Studies on Blisters Produced by Friction: I. Results of Linear Rubbing and Twisting Technics

Blisters produced by friction are distinctive forms of reactions which are practically confined to the human species. They are not found, as a rule, in lower animals. But these lesions of the human skin occur with such frequency that they constitute one of man's most common reactions to trauma. Practically everyone has at some time experienced discomfort from this commonplace skin lesion.Under special circumstances, individuals can be disabled by friction blisters. Such blisters on the hands or feet, while usually merely an annoyance or cause of discomfort, will, under certain conditions and in particular occupations, cause various degrees of disability. They can even lead to serious consequences. Certain military activities rank high among those in which friction blisters and their sequelae can actually produce casualties.Despite these facts, friction blisters have received little medical attention or scientific study. Notable exceptions are the precise investigations and detailed reports by Naylor (1, 2, 3) on the production of friction blisters under controlled conditions. Our present studies are mainly variations on and extensions of Naylor's fundamental work.The paucity of scientific investigations on friction blistering stands in remarkable contrast to the abundance of studies and reports on clinical bullous dermatoses {e.g., pemphigus, dermatitis herpetiformis, contact-type eczematous reactions, etc.) and on experimental blisters produced by heat, cold, ultraviolet rays, or chemicals such as cantharidin, croton oil, mustard gas, etc.Additional scientific information on the mechanism of friction blistering is needed. For quite apparent reasons, the many factors causing or contributing to clinical friction blisters cannot be precisely controlled nor readily analyzed under field conditions {e.g., soldiers on a march or persons engaged in sports, etc.). Since the skins of laboratory animals do not respond like the skin of man and do not generally form blisters in response to friction, we adopted the following methods for the experimental production of friction blisters on the skin of volunteers.materials and methodsLinear Rubbing. By modifying rubbing instruments used by Goldblum and Piper (4) and by Nalyor, (5) a special apparatus! for linear to-and-fro rubbing was designed and constructed (Figure 1).The instrument consists of a rubbing head to which various selected materials (leathers, cloths, plastics, etc.) can be firmly attached. The head can be moved over the surface of any chosen skin site at a chosen rate from 25 to 104 complete strokes per minute (counting each excursion and return to the original position as one stroke). The rubbing pressure against the skin can be determined by adding a known amount of weight from 215 to 1154 grams to the rubbing arm. Fluids, for example water or liquid emollients, can be accurately delivered to the rubbing interface by means of an attached perfusion pump. A strain gauge and a thermistor within the rubbing head permit continuous measurement of the frictional resistance and temperature. The temperature measured is that on the surface underneath the rubbing material where the thermistor is in place. Calculation of the frictional coefficient is obtained by the formula, /i = F/W, where “F” is the frictional resistance at the surface, graphically recorded in gram units on a multichannel Oscillo/t We are indebted to Dr. Irvin Levin of the Walter Reed Army Institute of Research for constructing the first of our linear rubbing machines, and to Mr. Emil Barish of the Research and Development Laboratory of the University of California Medical Center, San Francisco, for constructing the second, and also for our most recent twist blistering apparatus.456FRICTION BLISTERS457Fig. 1. Linear Rubbing Machine. (U.S. Army Photograph) a. Rubbing head with leather in place, b. Interchangeable rubbing-arm weight, c. Interchangeable gears for varying stroke weight, d. Roving probe thermistor, e. Perfusion syringe.riter,* and “W” is the amount of weight in grams pressing downward on the skin.Twist Rubbing. Friction blisters are readily produced on palmar skin using the eraser on an ordinary pencil. The pencil is held between the palms of the operator, perpendicular to the skin surface with the flat end of the eraser pressed down firmly on the area to be rubbed. The pencil is briskly rotated in clockwise-counterclockwise directions with the arc of each twist being about 270°. Although this technic constituted a distinct advantage over linear rubbing for some of the studies which will be described, the factors are not as precisely measurable as with the linear rubbing machine.Volunteers. All of our subjects were healthy male volunteers on active military duty with the U.S. Army at the Presidio of San Francisco and nearby installations, or (in one experiment) volunteers for studies carried out at the University of California