It is important to be able to measure the adhesion force between tissues for the medical profession because we can measure the adhesion of a thrombus to avoid embolism and measure the strength of a surgical glue so that we can quantify the glue's efficiency. The aim for this project is to design a device to measure the adhesion between tissues. Several adhesion measuring devices will be reviewed in this report such as peel test for measuring adhesions of pressure-sensitive materials and how could we modify it for measuring adhesion of soft tissues and we will look into cone and plate rotational viscometers and fluorescent labelling to measure the adhesions of a blood clot and vessel wall under shear rate. This report focuses on methods that measure tissues adhesion under atmospheric pressure (i.e. a piece of meat on leather) and tissues adhesion under continuous flow (i.e. blood clots on a vessel wall). We will compare the devices and methods in terms of cost, complexity and result data. We will study how tissue interactions could be affected by tensile force, shear rate, time, temperature and other parameters. In the end of this report we will choose a suitable method to modify and build an experimental rig to gain further understanding on tissue adhesion.
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1.1 interactions of tissues
Tissues are made out of same type of cells and the cells have same types of receptors and ligands. Receptors and ligands are the bonding parts of the cell and it is their interactions that form adhesion between tissues. It is our interest to study the interaction between different types receptors and ligands in varies conditions.
1.2 pressure sensitive adhesion
Pressure-sensitive materials are sticky and their adhesions don't involve chemical reactions, water or solvent. The adhesion of a pressure-sensitive material varies with the applied pressure, surface conditions and temperature. Some examples for pressure-sensitive materials would be surgical glue, note pads and pressure-sensitive tapes. Peel test and bulge and blister test are suitable to test pressure-sensitive materials, however, most soft tissues are not pressure-sensitive, they are delicate and the adhesion force could be significantly lower than pressure-sensitive materials. Modifications needed to be made for this tests to able able to measure the adhesion of tissues and cells.
receptor and ligand
receptors and ligands are protein molecules, they are very specific like keys and locks. If they meet and they fit together, they will form a ligand-receptor complex. We can use the law of mass action to measure how well the ligands and receptor are interacting by monitoring the dissociation constant. Not all kinds of ligand-receptor complexes show similar characteristics, for example, some of these complexes are more stable and the adhesion of some complexes ( blood clot/vessel wall ) are shear rate dependent. It is possible to label a ligand-receptor complex using fluorescent, however, it may not be specific enough if we couldn't find the suitable antibodies.
1.4 Blood clot and vessel wall
When there is a bleed, platelets and other proteins like clotting factor and fibrinogen will activate and a platelets/fibrin clot will stop the bleed. The bonding is complex, some of the adhesion bonds are time dependent such as a platelet GPIb/IX and collagen-von Willebrand factor complexes on the vessel wall, while most of the adhesion bonds are shear rate dependent. In a normal situation, blood clots will dissolve in blood after a wound is healed but some of the blood clots cannot be dissolved and it could get wash out to other organs to cause a stroke, internal bleeding and many serious damages. It will be useful to be able to measure the adhesion between a blood clot and a vessel wall since it can reduce the chance of embolism either during surgery or post-surgery.
2. Methods used for study
The idea of peel testing is that a tensile machine will apply a slow and constant peeling velocity to the two attached surfaces in a fixed angle; the peel is usually perpendicular to the surface so that it could simplify the calculations since the applied tensile forces will be the normal forces to detach the surfaces. Peel test is good at measuring adhesion force of pressure-sensitive materials but it needs modifications for measuring adhesion of soft tissues and cells. For example, The tissue sample is not going to be homogeneous so it will give an unusual result to standard peel test and since we are not using 'film' as it is hard to cut tissue films, we may have to take the weight of the tissue sample in account depending on the ratio of adhesion forces to the weight of sample, if the effect of the weight is insignificant, it could be neglected but when the adhesion forces are weak, the result need to be calibrated.
