Restore, Repair and Regenerate - Aspects and Prospects of Regenerative Medicine and Tissue Engineering :

Regenerative medicine and tissue engineering are rapidly growing fields of study that involves in repairing, restoring and regenerating the damaged or lost tissue. They are interdisciplinary fields of science that involves material science, cell biology, biochemistry and general engineering and medicine principles. According to National Institute of Health (NIH), Regenerative Medicine or Tissue Engineering is defined as "rapidly growing interdisciplinary area which involves physical and engineering sciences to develop functional cells, tissues and organs in order to repair, restore or regenerate or to enhance the lost biological function due to injury, abnormalities or ageing".

Components of Tissue Engineering

A general tissue Engineering product contains the following components: Cells, scaffold/biomaterial and biomolecules or signaling molecules. The procedure involves in seeding the suitable cell type on a biocompatible biomaterial often referred as scaffold together with the signaling molecules in order to grow the desired tissue.

Biocompatible materials - The choice of biomaterial used influence the growth, cell differentiation and proliferation in the process of tissue engineering.

Cells - The cells used in the tissue engineering process can be taken from the patient or from the same person to whom the regenerative medicine is applicable (autologous), from another person or donor (allogenic) or the animal cells (xenogenic) and the stem cells.

Cell-Biomaterial interaction for construction of synthetic Tissue Microenvironment :

Specific interactions between cell and biomaterials are required to control the cellular functions and for development of a cell (or stem cell) niche. These interactions provide rational designs for construction of specific tissue microenvironment for physiological and pathological conditions.

The design of biomaterials and the sourcing for appropriate cells are two integrated aspects of tissue engineering to construct a tissue implant for clinical applications. During the past decades, many innovative biomaterials with desirable biological and mechanical properties have emerged, while stem cells have been shown to be a promising cell source to differentiate into many cell types. However, the testing of these bioartificial tissue constructs in the clinical trials is far from satisfactory. How microenvironments in the biomaterials regulate cellular signaling pathways and functions, how stem cell-derived target cells respond to extracellular cues presented by the biomaterials, and how implanted tissue constructs interact with host tissues remain to be investigated.

The fundamental cross-talk between a cell and material to provide microenvironmental cues and in understanding the role (and interplay) of the cues in controlling cellular functions including stem cell for cell adhesion, proliferation, migration and differentiation. The information of the interplay between cell and biomaterials would be helpful to guide us in improving our current strategy to refine the tissue constructs for effective tissue repair in regenerative medicine.

Recent Advances in Tissue Engineering Strategies for the Treatment of Joint Damage : 

While the clinical potential of tissue engineering for treating joint damage has yet to be realized, research and commercialization efforts in the field are geared towards overcoming major obstacles to clinical translation, as well as towards achieving engineered grafts that recapitulate the unique structures, function, and physiology of the joint. Recent advances in technologies aimed at obtaining biomaterials, stem cells, and bioreactors that will enable the development of effective tissue-engineered treatments for repairing joint damage.

3D printing of scaffolds is aimed at improving the mechanical structure and microenvironment necessary for bone regeneration within a damaged joint. Advances in our understanding of stem cell biology and cell manufacturing processes are informing translational strategies for the therapeutic use of allogeneic and autologous cells. Finally, bioreactors used in combination with cells and biomaterials are promising strategies for generating large tissue grafts for repairing damaged tissues in pre-clinical models. Together, these advances along with ongoing research directions are making tissue engineering increasingly viable for the treatment of joint damage.

Nanofiber Scaffold for Utility in Bone Tissue Regeneration : 

Many variables serve to alter the process of bone remodeling and diminish regeneration including the size and nature of the wound bed and health status of the individual. To overcome these inhibitory factors, tissue engineered osteoconductive scaffolds paired with various growth factors have been utilized clinically. Bone is a highly vascularized and dynamic tissue that has an innate capacity for healing after injury. However, there are many variables that serve to alter the process of bone remodeling that diminish regeneration including the size and nature of the wound bed and chronic medical conditions. To overcome these inhibitory factors, tissue engineered, osteoconductive scaffolds paired with various growth factors have been utilized clinically in orthopedics and craniofacial surgery. However, many limitations still remain with commercially available products (e.g. rhBMP2) which can lead to rampant inflammation associated with injury or clinical intervention, ectopic bone formation, and ultimately graft failure. The ability for a nanofiber scaffold (Talymed), currently approved to augment cutaneous wound healing, to accelerate growth factor (rhBMP2) generated bone healing compared to the traditional absorbable collagen sponge (ACS) delivery system. To assess this healing after craniofacial fracture, 155 adult wild-type mice were randomly arranged in 16 groups by time, 4 and 8 week, and treatment, ACS or Talymed, loaded with control, low, medium or high dosages of rhBMP2. At experimental end points, skulls were subjected to microCT, biomechanical, and histological analysis to assess bone regeneration.

