What are the 6 advantages of urine measurement?
- Easy to come by
- Non invasive
- Large sample volumes
- Sterile, conductive analysis liquid
- many molecules slectively cleared by the kidneys
- Dugs syay in urine for a long time
What are the disadvantages of urine measurement?
- Sample not always available, not continous
- Samles easy to tamper with
- Conc could be low
- Does urine reflect blood levels?
- Conc depends on:
- Kidney function
- Molecular size
- not the same for related molecules
Describe Urine analysis with lateral flow assay
- a) Absorbant pad keeps the flow of liquid going
- b) Assay starts with ading liquid sample. capillary forces in paperpull liquid along the strip
- c) Antibodies conjugated to coloured nanopartices bind the antigen
- d) Particles with antigens bind to test line, particles without antigens bind to the control line.
What are the two types of sweat glands?
Eccrine: pccur all over the body and open directly to skin surface
Apocrine : feed into hair follicles and hence to the skin surface.
What are the pros of sweat measurement?
- Continuous access
- Ease of placement and comfort
- Can sample without external contamination.
- Its is not a digestive fluid that can degrade some analytes
- Can be sensed quickly.
- Can be produced on demand with iontophoresis
What are the challenges of monitoring sweat?
- Very low sweat production rates e.g nL/min/mm2
- Irregular sweat flow without inotophoretic stimulation
- Old sweat can mix and contaminate new sweat
Does sweat reflect blood
- Large analytes are filtered
- Some concs only represent skin
- Sweat rates can skew the concs of some analytes relative to the conc seen in blood.
What are the pros of blood monitoring
- Its what clinicians are used to
- Blood tests are quantitative
- Many standard tests are developed for blood measurement
- Can have large samples for adults
- Blood samples whole body system
What are the cons of blood monitoring
- Blood is a complex mixture so you may need plasma separatoin with centrifuge or pressure
- High protein levels means that some chemicals are stuck to the protein
- Blood is active so is susceptible to clotting and has its own metabolism
When are sensors fouled by blood clots and why
- When blood flow is low
Initially proteins like albumin stick to sensors, providing a diffusional barrier slowing response
time but not steady state reading
After a few hours blood platelets are activated and stick to he probe
- They're metabolically active so consume oxygen and glucose
- They give out CO2
What are the advantages of monitoring biomarkers in the tissue?
- Higher conc: not diluted
- Localisation: You know where the molecules are coming from
- Time locked to tissue function: can ask if its still working and what happens if I challenge it
- Links to other measurements: Electrical activity, blood flow etc
- Dynamic fingerprint: Can track disease progression
What is formula for flux? What are its units
- where D is the scaling constant m2/s
What is the einstein smoluski equation and what is its importance?
L = (2Dt)0.5
good for 1D approximation and it means and implanted device only measures stuff thats close to it.
What makes living tissue more complex than a beaker of solution due to the complexity of the extracellular space
- 1. Molecules have to diffuse around cells as they cannot penetrate cell walls. This creates a more tortuous diffusion path
- 2. Molecules inside the cells cant diffuse to your sampling point either since the cell walls stop them feeling the conc gradient outside the cell
- 3. Active uptake mechanisms exist for molecule taken up by or released by cells
- - This reduces conc seen away from site of release
- Give excluded volume effects again.
Where does lymphatic flow come from?
Lymphatic flow is from the difference between the arterial and venous blood pressures.
What are the in vivo measurement options?
- Microdialysis: A sampling method with ex vivo analysis
- Implantable electrodes: Direct detection in living tissue
What are the pros of microdialysis
- Good selectivity: You can be sure about what you're measuring.
- Can measure almost anything:use a range of techniques
- Can use in humans
Con(s) of microdialysis
Poor time resolution
Pro of in vivo implantable electrodes
Has best time sensitivity and time of response
Cons of in vivo implantable electrode
- Impossible to calibrate in vivo
- May detect other chemical species.
- Only a small no. of molecules detectable by direct oxidation (includes dopamine, noradrenaline, and seratonin)
What are micro-electrodes and biosensors best for?
- Short timescales
- Can fail at longer timescales cos of limited stability of in vivo response
Factors affecting the local conc of chemical in ECF of tissue in vivo
- Balance of delivery fluxes (release, delivery by blood)
- and removal fluxes (metabolism, blood flow).