Chapter 6: Movement
In the living body, elements relate each other, so that the whole body realizes the
movement as represented by the muscular-skeletal system.

In this chapter, we learn

basic engineering related to the interface: interaction between elements, lubrication, andwear.

6.1 Balance among forces and control of movement
6.1.1 Balance among forces“Force” is defined as a vector which changes the velocity or the form of an object.
When the force is not applied, the object keeps the shape and the velocity as the uniform
linear motion.
The hip joint consists of a combination of a sphere (the femoral head) and a cup(the acetabular) (Fig. 6.1).

In the standing position, the hip joint supports the bodyweight.

When you stand on both legs, each hip joint supports each half of the bodyweight.

When you stand on one leg, one hip joint supports the entire body weight.

Fig. 6.1: Force at hip joint.

© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, JapanAt the hip joint surface, a force that exceeds the weight is applied.
acceleration at the exercise.

It supports

It plays a role of damper for an impact force applied in ashort time.
The muscle generates the force to control the movement at the joint.

The line ofaction of the gravitational force by weight does not always coincide with the line ofaction of the supporting force at the hip joint.

When there is a deviation between thelines of action, the momentum force occurs around the joint. The balances both at theforce and at the moment demand the force exceeding the weight at the articular surface
as the fulcrum (Q 6.1) (Fig. 6.1).
The force acting apart from the fulcrum generates a large moment, which is called
the principle of leverage. The leverage reduces the force in inverse proportion to the
distance from the fulcrum.

The leverage increases the working distance (or speed) in

proportion to the distance from the fulcrum (Fig. 6.2).

Fig. 6.2: Principle of leverage.

© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japan

Even if the speed is constant, the direction of movement changes in a circularmotion.

In this case, a force is acting to the direction to the center of the arc.

The

force is called the centripetal force F in Eq. 6.1 (Fig. 6.3 (a)).

Fig.6.3: Circular motion and Centrifugation.
F＝m v2／r ＝m r ω2

(6.1)

In equation (6.1), m is the mass [kg], v is the speed [m s－1], r is the radius [m], andω is the angular velocity [rad s－1].
In a centrifuge, a centrifugal force is caused by the rotational movement.

The

centrifuge is applied to separation of solutes in the physiological liquid, to separation ofblood cells and plasma in the blood (Fig. 6.3 (b)), and to separation of cells suspended
in the medium.

By interposing a non-aqueous solution of the appropriate density, theerythrocytes can be distinguished in accordance with the density of the containing
polymer.
The deformability of red blood cells of the high density is low [26].
cell deforms from biconcave to ellipsoid in a shear flow field.

The red blood

The deformation can bequantitatively described by the deformation ratio y (Eq. 6.2).

© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japany＝(a－b)／(a＋b)(6.2)

In Eq. 6.2, a is the length of the major axis of the ellipsoid, b is the length of theminor axis of the ellipsoid (Fig. 6.4).

The value of y is zero at a sphere (a = b), andapproaches to unity as the deformation of the ellipsoid advances (a >> b).

The

deformation ratio of the red blood cell in the shear field can be measured in the counterrotating rheoscope (see 4.2.4) (Fig. 6.5).

Fig. 6.4: Deformation ratio.

Fig. 6.5: Erythrocyte deformation in shear field.

© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, JapanIf the maximum deformation ratio y0 and the characteristic stress τ0 are introduced
for the evaluation index of deformability, the deformation properties of the ellipsoid of
red blood cells in shear flow can be approximated as shown in Fig. 6.6 (see Eq. 6.4).

Fig. 6.6: Deformability of erythrocyte.

𝑦 = 𝑦0 �1 − 𝑒

−

𝜏
𝜏0

�

(6.3)

In Eq. 6.3, τ is the stress, and τ0 is the stress when y reaches to the 63% of y0.

A red

blood cell of high density has the small y0 (maximum deformation ratio) and the big τ0(characteristic stress).

The red blood cell of high density cannot deform greatly.

red blood cell of high density is not sensitive to the stress.

The

The deformability of red

blood cells may decrease during the circulation after the production at the bone marrow.
The centrifuge can be applied to realize the excessive gravity environment.

