Science Of Transdermal Alcohol Monitoring
Due to ethanol’s affinity for water, it is rapidly distributed throughout the body by the process of diffusion. Upon reaching equilibrium the fluids of the body will contain ethanol in proportion to their water content. The relationship between breath and blood alcohol is a constant ratio such that one volume of blood contains about the same amount of alcohol as 2100 volumes of alveolar air in normal healthy humans. This means that, in spite of a rather large concentration differences, alcohol excreted in the breath parallels that of the blood over the entire excretion phase (rising and falling). This is the underlying principle for using breath to predict BAC, and a similar process would be expected for perspiration.
The manner in which alcohol passes through the skin (pharmacokinetics) is very complex, and is not well understood. This complexity is in part due to the great number of applicable variables including the subject’s blood alcohol level, the rate of diffusion through the skin, the skin type and location, the thickness of the stratum corneum (the major barrier to water), the amount of eccrine sweating (sweat derived from glands found anywhere other than under the arms), and the cutaneous (within the skin) blood flow. There is also a very interesting delay in peak BAC and peak TAC. One study showed that this delay can vary from 30 minutes to 120 minutes. The amount of delay also depends on where the measurement is made, with the longest delay occurring in measurements taken from the subject’s forearm. It appears however that the manufacturers have yet to fully evaluate exactly how this time delay distorts the TAC curve, or how this distortion might vary with location, skin type and age.
Because the measurement of alcohol passing through the skin is complicated as well as attenuated, BAC cannot be accurately estimated from perspired alcohol content the same way that it is estimated from BrAC. Therefore, detection of alcohol consumption using a sweat collection system can only be regarded as a screening tool to help establish continued abstinence. How well it performs even this limited function is of course subject to debate.
The Science and Practice Of The SCRAM Bracelet
The SCRAM bracelet measures alcohol using the same fuel cell technology used by most portable breath testing devices. With this particular device, the fuel cell is manufactured by Draeger. A fuel cell is a device designed to continually convert fuel and an oxidant into direct current. The reaction that takes place in an alcohol fuel cell is alcohol oxidation, and for these purposes, the “fuel” is alcohol. So, as alcohol is converted in the fuel cell to acetic acid it produces two electrons for each alcohol molecule. This oxidation creates a current, and the intensity of the current correlates directly to the amount of alcohol consumed by the fuel cell. This measurement can be further converted into an alcohol concentration.
What makes SCRAM unique is that it uses this technology not to measure the amount of alcohol in one’s breath, but instead uses it to measure the amount of alcohol migrating through one’s skin. Once in place the device will monitor the wearer based on a schedule set by the monitoring agency. Then, at a predetermined time, the bracelet communicates with a home-placed modem via a 900 MHz radio signal. The readings are sent to a remote computer that acts as a central clearing house of data where it is monitored and interpreted. The data for a specific offender is then available to the home state’s monitoring agency through a secure Internet Web site.