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1

-    Ways    of    sticking    to    surfaces:-    

-    Mechanical
-    Suction
-    Viscosity
-    Capillarity
-    Friction
 

2

What    is    adhesion?

“The    action    or    process    of    adhering    to    a    surface    or    object”

3

Why is adhesion more common in the marine environment? 

Unlike on land, marine animals can be sessile because food is brought to them, waste is taken away and gametes dispersed. 

This saves energy, but their attachment must be strong as development causes mortality.  

 

4

Synthetic adhesives 

  • sticking underwater
  • does not require clean surfaces 

5

Mechanical    interlocking

Mechanical interlocking - lock and key type adhesion to the surface (although    electrostatic interactions are    also    required…).
•    At    the    micro-scale,    most    materials    are    rough.
•    The    glue    must    be    able    to    spread on    the    surface.
•    The    glue    must    be    able    to    enter    the    roughness    features.
•    The    glue    must    then    harden.
•    All    of    these    processes    present    challenges    underwater,    as    we    will    see!!

6

Mechanisms    of    adhesion:    
2.    Suction    &    Stefan    adhesion

Neither of these is really    ‘adhesion’    mechanisms per se,    in that there are no attractive forces. Rather, they are physical processes that interfere with removal.

Suction


•    Requires an elastic cup which is initially contracted and then expanded to form a    zone of reduced pressure.
•    Not attractive.
•    Theoretical maximum resistance of    1    atm.
•    Very weak in    ‘shear’. Easy to slide off.

Stefan adhesion

  • Resistance to viscous flow 
  • No zone of reduced pressure 
  • Not attraction 
  • Attachment pad - viscosity of medium and speed the pad is being moved


 

7

Capillary tubing

  • Depends on the surface energy    (determined by the chemistry)    of the surface and the surface energy    (called surface tension…)    of the liquid.
  • If the tube has more    ‘energy’    (is more reactive)    than the liquid,    the liquid will    ‘prefer’    to stick to the glass, rather than to itself,    so it is drawn up the tube. Glass has a    lot of free energy compared to water.
  • Can also draw surfaces together,    but liquid must have lower energy than the surface for this to work.
  • But like suction, capillary is very weak in shear. 

8

Friction

Octopus (among other things) - manage to attach very strongly using,    ostensibly,    a    suction mechanism. - stronger adhesion than 1 atm

  • This    supernatural    feat,    and    many    others,    can    be    ascribed    to    a    frictional    contribution    to    adhesion.
  • In  order    to    break    a    suction    bond,    the    seal    must    be    compromised    – usually    by    pulling    inwards.
  • If    this    can    be    prevented,    the    suction    force    is    theoretically    limitless.
  • Slip    of    the    perimeter    seal    is    governed    by    friction….
     

9

Many different unrelated organisms have developed similar ways in overcoming adhesion problems - give the example of tree frogs.

  • Tree    frogs    secrete    a    liquid    adhesive    (mucous)    from    their    soft    toe-pads,    producing    a    capillary    effect
  •  Capillary adhesion    is    weak    in    the    ‘shear’ direction,    however,    so    frogs    must    also    use    boundary-layer    friction    between     their    cuticle    and    the    surface    to    prevent    slip.
  • Toe pads subdivided into little grippers 

10

Many different unrelated organisms have developed similar ways in overcoming adhesion problems - give the example of bush cricket.

The    tarsi    of    the    cricket    Tettigonia viridissima also    have    smooth    flexible    pads    that    are    used    for    capillary    attachment.
A    liquid    (cuticular oil)    is    also    secreted    from    between    the    lamellae    of    these    pads,    mediating    adhesion    in    a    similar    way    to    tree-frogs.
Microscopic    droplets    of    water in    the    cuticular oil    form    an    emulsion that    stiffens    under    strain.    The    more    water,    the    more    resistance    to    shear.
This    ‘non-Newtonian    fluid’    hardens    ONLY    when    a    force    is    applied!
J.    R.    Soc.    Interface.,    2006;    3:689-697

11

Challenges od adhesion mechanisms for marine organisms 

  • Surface energy 
  • Surface hydration 
  • Surface contamination 
  • Surface roughness
     

12

Describe surface energy 

•    Surface energy is a    measure of the    ‘reactivity’    of a    surface;    how capable it is of forming new bonds with materials it comes into contact with.
•    Hydrophilic - high energy - love water - water spreads out across the surface.

  • Hydrophobic - low energy - hate water - water forms droplets 
  • Superhydrophobic - specific surface texture means water barely even comes in contact with it. 


 

13

Challenges - surface energy 

Glue gets outcompeted by water for contact with the substrate

what is true in air does not translate underwater….

• A hydrophilic surface, which would be optimal for adhesion in air, can be very difficult to contact underwater due to strongly bound water – a hydration layer.

