By SUE WALMER
CLINICAL STUDY DIRECTOR, PETER T. PUGLIESE & ASSOCIATES

Ballistometry measurements are made to determine the viscoelastic properties of the skin. The information gained from analysis of the data can be translated into measures of elasticity, stiffness, moisturization and suppleness of the skin. Several commercial instruments are available to probe viscoelastic properties of skin, and can be classified in terms of the basic measurement method: torsional stress, suction-cup stress, and ballistic hammer stress. Ballistometry using the ballistic hammer method offers a number of advantages for laboratory studies, and will be primarily discussed in this paper. For information about the other methods, the reader can search the Internet using the following terms: Dia-Stron ballistometer (torsional) and Dermaflex machine (suction- cup). The system employed in our lab is an IDRA® Ballistometer version 1.0, manufactured by Third Party Research & Development, Cortlandt Manor, NY. The hardware and software for this system are discussed in detail in the following paper:

Pugliese, P.T. and Potts, J.R., "Hardware and Measuring Principles:The Ballistometer", chapter 13 in Bioengineering of The Skin:Skin Biomechanics, Elsner/ Beradesca/ Wilhelm/ Maibach, Ed. , published by CRC Press, ISBN/ISSN:0849375215, 10/2002.

MEASUREMENT PARAMETERS
Ballistometry is a biophysical technique that measures the rebound of the skin when subjected to a repeated light force. In practice, a swinging arm, i.e., a light weight hammer (~8 grams) is used to deliver the force from a fixed height. This force, measured in Newtons' (kg/m2), and other various parameters are determined to be functions of the skin's viscoelastic properties. The swinging hammer is dropped onto the skin and as it rebounds produces a series of decreasing rebounds measured as Amplitudes of the shaft's movements. The series of rebounds are recorded by a microprocessor and then generated into a hard copy graph in Excel. The parameters generated in the graph are used to calculate skin stiffness, the cutaneous absorption coefficient (CAC) and the coefficient of restitution (COR). The parameters are defined below.

Cutaneous absorption coefficient or CAC is a dynamic time constant defined for exponential changes in the ballistic arm amplitude. The impact and the rebound energies can be calculated from the CAC value. The CAC is represented as k in the following equation. The CAC increases with age.
Y = Y0 e-kt

where Y0 is the initial amplitude of the hammer determined from the initial position of the shaft.

Coefficient of Restitution or COR is defined as the ratio of the ballistic arm impact speed to the rebound speed.

Skin stiffness, relates to skin "hardness", and is defined as the ratio of the hammer impact force to the skin impact.

The relative amplitude is a measure of the elasticity as it relates to the rebound energy of the skin.

Analysis of the data can then be interpreted as the elasticity, stiffness or firmness, and moisturization or suppleness of the skin.

BALLISTOMETRY MEASUREMENTS ON THE SKIN

Ballistometry is a relatively unknown technique for assessing skin. Briefly, a light-weight hammer is attached at one end to a rotary transducer, the hammer free falls onto the test surface, and the resulting oscillatory displacement-time data can be analyzed to determine distinctive physical parameters. Analysis of the skin's response to the hammer's impact and the resulting small energy forces can be correlated to the condition and age of the skin.

The skin is dynamic. Four parameters are looked at to access the skin.

1. AMPLITUDE

When the light-weight hammer is dropped onto the skin it stores the energy that is delivered by the falling object. This stored energy is released providing the rebound. If the skin was wholly elastic, the amplitude of the last rebound would be the same as the first amplitude. However, the skin is not completely elastic. It is both an elastic and viscous substance; therefore, some of the stored energy will be dispersed in the skin and not fully recovered, resulting in a smaller rebound.

As the subjects' age increases, the amplitude decreases.

Lower amplitude of the first rebound may be due to the deterioration of the network of elastic fibers in the dermis, often seen in sun-exposed, aged skin.

2. CAC

The Cutaneous Absorption Coefficient or CAC is the measure of elasticity defined as a dynamic time constant that assumes hammer impact energy is lost exponentially over time.

CAC increases with age, so there is no significant change in this parameter over a short period of time.

3. STIFFNESS

The stiffness parameter is defined as the ratio of the hammer impact force to the skin deformation and provides stress stain information. Skin stiffness relates to "skin hardness".

Skin that is elastic looks toned and is firm. It snaps back. This parameter is useful in evaluating hardened compacted cells in the outer layers of the skin, the softening hydrating effect on the skin cells at the surface, increased collagen and/or changes in the dermal tissues, and firmer skin tone and texture.

4. NUMBER OF BOUNCES

The number of rebounds or bounces is in direct proportion to the amplitude of the rebounds and is a good estimation of the skin's elastic properties.

Increases seen in this rebound effect (increased number of bounces) indicates that the skin may be more pliable or more resilient. Reduced number of bounces may be attributed to firmer skin tone or dehydrated skin, therefore it is important to assess your subject.