When

the muscle cells are cultured in excessive gravity, some of the cells are destroyed, arethinned out, and are subsequently promoted to form myotubes (Fig. 6.7) [40].

© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japan

Fig. 6.7: Myoblasts differentiate to myotubes.

6.1.2 Description of movement
The movement of an object in the three-dimensional space is expressed bytranslational and rotational movements.
the degree of freedom.

The number of independent variables is called

In the three-dimensional space, both three directions oftranslational movement (x, y, z) and three rotational motions around the three axes (x, y,z) are permissible, which makes six degrees of freedom (Fig. 6.8).

© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japan

Fig. 6.8: Degree of freedom (Translation, Rotation).

In the movement in the x-y plane, the degrees of freedom is equal to three: twotranslational movements (x, y), and one rotational movement around the z-axis. Whenthe movement of the object is constrained, the degree of freedom is reduced.

The

degree of freedom on the movement in a system, which consists of multiple objects, isrepresented by a set of the degree of the movement of each object.
Because the human joint has complicated movement, a special method is required todescribe the movement.

In the standing position of the human, three planes aredefined: the frontal plane (vertical plane between the front and rear), the sagittal plane(vertical plane between the left and the right), and the horizontal plane.

Severalmovements are defined around the trunk: flexion-extension, internal-external rotation,
adduction-abduction, and varus-valgus (Fig. 6.9).

Since the articular surface is not a

simple spherical surface, the center of rotation according to each curvature of thesurface is not fixed to one point [3].

© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japan

Fig. 6.9: Motion direction.

The standards of the position and direction are required to represent the movementin space. Coordinates are used as the reference for the position. For the movement ofa human body, the special definition is necessary for the coordinates.
In the durability test of the ceramic hip joint, the slightly shifted center of rotation
causes the partial contact of the friction surfaces, which leads to brittle fracture.

Onthe other hand, when it is implanted in a living body, the joint actuates along the friction
surface.

Adjustment of the center of rotation is important in the test in vitro (see3.1.2): a fixing method with “exploring the operating position in the movement”.
When the restoring force acts against the displacement, the repetitive movementback to the original position makes vibration.

The natural frequency of a vibrating

system depends on the mass and the restoring force.
large mass, and high with large restoring force.

The natural frequency is low with

The change of the natural frequency ofthe object can be applied to detect the adsorbed material on the object [41].

© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japan

6.2 Lubrication and wear
6.2.1 Machine elements and systems
The machine consists of multiple elements.

Between the elements, the force istransferred through fixation or relative motion.

The elements are classified into

several groups: the fastening elements, the transmission elements, the liquid transfer
elements, the sealing elements, the guiding elements, the motion conversion elements,
and the dumping elements.
Screws, bolts, nuts, washers, rivets, keys, and pins are included in the fastening
elements.

The transmission elements include gears, chains, belts, and pulleys.

liquid transfer elements include pipe fittings and valves.
included in the sealing elements.

The

Seals and О rings are

The guiding elements include shafts and bearings.

The motion conversion elements include cams, link mechanisms, and power springs.
The dumping elements include springs and dampers.
A cam is a disk-shaped element attached to a rotating shaft.

By variations of “thedistance between the rotation axis and the curved surface” in accordance with therotation angle, the cam converts the rotary motion to the linear reciprocating motion.
In the piston-bellows type of artificial heart, the amplitude of the reciprocating
motion of the piston is changed by the eccentric distance of the cylindrical cam (Fig.
6.10).

While the reciprocating distance of the piston remains constant, the waveformof the piston motion can be changed: the shape of the cam, or the removal of the part of
the reciprocating motion of the piston (Fig. 6.11).

© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japan

Fig. 6.10: Cylindrical cam.

Fig. 6.11: Motion of piston.
In accordance with the reciprocating motion of the piston, ventricular volume
changes periodically. The pulsatile flow waveform can be controlled by the motion of
the piston [42]. Is the pulsatile flow required for blood circulation?

What kind of

waveform of pulsatile flow should the artificial heart supply to the blood circulation?

© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japan

The damper attenuates vibration (Fig. 6.12).

The damping of the fastest approachto saturation without vibration is called the critical damping.

When the tracing of thedisplacement y is represented by the equation (6.4), t0 is called the time constant.