• Ironically, therefore, organisms may be expected to adhere better to low-energy surfaces underwater better than they can to high-energy surfaces.

• Some organisms settle with sticking to hydrophobic surfaces that they can contact easily. But some either remove water from the interface, or incorporate it into their glue, so that they can stick strongly to hydrophilic surfaces. •

14

Challenges:    2.    Surface    hydration

Removing    water    from    the    interface:

  • •    Mussels    and    barnacle    larvae    (cyprids)    both    use    lipids    to    remove    water    from    surfaces    which,    presumably,    enables    them    to    attach    more    easily    to    immersed    hydrophobic    surfaces.
    Incorporating    water    into    the    adhesive:
    •    Water    can    be    incorporated    into    adhesives    in    a    process    known    as    ‘complex    coacervation’,  mussels do this
     
  • Recognise that a high energy surface is the best to stick to, so remove the water 

15

Challenges:    2.    Surface    hydration

16

Challenges:    3.    Surface    contamination

Research does not have a good handle on how organisms deal with surface contamination. 

•    Just    as    was    the    case    for    dealing    with    water,    biofilm,    for    example,    can    be    either    incorporated    into    the    glue,    or    removed    from    the    surface.

•    It    seems    that    the    tubeworm    Hydroides may    do    the    former,    while    mussels    appear    to    brush    away    biofilm    with    their    foot    before    attaching.

•    Surface    contamination    also,    inevitably,    affects    the    surface    energy    and    the    wetting    properties    of    the    surface.

 

17

how does surface Surface    roughness affect adhesion

•    Generally    speaking,    rough    surfaces    have    stronger    adhesion    (when    mechanical    interlocking    is    used).

however,    whether    the    glue    penetrates    the    texture    depends    on    whether    the    surface    is    in    the    Cassie    or    Wenzel    state…

which,    in    turn,    depends    on    the    surface    energy    of    the    material.

tend to find the cassie state on superhydrophobic surfaces that has been roughened in a certain way. 
•    If    the    flat    material    has    a    contact angle when its smooth of  more  >90o,    then it may    enter    the    Cassie    state    when    rough it becomes more hydrophilic. 

    If    the    flat    material    has    a    contact angle of  less than 90o,    when it becomes rough it becomes more hydrophilic, wayter runs through nooks and cranny and spreads out. 


 

18

What are the different states a water droplet can be in? 

Water droplets on rugged hydrophobic surfaces typically exhibit one of the following two states:

(i) the Wenzel state in which water droplets are in full contact with the rugged surface (referred to as the wetted contact) or

(ii) the Cassie state in which water droplets are in contact with peaks of the rugged surface as well as the “air pockets” trapped between surface grooves (the composite contact)

19

Challenge 4 : surface roughness

•    Remember,    all    of    this    changes    underwater….
•    A    surface    in    the    Cassie    state    may    retain    trapped    air    when    placed    under    water    which    ‘may’    (the    jury    is    out    on    this...)    help    prevent    adhesion.
•    If    the    air    is    not    trapped,    however,    even    a    textured    hydrophobic    surface    will    enter    the    Wenzel    state    when    placed    underwater.
•    This    will    allow    entry    of    the    adhesive    into    the    texture    and    promote    strong    adhesion.
 

20

Challenges 5 reversibly 

- describe tube feet 

  • Not    many    adhesive    mechanisms    are    truly    reversible.
  • One    that    certainly    is    not,    but    looks    like    it    is,    is    the    tubefoot of    the    seastar….
  • Involves    a    3-gland    system.
    • Sfp1, (starfish foot protien 1)    a    large    protein    of    3853    aa,    is    the    second    most    abundant    constituent    of    the    adhesive.
    • Sfp1    is    translated    from    a    single    mRNA    and    then    cleaved    into    four    subunits   before being  linked back together    by    disulphide bridges in a different combination - mature adhesive protein - not sure why it does this.
  • Present in one of the glands and secreted and mixed with the contents of another one of these glands and produces a very strong teo component permandent adhesive bond. 
  • releases an enzyme from a third gald, digests the adhesive at the interface - brief permanent adhesion mechanims 

21

Suction remora

The remora’s ‘sucker’ is a modified dorsal fin.

The fin is flattened into a pad and surrounded by a thick lip of connective tissue that creates the suction seal.

The lip encloses rows of plate-like structures called lamellae, from which rows of tooth-like spinules emerge for mechanical grip.

22

Suction Cephalopods 

In the cephalopods, use of a liquid glue is an ancestral trait. More advanced cephalopods rely more on suction.

4 genera (Nautilus sp., Sepia sp., Euprymna sp. and Idiosepius sp.) use liquid adhesives.

One genus, Euprymna, also uses a deadhesive – i.e. produces one material to attach and another to detach!

