BIOPRINTING FOR SKIN - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 277

Furthermore, the hypodermis connects the upper skin layers to the lower tissues (e.g. muscles, fascia,
and bone). It is also where hormones are generated. The blood vessels in the hypodermis are even larger
than those in the dermis, and supply the glands and hair bulges with nutrients and oxygen ( Michael,
2013c ). Cell types, mainly adipocytes and multipotent stem cells ( Zuk et al., 2001 ), can be found in
the hypodermis.
13.2.4 CUTANEOUS APPENDAGES
As earlier mentioned, several cutaneous appendages can be found in the skin, which are formed by spe-
cific cells: hair bulges/hair, nails, sebaceous glands, perspiratory glands, scent glands, and mammary
glands. Mammary glands enables feeding of the offspring. Perspiratory glands produce sweat and are
therefore crucial for temperature homeostasis and regulation. Specialized, hormone-dependent types of
hair can be found in different parts of the body. These include eyelashes, eyebrows, beard hairs, pubic
and axillary hair, and hair in the nose or the outer ear canal. Sebum secreted by the sebaceous glands
keeps the skin and hair pliable. Apart from playing a role in temperature control, hair is also involved
in tactile sensation. Nails are a further specialized form of hair. They protect us against mechanical
injuries and also support the tactile ability of skin ( Michael, 2013c ).
13.2.5 SKIN SUBSTITUTES, APPLICATIONS FOR PRINTED SKIN
Skin substitutes/equivalents may have several applications. These comprise wound healing after skin
injuries as well as research of skin diseases or corresponding drugs.
13.2.6 INJURIES OF THE SKIN
If the skin is harmed superficially, it will normally heal by itself without complications. But under
certain circumstances, even the skin cannot regenerate anymore. This is true for large and/or deep
burn injuries. Burns result from various situations ranging from severe sun exposure to scalding with
hot water/coffee to electrical or chemical accidents. Direct heat, irradiation, or friction can also cause
burns ( Pallua and von Bülow, 2006 ; Herndon, 2002 , Chapter 2). Unfortunately, treatment of large and
deep burns is only of limited success. Autologous split-thickness skin grafts are normally used to cover
the wounds, but donor sites are rare in large burns and extensive scarring is often unavoidable. This
may lead to aesthetic and functional impairment. In case of deep wounds, dermal substitutes are used,
but insufficient vascularization may lead to rejection of the graft. Moreover, available skin substitutes
do not contain hair follicles, melanocytes, and sebaceous and sweat glands, which deprives them of
functions like temperature regulation as well as aesthetic appeal. This may in turn result in a low qual-
ity of life for patients as they struggle with fear of defacement, rejection, and feelings of failure and
worthlessness. In severe cases, scar formation may also lead to impairment of mobility and restrictions
in daily living.
Another problematic situation results from chronic wounds that are characterized by their patho-
physiology. The normally tightly controlled wound healing is impaired, resulting in badly or non-
healing wounds. Chronic wounds comprise three categories: an impaired venous drain due to a faulty
microcirculation (venous ulcers of the lower extremities); an increased mechanical strain (decubi-
tus); and a vascular, nerval, and/or metabolic tissue injury (diabetes). Often, chronic wounds are the