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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
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What are the disadvantages of urine measurement?
- Sample not always available, not continous
- Samles easy to tamper with
- Embarassment
- Conc could be low
- Does urine reflect blood levels?
- Conc depends on:
- Hydration
- Kidney function
- Molecular size
- not the same for related molecules
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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.
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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.
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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
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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.
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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
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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
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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
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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
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What is formula for flux? What are its units
- where D is the scaling constant m2/s
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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.
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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.
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Where does lymphatic flow come from?
Lymphatic flow is from the difference between the arterial and venous blood pressures.
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What are the in vivo measurement options?
- Microdialysis: A sampling method with ex vivo analysis
- Implantable electrodes: Direct detection in living tissue
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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
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Con(s) of microdialysis
Poor time resolution
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Pro of in vivo implantable electrodes
Has best time sensitivity and time of response
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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)
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What are micro-electrodes and biosensors best for?
- Short timescales
- Can fail at longer timescales cos of limited stability of in vivo response
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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).
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