Fig. 6.12: Damping of vibration.

𝑦 = 𝑦0 �1 − 𝑒

−

𝑡
𝑡0

�

(6.4)

In Eq. 6.4, y0 is the saturation value of the displacement, t is the time, and e is thebase of natural logarithms (e ≒ 2.72). The time constant t0 corresponds to the time,
when y reaches to 63% of the saturation value (see Eq. (6.3)).
A machine can be considered as a system: the machine consists of several elements,
the machine has relationships between the elements, and the machine realizes operation
as a whole.

In order to achieve the desired movement, the motion status should be

detected and the following movement should be adjusted.

The adjustment is called© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japan

control.

In the control, the subject is considered as a system.

The interaction

between the elements, the target value, and the situation of the operation are treated withsignal flows (Fig. 6.13).

Fig. 6.13: Control.

Cells assemble to make a tissue.
assemble to make system.

Tissues assemble to make an organ.

Systems assemble to make an individual.

OrgansThe larger levelis considered as a system, which consists of elements of the smaller level.

Themaintenance of homeostasis with adaptation to the surrounding environment can beconsidered as a control in the system.
For example, a cell can be considered as a system.
the cytoplasm, and the cell membrane.
are transferred.

The cell consists of the nucleus,

Between the elements, signals and substances

The cell has several behaviors: deformation, migration, proliferation,differentiation, absorption, and excretion.

The cell responds to the stimulations:deformation and movement of the surrounding tissue, the flow of the surrounding fluid,
contact with the surrounding cells and tissues, exchange of substances and signals withthe surrounding environment, and change of the internal state (Fig. 6.14).

© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japan

Fig. 6.14: Biological cell.

The individual death should be considered at the system level (Fig. 6.15).

The

death is conventionally determined with the cardiac arrest, the respiratory arrest, and thebrain function arrest.

Fig. 6.15: System in organ.

© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japan

The “cardiac arrest” cannot be determined on the beating of the heart alone.

The

cardiac arrest should mean the functional arrest of "blood circulatory system".

Abeating heart alone is not enough to maintain the life.
maintained, when a blood vessel is blocked.

The blood circulation cannot be

When the bleeding cannot be controlled,

it cannot be maintained, either. You can understand that the artificial heart alone
cannot make the permanent life.
The respiratory arrest and the brain function arrest are similar to the cardiac arrest.
The individual death should be judged on the system level rather than the organ level:
the respiratory system and the nervous system, rather than the lung and the brain.

6.2.2 Coefficient of friction
The relative movement between the objects, while keep contact each other, isclassified into two groups: the slip (including the rotation), and the rolling.

During therelative motion between the objects, the force acting in the tangential direction at theinterface is called the frictional force.

Between the objects, the stationary and therelative movements depend on the frictional force acting at the contact surface.

Theskin grabs the object by the frictional force.
The supporting force, which acts perpendicular to the contacting surface, is called
the normal force.

The ratio μ between the frictional force Fs and the normal force Fn,

which is calculated by Eq. 6.5, is called the coefficient of friction.
μ＝Fs／Fn(6.5)

The friction is called the static friction, when the object is stationary.

The friction,on the other hand, is called the kinetic friction during the motion.

When theinclination angleθof the slope increases, the object starts sliding slopes at the value θ0.
The θ is called friction angle (Fig. 6.16).

The tan θ0＝μ0 is called the maximum

coefficient of static friction.

© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japan

Fig. 6.16: Friction angle.
(M g sinθ0)/( M g cosθ0)＝tanθ0＝μ0(6.6)

In equation (6.6), M is the mass of the object, g is the gravitational acceleration.

6.2.3 Contact
On the solid surface, there are large and small irregularities (see 7.1.3 surface
roughness).

The area, where the solids contact directly each other, is called the "realcontact area".

The real contact area is very small compared with the "apparent

contact area", even when the apparent flat planes contact each other (Fig. 6.17).

© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japan

Fig. 6.17: True area of contact.

In the contact surface, solids exerts a force each other.

When a force operates froma solid pushing against the other solid, the resisting force operates as the reaction of theother solid. When the object is sustained in the gravitational field, the normal force
works to balance with the gravitational force (see 6.2.2).
Consider that the real contact area increases in proportion to the normal force by the
deformation of the solid surface.