23

Limpets

• Modern-day limpets also rely on suction for adhesion, using it for attachment during locomotion in addition to capillary/Stefan adhesion.

When exposed by the tide, however, limpets secrete a true adhesive hydrogel, containing:

o <97% water and several polar proteins,

o A 140 kDa glycoprotein complex for adhesion,

o Gives a tenacity of <500 kPa.

(all that's known about marine gastropods)

24

snake and slug 

• Similarly, the terrestrial slug Arion subfuscus produces a defensive secretion that is sticky and tough, despite being more than 95% water.

 In tensile tests, the glue sustains an average peak stress of 101 kPa (it is strong!), and fails at an average strain of 9.5 (and stretchy!). How??

25

  • What are some key constituents of slug hydrogels - . If calcium, magnesium and iron are removed the glue experiences a 15-fold decrease in storage modulus.
  • Proteins and carbohydrates have a negative charge but also crosslink positively charged sugars, stored separately in structures.
  • Metals act as counter ions to keep everything electrostatically neutral. When the stuff is secreted it is all rearranges and the metals crosslink the proteins and the carbohydrates but they do not carbohydrates to the protiens - slug glue - double network.
    • A network of protiens crosslinked by metals
    • A network of carbohydrates crosslinked by metals - interspersed by separate. 
  • Carbohydrate - runs through the protein network and is not stretchy - pull-on glue you break carbohydrate bonds but it is held together by strchy protien. 
  • Protein - stretchy held together by metals - on its own  

 

26

Adhesion of mussels: The byssus

• Mussels produce a byssus of adhesive threads

• These threads are formed along the ventral groove of the foot in a process resembling injection moulding.

• There are three major glands: the phenol, collagen and accessory glands. These feed specific amounts of their contents into the ventral groove to form the adhesive plaque, the collagenous (similar to a tendon) thread and the cuticle of the byssus.

• Those proteins forming the adhesive bond of the plaque to the surface are deposited first at the distal tip of the foot, followed by the bulk of the plaque and thread core components.

• Finally, just before the thread disengages from the groove, the structure is coated by a 5-μm-thick cuticle from the accessory gland.

• The byssus has as many as 20 different known protein components

27

Adhesion of mussels: Composition

  • The mussel foot proteins (important proteins for sticking) (Mfp)-2, -3, -4 and -5, originate from the phenol gland and are destined for the plaque.
  • Mfp-1 (Mytilus foot protein) and Mfp-6 have been localized to the accessory gland.
  • Collagen gland proteins, such as the prepolymerized collagens (preCOLs),
    • 3 types with distal (D), proximal (P) and nongradient (NG) distributions (preCOL-D, -P and -NG) are used in the thread core.
  • Although Mfps do exhibit some chemical diversity, most are glycine rich and contain DOPA, and all are moderately to strongly cationic.
    • DOPA is a post-translational modification of tyrosine and by far the most well-studied adhesive molecule in biological systems. DOPA is used as a precusor for melain etc in a human. Mussel take a chemical and uses it for something completely different.

28

Steps of adhesion mussels

J. Exp. Biol., 2017; 220: 517-530 

 

  1. Clean biofilm and mess that may interfere with adhesion 
  2. Foot extended
  3. Cavitation 
  4. pH adjustment - produces an acidic environment - DOPA is pH sensitive 
  5. Redox adjustment - introduces negative charges in the form of electrons into the distal depression 
  6. protein secretion  - once happy with the pH and redox conditions it will start producing protiens 
  7. Coacervation 
  8. Phase Inversion 
  9. Complete assembly 
  10. Solidification 

29

What happens in the two pH conditions whilst a mussel is sticking the collagen thread to the substrate? 

PH4 or lower

pH4 or lower 

  • DOPA is in its reduced form, has OH- groups on it, it's phenolic (has a phenyl ring) 
  • Mussel foot proteins can be up to 30 - 40 % - huge amino acid bias. 
  • OH- groups can interact strongly with mineralised surfaces (such as iron in rocks)
  • Thought that mussels use another amino acid - lycine which is extremely hydrophilic and interspersed with DOPA. The theory is that lycine gets the adhesive protein to the surface, once its at the surface the DOPA molecules adhere. 
  • After this, the mussel lifts its foot the surface and the collagen thread has adhered but not yet cured. 

 

30

What happens in the two pH conditions whilst a mussel is sticking the collagen thread to the substrate? 

ambient seawater

To cure the pH has to return to the ambient pH of seawater (8.2). 

  • at 8.2 DOPA behaves very differently  at the ambient pH 
  • DOPA is no longer in is reduced form, OH- ions pinch the H+  ions of the DOPA leaving two oxygens attached to the phenyl ring, called a dopaquinone. Dopaquinone is very good at sticking together, so links up all the unattached oxygens forming the solid mass that is stuck to the surface.