School of Medicine, San Francisco, California (see Footnote 4).To date, 162 attempts at blister production have been made with the linear rubbing machine on 54 different volunteers.f Over 150 experimental fric-* Texas Instruments, Inc., Houston, Texas.f Few of the experiments were on Negroes and most of the subjects were Caucasians. Therefore, no attempt was made to ascertain possible differ-tion blisters on the palms have been made with the pencil eraser technic on 47 volunteers.RESULTSRelationship between the Amount of Friction,the Number of Strokes, the Duration ofRubbing (work), and the BiologicResponse (Blister Formation)In our experiments with the linear rubbing machine, when added moisture, speed, head pressure and rubbing materials were held constant, closely similar amounts of friction were produced on corresponding areas on different individuals and on different symmetrically situated sites in the same individual. However, despite this equality in the level of friction, the time required to produce a blister on the palm varied in a very wide range. In some experiments a blister would be produced on the palm in three to four minutes; while in others 50 minutes or more of stroking with similar amounts of friction per unit of time failed to produce blistering.ences in response between the Negro and Caucasian skin.458THE JOURNAL OF INVESTIGATIVE DERMATOLOGY(Rubbing was not continued beyond one hour even if no evidence of blister formation had appeared in that period).Using the twisting eraser technic, blisters were often produced on the palm with only 30 seconds of friction and never required more than 3 minutes. This is to be compared with linear rubbing which never took less than 3 minutes of friction, and sometimes 50 or more minutes failed to produce blisters. Thus, on the palm, the prolonged rubbing time of our linear rubbing machine experiments was more apt to bring to light the variations in biologic response.Further studies on the palm with higher frictional twist-force applied under controlled and measured conditions will be required to verify this variability. For this purpose, a new “twist” blistering apparatus was designed and constructed to produce skin-twist rubbing on two sites simultaneously with measurable and recordable degrees of friction (Figure 2). Studies with this instrument are now in progress.Susceptibility of Different Skin Sites to Friction BlisteringExperiments with the linear rubbing machine and the twisting rubber eraser technic were carried out on the normal appearing skin of the back, buttocks, shins, forearms, upper arms, thighs, palms, and soles of the volunteers. Intact, fluid-filled blisters were very difficult to produce on any of these areas, except the palms and soles. Only rarely, and by chance, did we succeed in producing fluid-filled blisters on the back or on other “thin-skinned” areas with linear rubbing, and never with eraser twist. Further observations suggested the possible reason for this. On the less cornified and more fragile skin, it was not generally possible to give precisely the amount of frictional trauma required to produce the somewhat deeper epidermal damage which results in an intact blister without destroying or removing the superficial epidermal layers needed to form the blister roof. Therefore, no fluid-filled clinical blister could appear,Fig 2 “Twist” Rubbing Machine. This apparatus insures application of identical degrees of frictional trauma to two sites simultaneously. (U.S. Army Photograph) a. Twist rubbing head. b. Sleeve containing a calibrated spring, c. Motor control box. d. Two-channel recorder.FRICTION BLISTERS459but only an abrasion. In other words, on less cornified skin sites, frictional rubbing would either fall short of producing a clinical blister or would produce a “topless blister,” i.e., an abrasion.Our results on the palms were quite different. On these “tough” and thickly cornified sites, the superficial layers of the skin apparently resist the frictional trauma of rubbing so that this trauma does not destroy the superficial epidermal layers before the deeper process of blister formation can be initiated.Blister FormationThe beginning of blister formation was often heralded by a stinging or burning sensation. While a few of the volunteers voiced no expression of discomfort, a sudden painful sensation was spontaneously reported by some. The earliest objective sign of beginning blister formation on palmar skin produced by either the linear rubbing or twist rubbing technic was the appearance of a pale, narrow collarette around the rubbed, reddened central area. With additional stroking, the pale zone gradually enlarged cen-tripetally until it occupied the greater part of the rubbed site and eventually became elevated over the underlying skin to form the blister top. The space thus formed in palmar skin of a subject in the erect position usually filled with fluid within one hour. The exact location of this cleavage and the causal and limiting factors