If the friction force is proportional to the real contact

area, the friction force is proportional to the normal force.

In this case, coefficient offriction is constant, which is independent of the normal force (see equation (6.5)).
The interface state depends on the surroundings.

The state of the solid surface inthe gas depends on the atmosphere. The gas molecules are adsorbed on the surface.
Molecules, such as nitrogen or oxygen, are adsorbed on the surface in the air.
water molecules are adsorbed on the surface in the humid air.

The

The oxide film isformed on the solid metal surface by oxidation in the air (Fig. 6.18).

© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japan

Fig. 6.18: Surface of solid.

In the vacuum, on the other hand, the adsorbed gas molecules are eliminated.
When the metallic solids are pressed together in a vacuum, no adsorbing layer of gas
molecules interposed between the solids.
directly.

The metallic solids contact each other

When the real contact between the metal solids realized over the entire

surface, the real contact area is equal to the apparent contact area.
The friction generates the high pressure and the heat at the local area on theinterface.

The high pressure and heat destroy the adsorbed layer of gas molecules andthe metal oxide layer at the interface.
between the solids of metal.

The destruction causes the direct contact

The pair of the same metals may cause fusing at theinterface between the solids. The pair of the different metals is used to reduce thefrictional force.
In the roller pump (see 7.1.1), the friction between the tube and the roller isinevitable (Fig. 6.19).

The higher pressure for the blocking of the tube by the roller

may cause abrasion of the tube.

© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japan

Fig. 6.19: Roller pump.

In the screw pump, the fluid flows through the gap between the externalscrew-thread (rotor) and the internal screw-thread (stator) (Fig 6.20).

The frictional

force varies with the degree of the contact between the two screw-threads, which affects
the wear and the durability.

The incomplete contact causes the reverse flow.

Fig. 6.20: Screw pump.

6.2.4 Surface tension and hydrophilic property
In vivo, the membrane plays an important role as the partition for the core operationof life phenomena.

Therefore, it is necessary to focus on the nature of the interface.

Since the biological body consists of the aqueous electrolyte solution, the affinity withthe water is an index of the surface property.

© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japan

The droplet of the liquid tends to become a sphere of the minimum surface area by
the intermolecular force.
surface tension.

The tension to minimize the surface area is called the

The unit is N m－1.

The surface tension of the water in the air is

72.75 N m－1 at 293 K.
On a solid surface in the air, a water droplet tends to become a sphere by the surface
tension.

When the affinity between the water and the solid surface is high, on the other

hand, the water droplet is attracted to the solid surface to be deformed into a flat shape.
The angle between the water-air interface and the water-solid interface is called contact
angle θ (Fig. 6.21).

The surface of the contact angle smaller than π/2 is calledhydrophilic surface.

The surface of the contact angle larger than π/2 is calledhydrophobic surface.

Fig. 6.21: Contact angle: θ.
On the water-repellent surface like polytetrafluoroethylene, the contact angle isclose to π.

Walls and membranes in the living body, on the other hand, are hydrophilic.

The most of the surfaces, which contacts with the physiological liquid (for example,
artificial walls of blood vessels), are designed to be hydrophilic.
The hydrophilic surface is noted on the cell adhesion, the platelet adsorption, andthe thrombus formation.

The most of cells proliferate adhering to the extracellular© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japanmatrix.

Thus, cells are cultured on the hydrophilic surfaces.

The blood forms clots by the platelet aggregation and by the coagulation proteins(Fig. 6.22).

The clot formation is controlled by collaboration of subsystems: theplatelet aggregation, the coagulation protein, and the fibrinolysis (Fig. 6.23) [24].
The clot is formed to stop bleeding, and is dissolved to recover smooth surface.

Fig. 6.22: Clot.

Fig.6.23: Clotting system.

© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japan

The Clot plays an important role for closing the cleft of the blood vessel wall
(hemostasis).

The clot, on the other hand, prevents the flow, when it is formed insideof the vessel lumen (thrombus).

Although the operation is necessary to connect theartificial blood vessel to the biological vessel, it makes trouble by clogging the artificial
blood vessel lumen.

