Good morning everybody. I guess English is okay, right?

Yes. So, yeah, well, we thought that it might be good to just have a very, very brief introduction

into the topic of mechanics for those of you, especially for those of you who haven't been

exposed to this topic massively before, so i.e. mostly for biologists, medics, but also for

everybody else who's interested. At the same time, I would like to announce that it's not really

possible to cover every important aspect like in a single introductory lecture. So today I'm just

trying to explain a few key concepts and mention a few key terms that are very important to know

within the topic of EBM. And those pictures here, well they just show you how old I am.

Yes, so some of you might remember those movies, amazing movies, but it's all about force. All right,

so if we talk about mechanics, everybody will realize quickly that mechanics is about forces.

No force, no motion, outside of vacuum at least, but it's not just about force. And here is an

example that I like to show to my undergraduate students when I cover the topic. So what you see

here is a very brave guy jumping down the cliff looking forward to his impact with the water. So

the reason why he moves down towards the water is gravity, so there's a force acting on him.

But the outcome of this action does not only depend on the force, so mechanics is more about

forces. Mechanics is also about mechanical indirection. So you see if we do a gedankene

experiment, if we maintain the force but we change the environment slightly, then the same force will

lead to a very different outcome, right? So it's not enough just to study forces. If we want to

study mechanics, we also need to measure mechanical properties of the environment to understand

mechanical interactions, so what happens to this guy here, right? So before I dive into methods,

how we can measure forces and mechanical properties of the environment of cells, of tissues, for

example, I would just like to start with some definitions, just so that you know what I mean

when I use certain terms. So what's probably the most intuitive term is that most of you will know

is the force. I would like to mention or to emphasize that a force is a vector, so force

always has a direction, right? The force is not just a number, it always comes with a direction.

Same for tension, which you may encounter quite frequently when we talk about neurons.

Tension is a pulling force which is measured in parallel to a string on which it applies,

or in our case very often along the axon, for example, or along the cell membrane. This is

where tension is important, so axons are under tension, cell membranes are under tension.

In some projects, we mentioned mechanosensitive ion channels such as Piazza 1, for example.

Piazza 1, although it's not really understood how it's activated yet, there is evidence that at

least in vitro when you have the channel in a 2D flat membrane that if you increase membrane

tension this can lead to the opening of the channel. Tension refers to pulling force along,

like, parallel to a certain direction. And then this is all more or less like one-dimensional,

but of course tissues are three-dimensional objects, and this is where these terms force

and tension not always are applicable. And here we come to more complex terms, and this is what

engineers use a lot to describe mechanics of materials. So one is stress. Stress is basically,

well, I try to spell out everything as much as possible because we will share these slides with

you in the end, and so you can always quickly look up what these terms mean. Stress refers

to an internal force that acts within a deformable body, and it's basically force over unit area,

force per area. So the unit is pascal, or in our case piconewton per square micrometer.

This is stress acting within a tissue. And the deformation, the relative deformation arising

from stresses is called a strain, right? So it's a relative change in original size, so it's a

measure of the formation. It's a ratio of relative change over initial length, so it's unit-less.

So strain comes without a unit. And then to what we do in an experiment, where we measure tissue

mechanics, is we apply either stress, we measure strain, or the other way around, and the ratio

of stress and strain yields an elastic modulus. And this is what we refer to when we talk about

tissue stiffness, usually. It's a convention. It's not scientifically super correct, but it's

convention. Elastic modulus refers to stiffness. And please, all the engineers in the room,

feel free, if I say something too naive, too stupid, or something wrong, please feel free