The use of Talymed within the defect site was found to decrease the bone volume, bone formation rate, and alkaline phosphatase positivity compared to ACS/rhBMP2 combinations. Interestingly, the Talymed regenerated bone, although less, was found to have a greater hardness value than that of bone within the ACS groups. However, the difference in bone hardness between scaffolds was not detectable by 8 weeks.

Advances in Skin Regeneration Using Tissue Engineering

Advances in Skin Regeneration Using Tissue Engineering :

Tissue engineered skin substitutes for wound healing have evolved tremendously over the last couple of years. New advances have been made toward developing skin substitutes made up of artificial and natural materials. Engineered skin substitutes are developed from acellular materials or can be synthesized from autologous, allograft, xenogenic, or synthetic sources. Each of these engineered skin substitutes has their advantages and disadvantages. However, to this date, a complete functional skin substitute is not available, and research is continuing to develop a competent full thickness skin substitute product that can vascularize rapidly. There is also a need to redesign the currently available substitutes to make them user friendly, commercially affordable, and viable with longer shelf life.

Approaches for Tissue Engineering:

Different strategies, such as injecting growth factors and extracellular matrix, are being adopted towards tissue re-growth and wound healing. Some of the recent strategies are listed below.

• Cell Cocultures
• Cultured Epithelial Autografts
• Tissue Engineered Skin Substitutes

Tissue engineered skin substitute preparation. Bold lines indicate cell type for tissue engineered substitute and dotted lines indicate cell source

Types of Skin Substitutes:

• Acellular Skin Substitutes
• Cellular Allogenic Skin Substitutes
• Cellular Autologous Skin Substitutes
• Commercially Available Skin Substitutes

Tissue engineered skin substitutes

Biomaterials and Therapeutic Applications : 

Millions of patients worldwide have benefited from technological innovation from biomaterials. Yet, while life expectancy continues to increase, organ failure and traumatic injury continue to fill hospitals and diminish the quality of life. A number of organic and inorganic, synthetic or natural derived materials have been classified as not harmful for the human body and are appropriate for medical applications. These materials are usually named biomaterials since they are suitable for introduction into living human tissues of prosthesis, as well as for drug delivery, diagnosis, therapies, tissue regeneration and many other clinical applications. Advances in understanding disease and tissue regeneration combined with increased accessibility of modern technology have created new opportunities for the use of biomaterials in unprecedented ways. Materials can now be rapidly created and selected to target specific cells, change shape in response to external stimulus, and instruct tissue regeneration. Recently, nanomaterials and bioabsorbable polymers have greatly enlarged the fields of application of biomaterials attracting much more the attention of the biomedical community.

One such example is use of Biomaterials in Cardiac Therapy is mentioned below.

Synthesis and surface engineering of iron oxide Nanoparticles for Biomedical applications : 

Superparamagnetic iron oxide nanoparticles (SPION) with appropriate surface chemistry have been widely used experimentally for numerous in vivo applications such as magnetic resonance imaging contrast enhancement, tissue repair, immunoassay, detoxification of biological fluids, hyperthermia, drug delivery and in cell separation, etc. All these biomedical and bioengineering applications require that these nanoparticles have high magnetization values and size smaller than 100 nm with overall narrow particle size distribution, so that the particles have uniform physical and chemical properties. In addition, these applications need special surface coating of the magnetic particles, which has to be not only non-toxic and biocompatible but also allow a targetable delivery with particle localization in a specific area. To this end, most work in this field has been done in improving the biocompatibility of the materials, but only a few scientific investigations and developments have been carried out in improving the quality of magnetic particles, their size distribution, their shape and surface in addition to characterizing them to get a protocol for the quality control of these particles. Nature of surface coatings and their subsequent geometric arrangement on the nanoparticles determine not only the overall size of the colloid but also play a significant role in biokinetics and biodistribution of nanoparticles in the body. The types of specific coating, or derivatization, for these nanoparticles depend on the end application and should be chosen by keeping a particular application in mind, whether it be aimed at inflammation response or anti-cancer agents. Magnetic nanoparticles can bind to drugs, proteins, enzymes, antibodies, or nucleotides and can be directed to an organ, tissue, or tumour using an external magnetic field or can be heated in alternating magnetic fields for use in hyperthermia.