Three-dimensional structure of biological macromolecules on the

blood vessel wall might control the biological reactions.
A tube, which is inserted into a blood vessel, is called a catheter.
the catheter is treated to prevent the clot formation.

The surface of

One of the methods to prevent clot

formation is the coating of heparin, which is a biopolymer of anticoagulation.
The thrombus formation is a problem in the artificial organs, which contact with theblood.

When the blood circulates in the mock circuit (Fig. 6.24), the clot is formed in

the flow path (Fig. 6.25).

The formation of clot can be controlled by the flow (Fig.

6.26).
The clot tends to be formed at the bearing of the centrifugal type of artificial heart(Fig. 6.27).

The amount of clot can be formed in the stagnation area of the back sideof the impeller (Fig. 6.28). The thrombus formation is controlled by the shear rate
adjacent to the bearing of the impeller (Fig. 6.29).

Fig. 6.24: Blood circulation circuit.

© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japan

Fig. 6.25: Clot formation in tube (low flow).

Fig. 6.26: Clot formation in tube (high flow).

Fig. 6.27: Clot at bearing.

© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japan

Fig. 6.28: Clot behind impeller.

Fig. 6.29: Modified pivot of Artificial Heart.

The coating is applied on the synthetic material surface to prolong the clotting time
[43].

When the blood is sheared between concave-convex cones coated withsegmented polyurethane, clotting time is prolonged (Fig. 6.30) (see 4.2.4 and 4.2.6).
The clots are formed in the same amount of volume as that of non-coating.

The clotformation is prevented in the pulsatile flow, in which the period of low shear rate of <100 s－1 is shorter than 2 s.

© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japan

Fig. 6.30: Concave and convex cones.

6.2.5 Wear
The weight loss at the material surface by friction is called wear. The powder
exfoliated from the base material at the wear is called the abrasion powder.
The volume of wear per unit slide face load and per unit slide distance is called thecomparative abrasion quantity.

Unit of the relative abrasion is m3 N－1 m－1.

degree of wear is compared by the relative abrasion.
are not always reduced by the abrasion.
materials decreases in several cases.

The

The volumes of both base objects

The volume of only one side of the baseThe volume of the base material increases, when

the abrasion powders adhere to the base material.
Wear mechanisms are classified into two groups: the scraping of the surface, and the"adhesion-break".

The former is called abrasive wear, and the latter is called

adhesive wear (Fig. 6.31).

© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japan

Fig. 6.31: Wear.

The friction concentrated in real contact surface raises pressure and temperature in
the local area (see 6.2.3).

The rise causes peeling off the adsorbed molecules and theoxide film, and the base metal surface is exposed.
the virgin surface.

The chemical reaction may occur at

The atmosphere (gas molecules and temperature) around thefriction surface affects the reaction.
At the initial friction, wear occurs due to the unstable contact (initial wear).

At

the newly formed surfaces, wear occurs due to the high reactivity (virgin surface wear).
Wear decreases at the stable friction surface (mild wear) (Fig. 6.32 (a)).

© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japan

Fig. 6.32: Transition of wear.

The unstable friction surface due to fatigue modification of the material surface
causes vibration, which may be accompanied with the increase of wear (severe wear).
The friction between the wear particle and the base material cause the chipping and theadhesion.

The fresh surface during the wear may sometimes cause adhesion between

the base materials (seizure) (Fig. 6.32 (b)).

The wear phenomenon includes theunstable factors.
To study on the mechanism of the wear, not only the wear volume, but also theappearance of the friction surface and the wear debris should be observed.
Components of the counter base material may adhere to the friction surface.

The weardebris may consist of fusion of components of the both base materials in some cases.
Both the friction and the wear are important problems on the implanted joint
prosthesis (Fig. 6.33).

The combination of polyethylene and stainless steel-titanium

alloy is applied for the material of the sliding part.
of the joint.

The friction affects the movement

The wear powders may cause inflammation.

Ceramic is also used forthe prosthesis.

© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japan

Fig. 6.33: Joint prosthesis.

Long-term fixation between the artificial joint and the bone is also a challenging
problem.