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The final result of a peel test is usually a force against distance chart and sometimes force against time chat. The maximum force monitored from the peel test will represent the peel strength. The peel force can be normalised by the sample width and gives a better reading but for a non-homogeneous surface like a soft tissue sample, it is not important to do so that the adhesion force is not uniform due to the uneven distribution of fats and flaws on the surfaces.
2.1.1 effect of peel rate
The reason to apply a slow peeling velocity is because the system needs time to reach chemical equilibrium or there will be more unbroken bonds at the same distance. The peeling velocity can be decided by this mathematical model:
Where T is the applied tension
K is ratio association and dissociation rates
This model shows that for a fixed peeling angle, the logarithm of the peeling rate is proportional to the square-root of the applied tensile forces.
A four steps test  can be used to study the effect of peel rate. The peel rate increases from 7um/s, 70um/s, 0.7mm/s to 7mm/s over four regions with the same length. The follow figure shows the change in the normalised peel force versus position and we can obtain the error bar from the data. We can conclude from both the mathematical model and the four steps test that faster peel rate require greater peel force but it will generate larger error and might damage the delicate tissue sample. For a modified peel test for tissues, we have to apply a suitable peel rate that will not too fast so it would generate massive error or too slow and affect the freshness of the sample.
Four steps test
2.2 Bulge and blister test
Bulge and blister test is a combination of bulge test and blister test. This is an industrial test for measuring the adhesion energy; it is applied to the surgical field to quantify adhesion of surgical glues and other bioadhesives.
The test consists in applying pressurised water in one side of a film window through a hole in the film substrate. The sample was glued to an aluminium support with industrial glue. Water was progressively injected into the hole in the support and the bottom layer to create a blister with constant radius, increasing height and pressure. At a critical pressure, the system will fail and the radius and height of the blister increases and the pressure decreases, it is at this point that we can calculate the adhesion energy.
The adhesion energy can be measured at this point by using this equation
Gi is the adhesion energy
C is a constant, which varies slightly between 0.618 and 0.516
p is the pressure
h is the height
The preparations are the most difficult parts of the test. First, we need to fix the first pericardium layer on the sample holder with strong industrial glue, it is to avoid inï¬ltration of the pressurised water. A hold is cut on the first pericardium layer , it should be identical to the water inlet hold in the sample holder. A unbound zone that is smaller than the first pericardium layer but larger than the water inlet hole is prepared by a graphite on top of the hole. The unbound zone is to avoid glue filling the hole and affect the water injection. The second pericardium layer is to glued on the first pericardium layer and the zone. A wet pericardium however have a low elasticity , to avoid high deï¬‚ection and the steep slope, an extra thin layer of Gore-Tex is glued on the second pericardium layer to increases its stiffness.
the test uses a CCD camera to give top views of the sample illuminated by the laser fringes.
This method gives very quantitative data. However the preparation procedures are complicated, it involves cutting and gluing the layers and penetration of glue in the injection port will affect the result. This method also requires homogeneous surfaces which is hard to put into practical use. It is not suitable for tissue with weak adhesion force as the crack would happen too fast and the test would not work.
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2.3 Cone and plate Rotational viscometers
The cone-and-plate system consists of a cone, the tip
of which touches a flat-base plate. A blood sample with known platelet concentration is used and it is placed to the gap between the cone and the plate. The device uses rotation to generate a wide range of uniform and constant shear rate flow field. A smooth collagen coated surface is used and shear rate is applied in order to simulate the condition in a blood vessel. This in vitro test can measure the cell adhesion under controlled flow conditions to any smooth surface.
For a small angle
g = V / q
where V = cone rotation rate, q = cone angle, and g =fluid shear rate.
The test material (smooth collagen coated surface) was placed in the apparatus so that it formed the base of the wells. The four wells is then placed into the positioning bracket. Blood sample is then added to the wells. The cone then lowers and touches the test material. The cones will rotate and generate constant and uniform shear rate for a desired period of time. The cones will finally stop and raise, the test material will be ready to be examined.