Coating of Biomaterials with emphasis on microwave technology : 

Research in medical science is always seeking new technologies which meet the growing demands of better, more stable and durable implant in body environment. An ideal implant should excel in both the basic requirements i.e. in mechanical properties and in biocompatibility. Till now, very few materials can fulfill both the needs. To cope up with the emerging demands, new inventions are always required. Mechanical properties of implant like strength besides adhesion and integration of implant with human tissue is of paramount importance. Recent development in this field led to development of various biocoatings which exhibit enhanced integration of implant with the human tissue. Various recent technologies used in coating of metallic implants.These alternative coating techniques have shown better adhesion to varieties of substrates. Microwave processing is emerging as an innovative technology in efficient, economic, effective manner with many advantages. An example of Microwave-Assisted Dip Coating of Aloe Vera on Metallocene Polyethylene Incorporated with Nano-Rods of Hydroxyapaptite for Bone Tissue Engineering is shown below.

Stem-cell-based Tissue Engineering of Murine Teeth :

Teeth develop from reciprocal interactions between mesenchyme cells and epithelium, where the epithelium provides the instructive information for initiation. Based on these initial tissue interactions, we have replaced the mesenchyme cells with mesenchyme created by aggregation of cultured non-dental stem cells in mice. Recombinations between non-dental cell-derived mesenchyme and embryonic oral epithelium stimulate an odontogenic response in the stem cells. Embryonic stem cells, neural stem cells, and adult bone-marrow-derived cells all responded by expressing odontogenic genes. Transfer of recombinations into adult renal capsules resulted in the development of tooth structures and associated bone. Moreover, transfer of embryonic tooth primordia into the adult jaw resulted in development of tooth structures, showing that an embryonic primordium can develop in its adult environment. These results thus provide a significant advance toward the creation of artificial embryonic tooth primordia from cultured cells that can be used to replace missing teeth following transplantation into the adult mouth.

Topological design and additive manufacturing of porous metals for Bone Scaffolds and Orthopaedic Implants : 

One of the critical issues in orthopaedic regenerative medicine is the design of bone scaffolds and implants that replicate the biomechanical properties of the host bones. Porous metals have found themselves to be suitable candidates for repairing or replacing the damaged bones since their stiffness and porosity can be adjusted on demands. Another advantage of porous metals lies in their open space for the in-growth of bone tissue, hence accelerating the osseointegration process. The fabrication of porous metals has been extensively explored over decades, however only limited controls over the internal architecture can be achieved by the conventional processes. Recent advances in additive manufacturing have provided unprecedented opportunities for producing complex structures to meet the increasing demands for implants with customized mechanical performance. At the same time, topology optimization techniques have been developed to enable the internal architecture of porous metals to be designed to achieve specified mechanical properties at will. Thus implants designed via the topology optimization approach and produced by additive manufacturing are of great interest. This paper reviews the state-of-the-art of topological design and manufacturing processes of various types of porous metals, in particular for titanium alloys, biodegradable metals and shape memory alloys.

Caffeine-catalyzed gels : 

Covalently cross-linked gels are utilized in a broad range of biomedical applications though their synthesis often compromises easy implementation. Cross-linking reactions commonly utilize catalysts or conditions that can damage biologics and sensitive compounds, producing materials that require extensive post processing to achieve acceptable biocompatibility. As an alternative, we report a batch synthesis platform to produce covalently cross-linked materials appropriate for direct biomedical application enabled by green chemistry and commonly available food grade ingredients. Using caffeine, a mild base, to catalyze anhydrous carboxylate ring-opening of diglycidyl-ether functionalized monomers with citric acid as a tri-functional crosslinking agent we introduce a novel poly(ester-ether) gel synthesis platform. We demonstrate that biocompatible Caffeine Catalyzed Gels (CCGs) exhibit dynamic physical, chemical, and mechanical properties, which can be tailored in shape, surface texture, solvent response, cargo release, shear and tensile strength, among other potential attributes. The demonstrated versatility, low cost and facile synthesis of these CCGs renders them appropriate for a broad range of customized engineering applications including drug delivery constructs, tissue engineering scaffolds, and medical devices.

The 3D printing of gelatin methacrylamide cell-laden tissue-engineered constructs with high cell viability : 

In the present study, we report on the combined efforts of material chemistry, engineering and biology as a systemic approach for the fabrication of high viability 3D printed macroporous gelatin methacrylamide constructs. First, we propose the use and optimization of VA-086 as a photo-initiator with enhanced biocompatibility compared to the conventional Irgacure 2959. Second, a parametric study on the printing of gelatins was performed in order to characterize and compare construct architectures. Hereby, the influence of the hydrogel building block concentration, the printing temperature, the printing pressure, the printing speed, and the cell density were analyzed in depth. As a result, scaffolds could be designed having a 100% interconnected pore network in the gelatin concentration range of 10-20 w/v%. In the last part, the fabrication of cell-laden scaffolds was studied, whereby the application for tissue engineering was tested by encapsulation of the hepatocarcinoma cell line (HepG2). Printing pressure and needle shape was revealed to impact the overall cell viability.