Background
     I study bio-adhesives using a broad approach, analyzing their chemical composition, their mechanics, and how these factors affect the whole organism and its survival.  Bio-adhesives are especially interesting, because of their diverse functions and the mechanical demands placed on them.  Animals in the wave-swept intertidal must have powerful adhesives or they will be swept off the rocks.  Many bugs, tree frogs and geckoes use adhesives to walk on vertical surfaces, giving them access to a habitat that other animals cannot use.  Echinoderms and molluscs use adhesives to grip the substrate during locomotion.  In addition, adhesives are often used in feeding:  many filter feeding and deposit feeding animals, such as worms, echinoderms and cnidarians, must control the adhesiveness of their tentacles to trap particles.  Octopus and squid use adhesion to prey on large, active animals.
     For animals such as barnacles and mussels, it is sufficient to form a permanent, solid bond to the substratum.  Most other animals, however, need to maintain mobility as well as the capacity to form strong bonds.  Such temporary attachments are ubiquitous, but relatively poorly understood.  The requirements are impressive.  They must be strong enough to bear the weight of the animal, the brunt of a crashing wave, or to restrain struggling prey.  They must be able to release on demand and reform new attachments.  Finally, they can't use too much energetically expensive material.
     This research may have practical consequences.  Marine bioadhesives form strong bonds in wet, dirty environments.  In contrast, most artificial adhesives require extensive cleaning and drying of the surfaces before application.  Adhesives that work in wet environments would be useful in dental and surgical applications, and other applications where drying the surfaces is not practical.
 

Overview of current research
     We are currently investigating the nature of the adhesive secretions of a variety of molluscs.  These are gels:  they consist of a dilute, tangled network of polymers.  These adhesives are neither truly solid, like a cement, nor fluid, like the water under a sucker.  For lack of a better term, they are often referred to as mucus.  The structure of these gels varies widely, however, and the catch-all term "mucus" is probably inappropriate.  The function of these gels varies widely as well.  Some gels are outstanding lubricants.  Some are excellent adhesives.  Intriguingly, it appears that many molluscs can convert a slippery gel into a powerful adhesive.  Thus, we are comparing the structure of different adhesive gels with the goal of determining the functional significance of structural features.

     Our most exciting finding is that there are specific proteins that appear to convert gels into glues.  We have studied the limpet Lottia limatula from the rocky intertidal of California, the marsh periwinkle Littorina irrorata from the salt marshes of Florida, the garden snail Helix aspersa, and the terrestrial slug Arion subfuscus from the forests of central New York.  In all four species, the primary difference between adhesive mucus and non-adhesive mucus is the presence of one or two specific proteins.  We refer to these as "glue proteins".  These proteins may make up roughly half of the organic material in the gel.  This change in composition is associated with an increase in the adhesive tenacity of several orders of magnitude.  Finally, we have isolated the glue proteins and shown that they cause marked stiffening of gels.  They appear to be non-specific cross-linkers.
 

Summary of past research
     Most of my early research focused on suction.  Suction adhesives create a reduced pressure in water to pull themselves against a surface.  What makes them interesting is that they can actually reduce the pressure in water below zero atmospheres.  As long as there are no air bubbles present, the cohesive strength of water resists the tension.  The sucker literally pulls on the bonds between water molecules.  In theory, this can continue until the pressure falls several thousand atmospheres below zero (though no sucker could possibly pull this hard!).  In practice, there are always imperfectly wetted cracks on one of the adhering surfaces.  When the pressure is low enough, it will pull trapped air bubbles out of these cracks.  Once free, the air bubbles expand rapidly and disrupt the adhesion.  This event is called cavitation.  An understanding of the ability of water to sustain negative pressures led to a number of findings.

 The following is a summary of my findings:

     1) Previous researchers had assumed that negative absolute pressures (below 0 atm) were impossible underwater.  I demonstrated that this was not true;  octopus and squid suckers can generate negative pressures.  The lowest pressure that I recorded was -8 atm (-800 kPa) under a squid sucker.  Thus, cephalopods are capable of more powerful attachment than had been surmised.  Furthermore, I empirically determined the range of pressures that water can sustain under different conditions.  I then demonstrated that cavitation limits cephalopod suckers to the predicted range of pressures.

     2) In deeper water, the higher external pressure makes cavitation unlikely, therefore greater pressure differentials are possible.  There is variation among cephalopods in the ability to take advantage of this phenomenon.  Squid and cuttlefish suckers are considerably stronger than octopod suckers, and thus more able to take advantage of the effect of depth.  This is due to a fundamental difference in sucker design characteristic of these groups.  Squid and cuttlefish suckers are based on a plunger-like design, which works well for high strength and rapid attachment, but doesn't have the dexterity of an octopus sucker.

     3) Because negative absolute pressures are possible, the use of suction adhesion may be more widespread than was currently believed.  I demonstrated that limpets also use suction adhesion and can produce negative pressures.  Many researchers had dismissed suction as a mechanism because of the great strength of limpet adhesion.

     4) Although limpets use suction, they also use a gel-based adhesive.  Limpets use suction at high tide, then switch to gluing when the tide goes out.  This corresponds to their activity pattern;  when they are active, they use suction, and when they are inactive, they glue themselves down.  This pattern of alternating between attachment mechanisms has not been previously considered, and may be common in animals other than limpets.

     5) My work on negative pressures led me to address the question of sap ascent in trees.  My experiments and a review of the physics literature suggested that one would not predict the extreme negative pressures reported in the xylem elements of trees.  This has been relevant to controversy surrounding the magnitude of the tensions (negative pressures) in the xylem elements.  My view is that either the negative pressures in the xylem are typically overestimated, or plants have a novel mechanism for avoiding bubble formation.
 
 
 

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