After applying a constant shear rate for a desired period of time. The sample film will be counted for platelet concentration and compare with the original known platelet concentration and the platelet concentration under different shear rate. This device helps us study the relationship between platelet adhesions and shear rate. This in vitro test is not suitable to simulate actual surface condition of vessel wall as they are not smooth and homogeneous,
2.4Fluorescent labelling method
To understand the interaction between platelets and blood vessel wall, we can monitor their activity by prelabelling the platelets with fluorescent. In an experimental rig using collagen coated surface, the platelet area coverage, particle intensity and temporary arrest are monitored by computer. The area coverage reflects on platelet aggregation, the particle intensity represent the platelet adhesion. Temporary arrest is the weak and transient bond that platelets will form in the initial state of clotting; the adhesion for this bond is time dependent while other clot/vessel wall bonds are mostly shear rate dependent. We are able to monitor the temporary arrest at a particular time by monitoring the area coverage of the fluorescent images subtracting the area coverage of the fluorescent image after 5 seconds.
The data is only useful for comparing how platelets react in different environment as there is no actual quantity of the force needed to detach a blood clot. To further quantify the data, dubbed ReaLiSM  (Receptor and Ligand in a Single Molecule) can be employed. Binding a receptor from platelets and a ligand from the vessel wall and knock them apart using a molecular tweezers and measures the strength and lifespan of the bond. With the data of strength of a single receptor and ligand bond, and particle intensity, we can estimate the adhesive forces between the clot and the surface assuming all shear rate dependent bonds have similar strength or we can find antibodies that are specific enough to identify different types of receptor on the blood clots and uses ReaLiSM to measure the force of different types of ligand-reecptor bonds, the chance of actually doing that seems slim since the original research identify the weak platelet GPIb/IX and collagen-von Willebrand factor complexes(temporary arrest) by monitoring platelet interactions and not labeling. Further researches need to be done to modify this method but because it is too costly and time consuming, we will not consider developing this method.
This method measures the minimum tensile stress needed to detach or rupture the two surfaces in a field angle. An apparatus is designed using the principle of the ballistic pendulum to measure the tensile strength of blood clots.
A blood sample is allowed to clot in a cuvette which has a dumb-bell shaped cavity. After the blood finished clotting the two half cuvettes are linked by the blood clot, the principle of the ballistic pendulum is applied to measure the force applied to the clot in order to break the adhesion bonds. One half cuvette is in a carrier that The measuring pendulum hangs vertically from the bearing. The second half cuvette is in a carrier that attaches the traction pendulum which swings from the same bearing as the first but it is free to move. An electric winch drives the traction pendulum to swing and the measuring pendulum will follow the swing to they are linked by the clot, they will swing until the link breaks. The adhesion force is proportional to sine degree angle to the vertical. The maximum tensile strength is measured at the maximum pendulum deflection.
This apparatus uses blood clot as glue to link to metal surface. It measure the strength of the clots in a blood sample but not the tissue interaction. The data are quantitative but it indicate the likelihood of a patient getting thrombosis instead of measuring tissue adhesion.
Advantages and disadvantages of the methods
Low cost, easy to operate, can install a camera and monitor real time tissue interactions during the peel test, good for pressure-sensitive materials, gives quantitative data
Requires homogeneous surfaces, slow ( if production is involved)
Bulge and blister test
Novel method, good for pressure-sensitive materials, gives quantitative data
Cannot monitor tissue interactions, requires homogeneous surfaces, needs preparations
Cone and plate Rotational viscometers
Easy to operate, can adjust time period
Cannot monitor tissue interactions, in vitro ( for understanding platelet interactions only, cannot measure the adhesion force of a clot in a live patient)
Fluorescent labelling method
Can monitor tissue interactions, potential for further developments.
Cost, difficult to quantify the data
Low cost, easy to operate, gives quantitative data
Cannot monitor tissue interactions, it measures blood clot/metal adhesion using blood clot as glue (uncommon)
Peel test seems to be the best method for we to develop since it is easy to operate and only require a tensile machine and it will be interesting to monitor the peel surface using a camera while we changes the parameters such as peel rate and temperature.
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