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November 2016

Wetting Properties of Human Skin
Consider this scenario: a family sits down for dinner. Mom asks Johnny if he washed his hands. Naturally, Johnny says "yes". What do you expect him to say? Kids lie, especially when they're hungry. And technically he's not lying since he probably washed his hands yesterday. Dad says, "Let's test it." Johnny sticks his index finger into a small box that resides on the dining room table. A few seconds later a red light illuminates on the top of the small box. Busted. Johnny is sent back to the bathroom to wash his hands. How did this amazing invention work? By taking a quick water contact angle reading on Johnny's skin.

Skin that is freshly cleaned with soap and water is rather hydrophobic producing water contact angles over 100°. A dirty hand that is either soiled with contaminants or natural skin oils will produce a much lower contact angle - between 55 and 65°. Thus, while cleanliness may not always be readily visible to the eye, it can be detected by contact angle.

Water Contact Angle of my Unwashed Finger

One of the earliest works that addresses contact angle on human skin was done by Armour & Company 50 years ago with the help of a ramé-hart contact angle goniometer Model A100.1 The researchers at Armour determined that cleaned human skin behaves a lot like polyethylene. However, residue from hand lotion and other pretreatments can greatly increase wetting properties. Interestingly, there were no significant differences in wetting behavior of skin from one person to the next - and of either gender.

Developers of pressure-sensitive tapes and adhesives used in bandage and medical applications are keenly interested in adhesion-to-skin behavior and the surface energy of human skin.  

So, too, makers of skin care products such as lotions and creams are interested in how their products, which are made from complex arrays of ingredients, interact with and affect the wetting properties of skin. These ingredients include pigments, water, surface active agents, humectants, emollients, solvents, and more. In addition to the physical and chemical properties of the materials, environmental conditions can also affect the skin's surface energy.

Dozens of approved and investigational skin and soft tissue substitutes are or will become available for reconstructive surgery, and the treatment of ulcers, burns, and diseases (such as dystrophic epidermolysis bullosa). Researchers of these bioengineered skin substitutes are motivated to offer products that emulate human skin and act as a temporary or permanent replacement.

Despite myriad applications that could benefit from a more comprehensive explanation of the wetting behavior on human skin, there is a dearth of research on the topic. Partly, this may be due to the variability of human skin and the complexity of its structure. The outermost layer that affects wetting behavior, the nonviable epidermis, can have a water content between 25% and 75%. The topography (i.e., roughness) and environmental conditions can further complicate wetting behavior. In fact, so irregular and unrepeatable is human skin that some researchers have developed synthetic model skin, made from protein, water, and a synthetic lipid-like material, which may be more suitable and less costly for testing adhesives and products that interact with or are designed to emulate human skin.2 And the synthetic skin certainly doesn't move around as much on the specimen stage as in vivo samples do.

Whether you're interested in the wetting behavior or human skin under specific conditions, or if you develop products that interface with skin, we have a tool for you. We've been measuring skin contact angle for over 50 years - as well as the contact angle of 1000s of other surfaces.

1 M. E. Ginn, C. M. Noyes, and E. Jungermann, The contact angle of water on viable human skin, J. Colloid Interface Sci., 26, 146-151 (1968).
2 John C. Charkoudian, A model skin surface for testing adhesion to skin, J. Soc. Cosmet. Chem., 39, 225-234 (July/August 1988).

Product of The Month: Model 250
This month our featured product is one of our best selling models, our Standard Goniometer Tensiometer Model 250. This powerful and versatile tool will measure contact angle of skin and just about any other material you can put on it's 3" x 2" specimen stage. Surface tension can also be measured with the DROPimage Advanced software which includes a methods-based experiment design tool.

ramé-hart Model 250 Standard Goniometer / Tensiometer

Model 250 is one of the most modular and upgradeable instruments available. Add the Automated Dispensing System to form more precise sessile and pendant drops and improve repeatability. It can also be used for dynamic and constant volume time-dependant studies. Add the Automated Tilting Base for determining advancing and receding contact angle as well as roll-off angle. Add the Environmental Fixture for interfacial tension and captive bubble studies. Several elevated-temperature options are also available for Model 250: the Hot Plate, Peltier Environmental Chamber, and Heated Environmental Cell.

The 750 FPS SuperSpeed U2 Series Camera Upgrade Kit is recommended for high-speed applications. Add the Overhead Optical Imaging Kit to capture extremely low contact angles. Add the Oscillator to capture surface dilatational elasticity and viscosity.

Please contact us today if you're interested in a quotation for Model 250 or any other model.


Carl Clegg
Director of Sales
Phone 973-448-0305
Contact us


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