Against the foreign materials, the living tissue has several reactions:disassembly, isolation, and exclusion. These reactions make a gap between the living
bone and the prosthetic stem (Fig. 6.23), which causes loosening.
A cardiac valve prosthesis is used to replace a heart valve.

At one of theprostheses, the two valve-leaflets incline at opening to pass blood in one direction.
The wear at the bearing between the leaflet and the valve seat makes malfunction of thevalve (Fig. 6.34).

© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japan

Fig. 6.34: Cardiac valve prosthesis.

The heart continues operation as a pulsatile blood pump.
operation in synchronization with the pulsation of the heart.

Valve continues

One beat per second

corresponds to the beat of 3 × 107 beats (60 seconds × 60 minutes × 24 hours × 365days) in one year.
The wear at the strut or at the leaflets causes malfunction of the valve has beenreported in a few cases [44].

Several metallic materials, such as nickel-chromium

alloy and stainless steel, have also been used for the struts.

The metallic material

surface becomes unstable due to corrosion in the saline solution.

The blood contains

ions, and is corrosive environments.
The effect of wear debris from the valve prosthesis on the living body is notreported.

This is because the volume of the wear of the prosthetic valve is small

compared with that of an artificial joint.

6.2.6 Lubrication
In the relative motion between the objects, the aims of lubrication are as follows:
reduction of frictional force, stability of friction, and reduction of the wear.
Lubrication achieves the smooth movement at the joint.
The lubrication at the contact between solids is called “solid lubrication”.

Thelubrication sandwiching the fluid between solids is called “fluid lubrication”.

Thelubrication of mixture of two types of lubrication (solid lubrication and fluid
lubrication) is called “mixed lubrication”.

Substance, which is interposed at friction

surface for lubrication, is called “lubricant”.
When fluid is sandwiched between solids, the flow resistance of the fluid governs
the friction.

The biological joint wrapped in the joint capsule includes the synovial

fluid between the articular surfaces.

When the local contact between solids occurs

through the thinned synovial fluid film at lubrication surface, the frictional force© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japanbecomes larger than the flow resistance of the fluid.

The lubrication state is called“boundary lubrication”.
In the human joint, the direct contact between the articular surface is prevented byseveral effects: drawing of the fluid by the relative movement between articular surfaces(wedge-film lubrication), the occurrence of pressure in the fluid by approach between
the articular surfaces (squeeze film lubrication), and the elastic deformation of thearticular surface (Fig. 6.35).

Therefore, the frictional resistance in the human joints ismuch smaller than that between the solids.

The fluid lubrication accompanied with theelastic deformation of the surface is called “elastohydrodynamic lubrication”.

Fig. 6.35: Articular surface lubrication.

The contact between solids must be avoided to keep the fluid lubrication.

The

thickness of the fluid film must be kept greater than the roughness of the surface.

Thesupply of the fluid to the friction surface must be larger than the discharge of the fluid
from the friction surface.

The lower viscosity coefficient of the fluid reduces the flow

resistance of the fluid itself.

The discharge of the fluid lubricant from the friction

surface, on the other hand, decreases at the higher viscosity coefficient of the fluid.
The friction at the bearing of the rotary type of the artificial heart generates heat,© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japanwhich denatures the blood and causes thrombus formation (Fig. 6.19 (d)).

In somerotary pumps, the shaft keeps floating from the bearing by the magnetic effect.

Questions

Q 6.1: Calculate the force applied on the hip joint in standing position by one leg at thefollowing case: the gravitational force by body weight is 600 N, the deviation is 1 cm
between the line of action of the gravity and the supporting force, the deviation is 2 cm
between the line of action of the supporting force and the muscle force, and the positionof the supporting force is between the gravitational force and the muscle force (seeFigure 6.1).

Q 6.2: Give examples of levers to increase the force, and to reduce the force.

Q 6.3: Calculate the centrifugal force acting at the distance of 30 cm apart from the axis
with rotational speed of 1000 revolutions per minute. How many times higher is itcompared with the gravitational acceleration?

Q 6.4: List the type of the pressure generated in the fluid lubrication. Discuss on the
lubrication in the human joints.

Q 6.5: List problems related to lubrication in the artificial heart.

© Shigehiro Hashimoto 2013, Published by Corona Publishing Co., Ltd. Tokyo, Japan