APMDA boutelle L1&2

• 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

• 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

• 
• 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.

Eccrine: pccur all over the body and open directly to skin surface

Apocrine : feed into hair follicles and hence to the skin surface.

• 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

• 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.

• 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

• 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 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

• 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

• 
• where D is the scaling constant m²/s

L = (2Dt)^0.5

good for 1D approximation and it means and implanted device only measures stuff thats close to it.

• 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.

Lymphatic flow is from the difference between the arterial and venous blood pressures.

• Microdialysis: A sampling method with ex vivo analysis
• Implantable electrodes: Direct detection in living  tissue

• Good selectivity: You can be sure about what you're measuring.
• Can measure almost anything:use a range of techniques
• Can use in humans

Poor time resolution

Has best time sensitivity and time of response

• 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)

• Short timescales
• Can fail at longer timescales cos of limited stability of in vivo response

• Balance of delivery fluxes (release, delivery by blood)
• and removal fluxes (metabolism, blood flow).