Lec 5-6

Question 1

In measuring the bulk modulus of a cell or nucleus, as it is compressed more, the bulk modulus appears to

Answer 1

increase

Question 2

Whats bulk modulus

Answer 2

bulk modulus K measures a material’s resistance to uniform compression

the stiffer a material, the more pressure it takes to shrink it a little

Question 3

What happens when increase lamin and osmotic pressure

Answer 3

higher Lamin A/C makes nuclei stiffer
Higher lamin has lower volume change with increasing osmotic pressure

Question 4

What is the smallest resolvable feature

Answer 4

abbé diffraction limit

Question 5

Q: What is confocal microscopy?

Answer 5

A: A fluorescence imaging technique that uses a pinhole to eliminate out-of-focus light, improving optical sectioning and Z-resolution (~100 nm).

Question 6

Q: What is widefield (epifluorescence) microscopy?

Answer 6

A: A fluorescence technique where all emitted light is collected, resulting in lower Z-resolution but faster imaging.

Question 7

Q: How can microscopy be used to determine filament stiffness?

Answer 7

A: By tracking thermal fluctuations of the filament’s shape and extracting persistence length → derive modulus.

Question 8

How does a microfluidic device apply shear stress?

Answer 8

A: By controlling flow rate through channels of known dimensions, shear force is applied to cells on the channel walls.

Question 9

What is a main limitation of bulk rheology in bioengineering?

Answer 9

A: Requires large, homogeneous samples (>200 mm³), which is incompatible with studying individual cells or filaments.

Question 10

What is Atomic Force Microscopy (AFM)?

Answer 10

AFM is a technique that uses a nanoscale cantilever tip to physically indent or scan a surface, measuring deflection to determine mechanical properties like stiffness, force, and topography at nanoscale resolution.

Question 11

How do optical tweezers measure force?

Answer 11

A: Force is measured by tracking the displacement of the trapped particle from the laser’s center. Since the laser trap acts like a spring:

Question 12

MSD slope for pure fluids
MSD slope for pure solids
visco

Answer 12

always 1
always 0
Slope between 0 and 1: viscoelastic material

Question 13

What are the only forces acting on a system in equilibrium

Answer 13

Thermal motion

Question 14

What is N/m^2 equal to

Answer 14

pN/µm^2

Question 15

advantages of PDMS

Answer 15

• Optically transparent
• Well defined mechanical
  properties
• Linear elastic property in the
  range of our interest
  (~0.8-100kPa)
• Easy to synthesize
• Stable & inert material
• Long shelf-life

Question 16

loss tangent tells us

Answer 16

LT >1 principally viscous
LT < 1 principally elastic

Question 17

What is bead mvm in cells ?

Answer 17

superdiffusive, due to active forces

Question 18

Phase angles

Answer 18

90º for ideal fluid
0º for elastic solid
60º or other is viscoelastic

Question 19

Substrate for tfm

Answer 19

Substrate must be:

Elastic

Isotropic

Homogeneous

Question 20

What is the purpose of microrheology?

Answer 20

To measure the mechanical properties (viscosity, elasticity, stiffness) of materials like the cytoplasm using small probes (e.g., beads).

Question 21

What is passive microrheology?

Answer 21

A method that tracks the random (Brownian) motion of embedded beads to infer mechanical properties without applying external forces.

Question 22

What kind of data do you collect in passive microrheology?

Answer 22

Mean squared displacement (MSD) over time.

Question 23

What physical property can be derived from bead diffusion in a fluid?

Answer 23

The viscosity of the surrounding material.

Question 24

Why does cytoplasmic bead motion differ from pure fluids like glycerol?

Answer 24

Because the cytoplasm has structural elements (like actin) and motor activity that alter passive diffusion.

Question 25

What happens to bead motion when ATP is removed from cells?

Answer 25

Beads become mostly immobile, indicating the cytoplasm behaves like a passive elastic gel.

Question 26

What conclusion is drawn from observing increased bead movement in ATP-present conditions?

Answer 26

Active processes (e.g., motor proteins like myosin) are contributing to internal forces.

Question 27

What is active microrheology?

Answer 27

A technique where external forces are applied (usually via optical tweezers) to probe material stiffness or viscoelastic response.

Question 28

What is the principle of optical tweezers?

Answer 28

A focused laser beam creates a potential well that traps a bead and allows force application by moving the trap or the sample.

Question 29

What do you measure in an optical tweezer experiment?

Answer 29

Bead displacement in response to a known, oscillating trap movement.

Question 30

What does a small phase lag between trap and bead indicate?

Answer 30

The environment is more elastic or solid-like.

Question 31

What does a large phase lag indicate?

Answer 31

The environment is more fluid-like (viscous).

Question 32

How do you determine if a cell behaves more like a solid or fluid using optical tweezers?

Answer 32

By comparing the phase lag between applied force and bead displacement — smaller lag = more elastic.

Question 33

What is the purpose of loss tangent (tan δ) in microrheology?

Answer 33

It quantifies the ratio of viscous to elastic behavior — values <1 indicate elastic dominance, >1 indicate viscous dominance.

Question 34

What is the general mechanical behavior of cytoplasm at short vs. long timescales?

Answer 34

Solid-like at short timescales, more fluid-like at long timescales — like silly putty.

Question 35

What is Force Spectrum Microscopy (FSM)?

Answer 35

A method to quantify total internal forces in cells by measuring spontaneous bead fluctuations and local stiffness.

Question 36

How does FSM separate passive and active forces?

Answer 36

By comparing forces in normal, ATP-depleted, and drug-treated (e.g., myosin-inhibited) cells.

Question 37

What happens to the intracellular force spectrum when ATP is depleted?

Answer 37

Only thermal fluctuations remain — active forces disappear.

Question 38

How does myosin inhibition affect FSM results?

Answer 38

It selectively reduces low-frequency active force generation.

Question 39

What is the difference in mechanical phenotype between benign and malignant cells?

Answer 39

Malignant cells are softer and show higher levels of active intracellular force generation.

Question 40

What is Traction Force Microscopy (TFM)?

Answer 40

A technique to measure how much force a cell applies to its substrate by tracking deformations of that substrate.

Question 41

What kind of substrates are used in TFM?

Answer 41

Soft, elastic, and optically clear materials like PDMS or polyacrylamide with embedded fluorescent beads.

Question 42

What key measurements are needed in TFM?

Answer 42

Displacement of the beads caused by the cell and the stiffness of the substrate.

Question 43

How is force calculated in TFM?

Answer 43

From the displacement of embedded beads and the known stiffness of the substrate (using F = kx).

Question 44

What do you need to do before applying cells to a TFM substrate?

Answer 44

Take a "force-free" reference image of the gel without the cell.

Question 45

What are the two modern setups for TFM?

Answer 45

(1) Gel substrates with embedded fluorescent beads; (2) Micropillar arrays that deflect under cell force.

Question 46

Why must TFM substrates be elastic, isotropic, and homogeneous?

Answer 46

So that the force-displacement relationship is predictable and accurate.

Question 47

How do you determine whether a PDMS gel is elastic enough for TFM?

Answer 47

Perform a frequency sweep to see if the storage modulus (G′) is flat at relevant frequencies.

Question 48

What does a rising modulus with frequency indicate in rheology?

Answer 48

Viscoelastic behavior — the material becomes stiffer with faster deformation.