Surfactant: Easing Tension on Surface

Ever wondered why raindrops were spherical or what made those tiny water striders walk on water? It is surface tension that makes these things as they are. When a liquid such as water, is in contact with air, the surface of water that is in contact with air, contracts. This is attributable to the interaction among dipole (polar nature) water molecules. It may be thought of as an elastic sheet that is trying to shrink.

Imagine any water particle, that is not part of any surface: air or that of the container. This imaginary water particle will be subjected to forces from water particles from all directions. The resultant force acting on this particle will thus be zero. But it is not so for water particles lying on the surface.
As a result, cohesive forces among surrounding water particles predominate and surface tension ensues. When an insect (as shown in the picture) or even a small metal coin is put on its surface, it doesn't sink. Buoyancy (Archimedes' Principle) is not the contributing factor here, since the amount(weight) of the displaced water (liquid) is very small. You can do a small experiment to visualize surface tension. Cut a strip of paper into the following shape.
Place it gently on some water surface and let it float. Add a small drop of soapwater into the hollow of the arrow. See how the arrow darts ahead. Its because this soap water (or camphor) reduces the surface tension at the back, while the surface tension ahead remains unabated, causing this pull. Now add a bit of oil: it stops moving further. Oil increases surface tension. The above is one example where physics and biophysics play an important role in biology.

Our body has plenty of water molecules in it. Surface tension thus plays important role in the lungs, where water comes in contact with air. The interior of the alveoli is lined with fluids resembling water. Predictably, the inner side of the alveoli will also try to shrink due to the surface tension arising from the air fluid interface. This pull would be about 72 dynes/cm, had it been pure water. However, the alveolar fluid contains detergent like 'surfactant' molecules to prevent it from collapsing (shrinking). They reduce the tension to 5-30 dynes/cm. These surfactant molecules are secreted by type 2 alveolar cells in the lungs. Chemically, they are a combination of proteins and phospholipids; di palmitoyl phosphatidyl choline (DPPC) being the most important. These molecules are amphipathic, meaning that they have both hydrophilic and hydrophobic domains. The hydrophilic domains remain in touch with the fluid while the hydrophobic domains remain in contact with air. Thus while surface tension (T) is trying to collapse the alveoli, surfactant is trying to diminish it. The pressure on the alveoli can be mathematically given as P=2T/R, where P is the pressure that needs to be applied externally, in order to prevent any change in its volume, due to surface tension (T). R refers to the alveolar radius. The above equation known as the Laplace's equation clearly depicts that when the alveolar radius becomes narrow, as in expiration, P becomes great. This is where surfactant does its duty. In premature neonates, the amount of surfactant may be much lower than normal, owing to their immature lungs. Hence they have high risks of having collapse of the lung (atelectasis). Severe respiratory distress may occur and this condition is known as hyaline membrane disease (also called infant respiratory distress syndrome, IRDS, respiratory distress syndrome of the neonates). This condition needs to be treated urgently.

"Warm Ice" may help us achieve a Better Prosthesis

Lets face it. We have heard of "hot ice" and now we are discussing "warm ice", before even the cool shock of the former is yet to wear off.

Water is a fascinatingly strange molecule even though its molecular structure is apparently simple, consisting only of 1 oxygen and 2 hydrogen atoms. We will restrict ourselves with the topics relevant here, rather than digressing about the properties of water, in general.

A planet, called GJ 436, situated some 3o light years away from earth, was speculated to contain "hot ice" in it. This Neptune sized exoplanet (extra solar planet) has a surface temperature of about 300 degrees Celsius (centigrade). At this temperature, water should exist in gaseous state. But the high pressure of the planet (due to its high density), let water remain in ice form. (for example, in a pressure cooker, water remains liquid at temperatures exceeding 100 degrees Celsius, due to high pressure inside the cooker).

Exploiting a principle of biophysics, researchers (Alexander Wissner-Gross and E. Kaxiras) at Harvard University, US, have shown it by computer simulation, that by covalently bonding sodium atoms to diamond (an allotrope of carbon), some nanoscale ice would form on its surface. The sodium atoms would allow dipole interactions to occur between adjacent water molecules. A frosty layer of about 2 nm (nanometer=one billionth of a meter) would form over the surface of diamond coating. Thus, while on one hand, you can take the advantage of diamond for its 'wear and tear' resistant properties (for which it is already being used in artificial implants like heart valves, joint replacement prostheses etc.), but also we stand a better chance to ward off any possible thrombus (clot) formation by virtue of the smoothness of the warm ice. This ice is called 'warm ice' since it remains in ice form at our warm body temperature of 37 degrees Celsius. Diamond though corrosion resistant, is prone to induce or aggravate blood clotting and hence it is part of the therapeutic protocol that the patient continues taking anti clotting (blood thinning drugs: e.g. Warfarin, Ticlopidine) drugs to prevent any such calamity. Warm ice may, in future, simply make these anticoagulant drugs unnecessary.

Seems like a cool ending to a hot story!