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What has the lowly lizard that we can learn from? Most of us despise it at home, yet it holds a fascination for us. Hindu almanacs (panchamgam) interpret the sounds the lizard makes and the direction from where it falls (on our body), and predict the (often dire) outcome. We grudgingly marvel at its ability to scurry on vertical walls and across the ceiling.

It is this ability of the lizard and its larger outdoor cousins — the geckos — that researchers have been trying to understand and mimic. Work during the last decade has explained this remarkable ability. It is not because of any suction or temporary vacuum produced by the footpads, as our school books suggest.

The secret lies in the rows of tiny hairs with multiple split ends at the bottom of each toe pad. Drs Kellar Autumn and Robert Full showed in the 8 June 2000 issue of Nature that it is the sticking capacity of each of these split hairlets that does the trick.

Each toe pad has thousands of such hair or fibrils (called setae). Each seta has numerous projections, or tendrils at the end, called spatulac.

Each seta is thus like the stem of a feather duster, a chamaram, or more prosaically — a cobweb duster, fanning out at the end. Each tendril or spatula of the seta is able to grab even the smoothest of surfaces-dry or wet. The collective action of thousands of these can thus produce a force of attachment, per seta, that can lift an ant. Millions of these can lift a small child. And the material they are made of is keratin, the same protein of our hair or the feathers of birds.

If this is the situation with a lizard, imagine the scale involved with a gecko that is a foot or bigger in size, with each toe the size of a human finger. No wonder Shivaji was able to climb the Raigarh fort using a gecko (udumbu in Tamil), tied to a rope.

What is the nature of the force that allows each spatula to hold on to a surface? Surprisingly, it is a remarkably weak one, referred to as van der Waals force.

This is a non-specific (anything to anything) attachment that is thousand times weaker than a chemical bond. It is a two-faced one, operating as attractive at distances as short as a fourth or fifth of a nanometre. But get closer than that, attraction gives way into repulsion.

It is the van der Waals force that makes even the inert atoms of the noble gas helium come together to form liquid helium.

The energy of attraction is so feeble that one has to cool the gas way down, almost 300 degrees below room temperature to make the liquid.

In addition, given the nanometre sizes involved, capillary action also contributes to the sticking.

This is the force that makes a liquid climb up the tube, when a hollow tube with millimetre hole size is dipped in the liquid. Widen the hole, the liquid climbs less.

The force is too feeble for one spatula of the seta. But when you have hundreds acting together, the strength shoots up.

If the strength is so high, how does the lizard move? Just bend the spatulae a bit and the stickiness weakens.

It is this sequence of straight–bent, straight–bent that lets the animal move effortlessly.

This feature of lizards and gecko has inspired material scientists and chemical engineers to try and make similar materials that would be both biocompatible (can be used in the human body safely, with no ill effects) and biodegradable (after its use to repair and heal the damage or wound, enzymes and cells in the body would dispose it off).

While keratin or similar tough proteins would be ideal for the purpose, solubility and machinability appear to pose challenges.

Synthetic polymers such as polyglycerol sebacate acrylate (PGSA) seem to suit the purpose well. PGSA was first introduced by Robert Langer and associates of Harvard/MIT Joint Division of Health Sciences and Technology.

In a recent paper (Mahdavi A, et al. PNAS (US), 105, 2307, 2008), the group has reported the development of a biodegradable and biocompatible tissue adhesive made of PGSA.

First, they had to mimic the setae and spatulae. This was done using a silicon template with nanomould cavities on which the PGSA was spread.

UV light shone on it generated the ‘pillar’ pattern on the elastic polymer, mimicking the gecko toe pad. The thinner each pillar diameter and the further apart the pillars are, the greater the adhesion, when the PGSA template was placed on a biological tissue.

Second, the synthetic gecko mimic should also work when wet. In order to achieve this, they chemically coated the PGSA nanopatterns with a layer of oxidized dextrom.

To further optimize this wet adhesion, they ‘doped’ the PGSA with another polymer PEG, and then coated as above.

The so obtained synthetic gecko mimic was found to adhere very well on pig intestinal tissue in vitro, and rat abdomen in vivo, with no ill effects. The material is thus biocompatible.

The ultimate aim of such tissue adhesives is to enable nature repair the damage, and in so doing have the synthetic material be degraded and disappear with no trace or any ill effects.

Testing the material with an enzyme found in macrophages (foreign material degrading cells found in must animals) showed the material started dissolving away in a week or so.

The researchers conclude, “This gecko-inspired medical adhesive may have potential applications for seeding wounds and for replacement or augmentation of suture or staples.”

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