Soft tissue samples

Dear Danielle, Less than 1kPa is really soft... Then the only cantilevers use can use are MCLTs. You should use the triangular longest one (spring constant 0.01 N/m). You migh have to wait 30 min before engaging to the surface because of thermal equilibration (when the vertical drifts stops, you can engage). Also deflection sensitivity should be between 25 and 100 nm/V. For your information, we recently developped peak force tapping, which includes peak force QNM that allows getting quantitative info (Youn's modulus, deformation, dissipation,...). It seems to wok also very well on super soft samples. Now coming back to you second question, I need to know more about your sample. What is it ? Living cells, bacteria? If you want you can contact me directly at the following email address: aberquand@veeco.de Best regards, Alex.

Hi Danielle,

An elastic modulus of 1kPa really is very soft. All the papers that I've read measuring moduli in that range report using cantilevers that have been modified with microspheres in the range of several micrometers in diameter. If you start doing some rough calculations you start to see why. A common indentation model is the DMT/Hertz model:

F = (4/3)*E/(1-v^2)*(R^0.5)*(d^1.5)

where:

F=Force, E=Elastic modulus, v= Poisson's ratio, R=Tip radius, d=indentation depth (i.e. deformation)

Just for some quick estimates, assume that v=0.5 (actually it's not usually well known) and let d=R (worst cast- if d>R the model doesn't work).

First let's see what happens when E=1kPa and R=20nm. You find that F would be only 0.7 pN, certainly not practical to measure with AFM. Smaller deformations of course would require even smaller forces.

But if you let R=2000nm and leave E=1kPa, then you get F=7.1nN. That's certainly possible to measure with AFM. Of course we're still assuming that d=R, and you probably don't want to indent your tissue 2000nm. But even if you reduce that to d=100nm, you get F=79.5pN, which is still reasonable. If you want even smaller indentations, then you need to increase the tip radius.

There are some commercial sources of probes already modified with microspheres. It's also not too hard to do yourself. Veeco Probes sells tipless cantilevers (link here) to which you can glue the spheres. I'll post and link a couple documents describing different attachment methods.

Working with tissue samples can be tricky with AFM. Even once attached, they tend to be quite rough. The Catalyst has a lot of Z range, >20um, but it could still be an issue. You also need to consider how well the real indentation situation will approximate the indentation model (a sphere indenting a flat plane). If you are using a larger particle modified tip then you want the sample to be flat over areas larger than the particle.

There are many possibilities for attaching tissue sections to slides, including  "tissue adhesives" (Cell-Tak), silane treatments (protocol here), and commercially available slides (several here). It might just take some trial and error to find something that works with your samples.

 

Regards,

-Ben