**5. Biophysical examination of the skin and subcutaneous tissues**

The fundamental role of human skin is to protect the body from invasion by external factors. Biological and chemical invasions of the body could be prevented by the skin, which include circulating cells of the innate immune system. Furthermore, protection from physical invasions—such as mechanical force and thermo injuries—are also important to maintain the homeostasis of the human body.

Viscoelastic Properties of the Human Dermis and

0

20

Delta , δ / degrees

40

60

Other Connective Tissues and Its Relevance to Tissue Aging and Aging–Related Disease 163

10 15 20 25 30

Oscillation Stress / KPa

**Figure 7.** The composition of a pressure ulcer model and the positions of strain gauge probes. The physical model for the pressure ulcer is made of sponges; the probes are placed around the hole

Figure 8 shows the data observed from RTSSM when a tensile load is applied toward the channel 2–4 direction. From the results, it can be observed that this method is able to measure the strain force during the loaded state (0–0.3 seconds) and the relaxed state (after

We further examined our model by testing the strain force around a pressure ulcer in a patient. This study was approved by the ethics committee of our institution and performed following written informed consent was obtained from the patients. As shown in Figure 9,

0

mimicking pressure ulcer.

0.3 seconds).

100

200

300

400

500

/ KPa

Storage modulus , G'

Loss modulus , G"

600

Storage modulus , G' Relaxation Strain

Relaxation Strain

Relaxation Strain

**Figure 6.** Viscoelasticity measurement by muscle contraction.

Loss modulus , G"

Delta , δ

Skin consists of 3 layers, which include the epidermis, dermis, and subcutaneous tissue. Skin covers most of the body's surface, except for some "holes such as oral cavity". Thus, the physical barrier that skin provides is crucial to protect the human musculoskeletal system and internal organs. The physical properties of skin have been measured using several devices (17). In this study, the authors measured the mechanical properties of the skin by dynamic indentation. This study noted that the measurement of these mechanical properties by indentation is not well correlated with that by suction. (17). Furthermore, they also reported the aging-associated alteration of mechanical properties of the skin (18). The CutemeterTM has been used to measure the viscoelasticity of skin. Additionally, we have recently established a novel method to measure the viscoelasticity of skin using a rheometer (AR instrument, AR 550) (Figure 5).

**Figure 5.** A rheometer can be used to determine the viscoelasticity of skin.

Using this method, skin is treated as a complex of different materials. The skin surface at the bottom of an appendage is immobilized so that deformity is only obtained by the external force generated from the upper probe. From the results shown in Figure 6, viscoelasticity of skin (and subcutaneous tissues) was estimated to be approximately 30 kPa. This data was not influenced by muscle contraction, thus indicating that the origin of the physical properties of skin could be the fascia (19).

Next, we developed a physical model for pressure ulcers and mechanical force around the ulcer was measured using our new device, real time skin strain monitor (RTSSM). A pressure ulcer is characterized as a skin and soft tissue injury caused by an external force on a bony prominence. However, it is not clear how a pressure ulcer is strained by external force. Previous studies have reported the similarity in strain properties of human soft tissue and industrial buffer materials. Therefore, we utilized a cell sponge as a testing material for its physically similar attributes to soft tissues. As shown in Figure 7, a physical model for pressure ulcers was developed. Strain gauge probes were stitched around the pressure ulcer model as indicated.

Viscoelastic Properties of the Human Dermis and Other Connective Tissues and Its Relevance to Tissue Aging and Aging–Related Disease 163

**Figure 6.** Viscoelasticity measurement by muscle contraction.

162 Viscoelasticity – From Theory to Biological Applications

the homeostasis of the human body.

**5. Biophysical examination of the skin and subcutaneous tissues** 

the viscoelasticity of skin using a rheometer (AR instrument, AR 550) (Figure 5).

**Figure 5.** A rheometer can be used to determine the viscoelasticity of skin.

properties of skin could be the fascia (19).

model as indicated.

The fundamental role of human skin is to protect the body from invasion by external factors. Biological and chemical invasions of the body could be prevented by the skin, which include circulating cells of the innate immune system. Furthermore, protection from physical invasions—such as mechanical force and thermo injuries—are also important to maintain

Skin consists of 3 layers, which include the epidermis, dermis, and subcutaneous tissue. Skin covers most of the body's surface, except for some "holes such as oral cavity". Thus, the physical barrier that skin provides is crucial to protect the human musculoskeletal system and internal organs. The physical properties of skin have been measured using several devices (17). In this study, the authors measured the mechanical properties of the skin by dynamic indentation. This study noted that the measurement of these mechanical properties by indentation is not well correlated with that by suction. (17). Furthermore, they also reported the aging-associated alteration of mechanical properties of the skin (18). The CutemeterTM has been used to measure the viscoelasticity of skin. Additionally, we have recently established a novel method to measure

