss modulus) by best matching the force harmonics. Please refer to the supplementary material page 11 for in detail explanation. Cheers; Alexander Cartagena

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