MAKING THE MEASUREMENTS

The instrument must be calibrated as to ballistic hammer initial and armed position. No details regarding the hammer used are needed if only the Amplitude, CAC, and COR are recorded. The weight, length, center of gravity and other hammer details must be known, if STIFFNESS measurements are also of interest. These parameters are built into the computer program and are designed to adjust the results according to the values obtained in the calibration procedure. The two critical positions are 0 degrees and angle set on the faceplate (degrees) of the ballistic arm. The instrument should be calibrated daily before any measurements are made.

Third-Party Research and Development supplies a test tape that serves as a test surface to check on system performance and the state of calibration. The tape has an adhesive on one side which permits adhering it to a weighted block. The weighted block keeps the tape from moving during testing, which allows its mechanical properties to be probed. A rubber glove or a dental dam is flexible elastic material that can also be used as the positive control for determining elastic properties. A block of wood or metal block can be used as the inelastic or negative control. Experiment with fruit (the skin of a banana) clay, gelatin, or ultrasound standing gel pad to measure the deformation with the light-weight hammer's impact.

The subject should be relaxed and comfortable while seated in an ambient room. Their position is extremely important because if they are seated too high or too low to the ballistic arm they may be tense or straining their muscles in the measurement which will result in increased stiffness or less # bounces. Likewise, if they are too relaxed, it can have the opposite effect on the data. An armed chair or pillow to rest their head or chin will help make more accurate measurements. If the subject is standing for measurements make sure they can rest their body onto a stationary object for balance.

Make 3 measurements on the skin. Take the average of 3 readings.

Do a test run on the subject to let them feel the impact of the hammer on the skin. Instruct them not to move during the measurement.

The skin can be evaluated on any area that has a relatively flat surface. These areas include the forearm (volar and dorsal surface), cheeks, wrist, hand, forehead (may be too bony), cheeks, outer thighs, and outer canthus (crow's feet area).

The error "Caution, test subject may have moved" is commonly seen when there is a slight variation in subject's starting to ending position during the reading of the measurement, usually due to breathing.

COMMENTS

Know your subject. Take a detailed medical history prior to making any measurements on the skin.

Various age groups and ethnic groups should be used to effectively evaluate test products.

AGING SKIN

Young skin has a thicker epidermis. The cutaneous matrix is dense and collagen fibers are well structured. The papillary dermis is wavy structured providing good skin elasticity.

Aging skin is thinner and cell renewal is slower. With time, the production of collagen slows down and the skin matrix isn't as tight as before. Skin loses its firmness and elasticity. The epidermal junction becomes flattened.

Premature skin aging can be recognized from excessive sun or tanning bed exposure, and lifestyles factors such as stress, lack of sleep, too much alcohol, tobacco use, substance use, etc.

The inevitable aging process has a marked impact on the skin's hydration properties for skin becomes drier as we age.

OTHER FACTORS

Subjects that have increased lipid content in their skin will have decreased stiffness.

Black skin has increased skin elasticity even as they age because they have more elastin in the skin than skin of other ethnic groups.

Dehydrated skin looks dull and feels tight. It lacks elasticity, may have a rough texture and may look irritated. Astringent lotions that contain alcohol may be drying and excess sun exposure may cause sun burned skin.

Deposition of fat in the skin must also be taken into consideration for the amount offat padding will affect the measurement.

Environmental factors such as sun (affects elastin fibers), wind, pollution, temperature and relative humidity can affect readings.

Drugs, such as diuretics or anabolic steroids, will affect the skin. Other conditions include hormonal imbalances, illness, edema, etc.

This statement could be adapted for informed consent form:

{Ballistometry measures the viscoelastic properties of the skin. A small light weight hammer is used to deliver a force to the outer skin from a fixed height. Measurements will be made on the (designate face, buttock, cheek, leg, volar or dorsal forearm, etc) and computerized readings are obtained. The data obtained
from this study will be analyzed for changes in skin elasticity, stiffness, moisturization properties, and suppleness.}

INTERPRETING RESULTS

The average of the 3 measurements is used for the final data analysis. A student's t-test or z-test can be run to determine if the results are statistically significant.

Decreases in skin stiffness can be attributed to on the skin an active product's moisturization qualities and its hydration effect of the skin seen by changes in the dermal tissues which may indicate firmer skin.

The number of bounces or rebounds is a good indication of skin elasticity. Increases in the rebound effect in a treatment group may indicate that the skin is more resilient resulting in improvements in the extracellular matrix of the skin

As skin ages, a decrease in the amplitude measurement is usually observed.

The weather can have a profound effect on the ballistometry data. For instance, a 6 month study that starts in winter will have low temperatures with decreased relative humidity values. The subjects' indoor heating systems will vary considerably (electric heat, coal heat, wood-burning stoves, etc). Typically, skin becomes drier skin as colder weather progresses. As the study progresses the season changes bringing increased temperatures and relative humidity changes too. Therefore, it is essential that the data be carefully evaluated to ensure that the test product(s) truly has an effect or if improvements seen in the skin's condition are solely attributed to climatic changes.

Evaluate the subject and their skin before and after the ballistometer study to ensure accurate reporting of the study data.