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International Journal of Bioprinting 3D bone: Current & future

have displayed promising enhancements in biomaterial The human body comprises 206–208 bones, which
strength and biodegradability, while eliciting an adequate can be tubular, flat, or irregularly shaped. The outer layer
host immune response. Furthermore, there is currently an of bones is made of a dense fibrous connective tissue,
increasing utilization of decellularized ECM (dECM) in called the periosteum, which functions to protect and
scaffolds, hydrogels, and bioinks due to its resemblance to nourish the bones and also plays a vital role in bone
native ECM. Mesenchymal stem cells (MSCs) derived from regeneration. The periosteum layer comprises an outer
bone marrow and adipose tissue are among the widely and fibrous layer (stratum fibrosum) and an inner cellular layer
successfully bioprinted cell types used in bioinks. 7,8 (stratum osteoblasticum). The layers comprise parallel
Several studies have reported the clinical application plates composed of collagen fibrils with intervening
of 3D-bioprinted bone tissues as replacements and proteoglycans. The plates are arranged in a few layers
implants, but the number of related clinical trials remains parallel to the outer and inner surfaces of the solid bone
low, warranting further research and development in the (i.e., lamina generales externae and lamina generales
field. Herein, this article briefly introduces the structure, internae, respectively); in the other parts of the bone, they
function, and development of bone tissues, followed form columnar units called osteons (or Haversian plate
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by a detailed discussion on 3D bioprinting techniques, systems). Haversian canals are cavities within the osteons,
materials, and their recent advancements for clinical through which veins and nerves pass. These canals are
applications. interconnected with the periosteal (outer) and endosteal
(inner) spaces via Volkmann canals, which radiate outward
2. Structure, function, and development of from the bone’s surface. Extensions of these canals connect
neighboring cells through tiny channels called canaliculi
bone tissues within the bone plates. Their extensions are connected to
2.1. Structure and composition of bone tissues the wings of adjacent cells by tiny canaliculi through the
Bone is the only tissue type in the body with a mineralized plates. A trabecular bone has a similar structure to a solid
intercellular matrix, resulting in a tough and resistant tissue bone but has smaller and larger cavities that resemble a
that constitutes the body’s skeletal framework. Bones can sponge. Hence, the trabecular bone is both strong and light.
be divided into four groups according to their morphology: Moreover, the cavities contain part of the bone marrow, an
essential tissue for blood formation. In the case of tubular
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(i) The tubular or long bone group refers to most limbs, bones, the spongy substance is reduced in the diaphysis,
where the epiphysis (end of the bone) has an ironlike and the large marrow cavity is filled with yellow marrow,
cartilaginous articular surface and the diaphysis fatty tissue, and a few large vessels (Figure 3). 9
(middle part of the bone) is tubular. The cavities
(interior) of the epiphysis are filled with spongy The mechanical properties of bone tissue are essential
bone tissues, corresponding to the red bone marrow for its function and overall health. The unique composition
in young bones. Likewise, the diaphysis has a strong and hierarchical structure of bones imbue the ability to
cortex with a cavity of yellow bone marrow. withstand external forces, resist fractures, and adapt to
changes in response to mechanical stress. Bone strength
(ii) The short bone group is found in the wrists, knees, refers to the ability of the bone to withstand stress without
and ankles. These bones resemble the epiphyseal breaking and is determined by the bone composition
part of the long bones and are filled with spongy and structure. The inorganic component of bone, mainly
bone tissues without a medullary cavity.
calcium phosphate, provides compressive strength, while
(iii) The flat bone group includes the cranial and pelvic the organic component, mostly collagen fibers, provides
bones, as well as the scapulae, sternum, and ribs. tensile strength. The unique arrangement of these
These bones are typically shaped like a blade or shell components in a hierarchical structure allows for a balance
and are fitted together to enclose a cavity, thereby between strength and flexibility and simultaneously
protecting essential organs from mechanical stress increases the fracture resistance of bones.
and providing a large muscle contact surface. Like Bone stiffness refers to the resistance of bone to
tubular bones, flat bones have a robust cortical deformation when a force is applied. Stiffness is directly
structure and are filled with spongy bone tissues related to the elastic modulus of bone that measures its
without large cavities. ability to return to its original shape after being deformed.
(iv) Irregularly shaped bones are found in the face, Bone tissue has a high elastic modulus to withstand large
vertebrae, patella, feet, and hands. These bones forces without permanent deformation, maintaining the
usually have a thin cortex with an interior of spongy shape and stability of bones and preventing them from
bone tissues. bending or breaking easily. In contrast, bone toughness

Volume 10 Issue 3 (2024) 143 doi: 10.36922/ijb.2056