Using this method, skin is treated as a complex of different materials. The skin surface at the bottom of an appendage is immobilized so that deformity is only obtained by the external force generated from the upper probe. From the results shown in Figure 6, viscoelasticity of skin (and subcutaneous tissues) was estimated to be approximately 30 kPa. This data was not influenced by muscle contraction, thus indicating that the origin of the physical

Next, we developed a physical model for pressure ulcers and mechanical force around the ulcer was measured using our new device, real time skin strain monitor (RTSSM). A pressure ulcer is characterized as a skin and soft tissue injury caused by an external force on a bony prominence. However, it is not clear how a pressure ulcer is strained by external force. Previous studies have reported the similarity in strain properties of human soft tissue and industrial buffer materials. Therefore, we utilized a cell sponge as a testing material for its physically similar attributes to soft tissues. As shown in Figure 7, a physical model for pressure ulcers was developed. Strain gauge probes were stitched around the pressure ulcer

**Figure 7.** The composition of a pressure ulcer model and the positions of strain gauge probes. The physical model for the pressure ulcer is made of sponges; the probes are placed around the hole mimicking pressure ulcer.

Figure 8 shows the data observed from RTSSM when a tensile load is applied toward the channel 2–4 direction. From the results, it can be observed that this method is able to measure the strain force during the loaded state (0–0.3 seconds) and the relaxed state (after 0.3 seconds).

We further examined our model by testing the strain force around a pressure ulcer in a patient. This study was approved by the ethics committee of our institution and performed following written informed consent was obtained from the patients. As shown in Figure 9, the probe was adhered onto the dressing and the strained force was measured in the bedridden patients.

Viscoelastic Properties of the Human Dermis and

Other Connective Tissues and Its Relevance to Tissue Aging and Aging–Related Disease 165

Point B (Left)

**Figure 10.** Changes in strain force at the buttocks are dependent on a positional change in head lifting.

Using the RTSSM, we next determined the direction of force by coordinating data from several probes. To this end, multiple probes can be adhered around the wound (Figure 11) and the measured force can be generated. In this case, it was reasoned that the different vectors, representing the strain force between the right and left sides, were generated due to the contracture of the right leg. Thus, the data obtained can be used to determine the

positioning change that is ideal in the care of the patient with a pressure ulcer (20).

Point A (Right)

15deg. 15deg. 15deg. 15deg. 30deg. 30deg. 0deg.

15 degree

30 degree

**Figure 11.** Positioning changes generate strain force on a pressure ulcer toward a specific direction. When the position of head is lifted at the indicated degree (15 or 30) a strain force is promptly changed.

**Figure 8.** RTSSM measured by the strain distribution of a pressure ulcer model. The loading force is increased by 100 μ strain/s from the initial load at 50 μ strain/s and a maximum force of 250 μ strain/s is maintained.

**Figure 9.** Measurement of strain force around pressure ulcers.

It has been noted that the head lifting position of the patient can occasionally worsen a pressure ulcer. Therefore, the manner in which positioning changes influence a pressure ulcer is an important issue for the care of a patient. To address this issue, we measured strain forces around the pressure ulcer during positioning changes. Measurement using RTSSM indicates that a positioning change can generate a strain force around the wound (Figure 10).

Viscoelastic Properties of the Human Dermis and Other Connective Tissues and Its Relevance to Tissue Aging and Aging–Related Disease 165

164 Viscoelasticity – From Theory to Biological Applications

bedridden patients.

maintained.

(Figure 10).

the probe was adhered onto the dressing and the strained force was measured in the

**Figure 8.** RTSSM measured by the strain distribution of a pressure ulcer model. The loading force is increased by 100 μ strain/s from the initial load at 50 μ strain/s and a maximum force of 250 μ strain/s is

It has been noted that the head lifting position of the patient can occasionally worsen a pressure ulcer. Therefore, the manner in which positioning changes influence a pressure ulcer is an important issue for the care of a patient. To address this issue, we measured strain forces around the pressure ulcer during positioning changes. Measurement using RTSSM indicates that a positioning change can generate a strain force around the wound

Point B (Left)

**Figure 9.** Measurement of strain force around pressure ulcers.

Point A (Right)

**Figure 10.** Changes in strain force at the buttocks are dependent on a positional change in head lifting.

Using the RTSSM, we next determined the direction of force by coordinating data from several probes. To this end, multiple probes can be adhered around the wound (Figure 11) and the measured force can be generated. In this case, it was reasoned that the different vectors, representing the strain force between the right and left sides, were generated due to the contracture of the right leg. Thus, the data obtained can be used to determine the positioning change that is ideal in the care of the patient with a pressure ulcer (20).

**Figure 11.** Positioning changes generate strain force on a pressure ulcer toward a specific direction. When the position of head is lifted at the indicated degree (15 or 30) a strain force is promptly changed.
