University of Florida Research Foundation, Inc. (Gainesville, FL)

In this article are described methods and equipment for creating tissues replacements, for instance bone replacements, which can be used to fix damaged or missing parts of bone, as can occur during wound healing or in the treatment of a congenital anomaly. The methods employ three-dimensional (3D), cell growth medium constructed from yield stress materials. This permits cells and other structures to easily be deposited and placed.

The remarkable self-repair ability of skeleton bones is impressive. However, severe traumas (e.g. blast injuries) or cancer surgery when large portions of bone are destroyed or removed, full functional repair is required by the implantation of a bone graft that is exogenous and has the appropriate dimensions and load-bearing capabilities. Vascularized autogenous bones are the best choice since it readily assimilates into neighboring tissues and is fully transformed into healthy tissue. Given that the supply of healthy bone available for harvesting is a limited supply the decellularized bone of donor cadaveric tissue is commonly utilized to repair large-scale defects. Although it provides the original structure, structure, and mechanical support, bone isn’t able to be remodeled into living tissue due to the absence of vascularity and can become fragile and prone to breaking.

The use of a three-dimensional scaffold as a model to regenerate cells is fundamental to tissue engineering. As biotechnology advances from 2D to 3D cell culture, it is becoming more important to comprehend the relationship between cell response to surfaces (2D) and to a 3D object (3D). This allows for better replication of in the vivo counterparts.

Modern day production of micron-to millimeter scale topography is based upon micropatterning, lithography techniques, or rapid prototyping using the combination of 3D computer-aided design (CAD) and 3D printing or anadditive layering technique that requires special equipment or facilities and often involve tedious procedures. Substrates that have complex geometrical shapes, like cylindrical, toroidal, or spherical structures are difficult and take a long time to design using the aforementioned methods. Methods for processing scaffolds have advanced from traditional subtractive techniques, which involve constant removal of materials, to conventional additive approaches that use solid-free formfabrication, which involves adding layers of materials. The present 3D printing technology permits the printing of an additional support layer at the same time, along with the printed structure. The support material is washed away and discarded after printing.

Electrospinning is a free form process that produces nonwoven fibrous structures. It creates fibers with dimensions that range from tens to micrometers. The solution for polymer is fed through a syringe and extruded from the spinneret (needle tip) connected to a high voltage, where nanofibers are created. In 3D printing, a dispensing head (inkjet printer head) is utilized in the free forming process to place every layer of powdered polymer and binding fluid.

The methods described herein comprise equipment for making tissue replacements, such as bone replacements which can be used to repair damaged or missing sections of bone, such as may occur in wound repair or to repair a congenital anomaly.These methods require three-dimensional (3D) cell growth medium made from an yield stress material that lets structures and cells be easily placed and moved. In some cases it involves a solid to liquid phase change in a specified location within the region of yield stress material in such a way that the yield stress material flows and then be moved when cells or structures are placed in the location desired. It is then transformed into a solid phase that supports the cells/structures when it’s completed. The disclosed 3D growth medium allows the placing of tissue scaffolds or vessels to produce vascularized substitutes for tissue.

This invention discloses a method to design vascularized tissues. It involves the creation of a 3D-cell medium, which includes a variety of hydrogel particles and liquid medium for cell growth. The hydrogel particles are then dissolving within the medium of liquid cells to form an solid jelly.

This can be done by inserting a biocompatible support structure into the granular gel. The scaffold could be synthetic or natural. For example, the scaffold can be derived from an autologous, allogenic, or the xenogenic transplant. The scaffold could also be made from biocompatible substances. In some instances, 3D printing is used to create the scaffold. This is done before depositing into the granular jelly , or directly in the gel. In the case of example in the case of tissue comprised of bone, then the support scaffold may be mineralized. In certain instances, the support scaffold comprises bone that has been decellularized, e.g. from a cadaveric donor. The support scaffold may be constructed from an osteoconductive, osteogenic or osteoinductive material. For example, the scaffold could be made up of hydroxyapatite.

The process can also include depositing into the gel a vascular structure that is capable of helping to support blood flow. This can be done either before, after or in conjunction when the scaffold is placed. The vascular structure has an proximal and distal end for fluid communication. In preferred embodiments, the distal portion of the vascular structure is positioned in the granular gel a location suitable to provide oxygen to the scaffold when blood is pumped into distal end of the vascular structure. In certain embodiments, the vascular structure comprises an arterial or venous segment of an individual. Another embodiment employs a tissue-engineered vessel as the vascular structure. For instance the vessel could be created using autologous or allogenic endothelial cells, progenitors cells, or stem cells.

The method could include the addition of a cellular component into the gel. This can be done either prior to, during, or after the formation of the scaffold, and/or the associated vascular structure. The type of tissue being engineered will determine the choice of the cellular components. In certain cases the cellular component is at least one stem cell or progenitors capable of forming the desired tissue in the support scaffold. The cellular component can be placed, for instance within the support scaffold, or between the support structure as well as the vascular structure. For instance, when the bone tissue is present, the cellular component can include bone marrow cells or Marrow-like cells. In certain embodiments the cellular component could comprise mesenchymal stromal cells (MSCs) as well as human hematopoietic stem cells (HSCs), osteoblasts or Osteocytes. The cellular component may also comprise growth factors selected from vascular epithelial growth factor (VEGF), basic fibroblast growth factor (bFGF), epidermal growth factor (EGF), or any combination of them.

This is accomplished by attaching a flow device near the proximal end of the vascular structures and then circulating blood-like media through the vascular structures to aid in the growth of tissue. The blood-like medium could contain blood or a substitute for it with oxygenated erythrocytes.

In some instances, the cellular component is circulated through the vascular structure rather than depositing it directly into the granular gel. In some methods the growth factors are transported through the vascular structure. A growthfactor gradient can also be created in the gel’s granular.

In some instances, the three-dimensional growth medium is characterized by yield stress in which the medium for cell growth undergoes a transition from a solid phase to the second liquid phase on application of a shear stress that is greater than theyield stress. The yield stress could be as high as 10 Pa+/-25 percent. In some cases, the concentration of hydrogel particles can range from 0.05% to about 1.0 percent in weight. In certain embodiments, hydrogel particles have a size that ranges from 0.1 .mu.m to about 100 .mu.m when swollen with the liquid culture medium. In some cases, the hydrogel particles have a size in the range of approximately 1 .mu.m to around 10 .mu.m when they are swollen by the liquid cell culture medium.

In certain instances, there are molecules that are dispersed in the granular gel particles , and throughout the granular gel. For instance, the molecules can be proteins or small molecules (e.g. Growth factors. In certain instances some cases, the molecules are small molecules which are small, and the smaller molecules are composed of nutrients or dissolving gasses.

The following description and the accompanying drawings provide information about the various embodiments. Other aspects, features and benefits of the invention will be apparent from the description and drawings as well as from the claims.

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