The Time Of Modern Biomaterials Health And Social Care Essay

To restore body defects, nature has engineered products since ever, "at no costs and introducing no inflammation or rejection side effects" [1]. The efficiency of these tissue engineered products is however limited to occur only during the healing process in healthy individuals, and not in compromised patients [1]. While contemplating what nature is doing, human intelligence has been challenged from the prehistoric times to find solutions for filling bone defects, replacing body parts, and healing wounds. Materials such as bone, teeth, wood, metal, ivory and even coral were handled in the aim of restoring body aesthetic and functional integrity. These materials can be considered the first biomaterials ever used; they were used long time before the field of biomaterials emerged in the modern scientific research. In a recent review [2], Prof. Dorozhkin captures our attention by resuming the most remarkable historical evidences on the existence of biomaterials. "The artificial generation of tissues, organs or even more complex living organisms was throughout the history of mankind a matter of myth and dream. Unfortunately, due to the practice of cremation in many societies, little is known about the prehistoric materials used to replace bones lost to accident or disease. Nevertheless, according to the available literature, introduction of non-biological materials into the human body was noted far back in prehistory."[2] The list of archeological evidences supporting this statement starts with the "Kennewick Man" up to 9000 years ago. With this respect, Prof. Dorozhkin [2] commented: "This individual, described by archeologists as a tall, healthy, active person, wandered through the region now known as southern Washington with a spear point embedded in his hip. It had apparently healed in and did not significantly impede his activity. This unintended implant illustrates the body’s capacity to deal with implanted foreign materials. The spear point has little resemblance to modern biomaterials, but it was a tolerated foreign material implant, just the same" [2].

Other similar examples have fascinated the specialists in biomaterials and regenerative medicine; among them there are archaeological testimonies on the innovative spirit of Hindu, Egyptian and Greek civilizations with respect to body parts replacement or repair using interesting techniques or materials. The following examples captivated the attention of high tenure specialists in the biomaterials field and are included in most of the lectures and books on the history of biomaterials:

(1) Wooden prostheses - designed by ancient Egyptian physicians to help their ailing patients. The oldest known prosthesis is a wooden toe fixed with textile lace on the foot of a ~3000 years old female mummy (studied by the German researchers at Ludwig Maximilians University in Munnich) [2-3]. The researchers found evidences on low tissue remodeling after the undoubted toe amputation presumably due to an artery disease [3]. It should be mentioned here that, unless other added fake parts of the body exclusively aimed to provide body integrity for the afterlife journey, this wooden prosthesis presented clear evidence of being used by that woman during her life.

(2) Other biomaterials used by our ancestors include: (i) gold wires and linen thread for ligatures in the repair of bone fractures (apparently used by Hippocrates ca. 460 BC–370 BC), (ii) lint for filling of large cavities, (iii) artificial dental bridges carved from oxen bones by Etruscans, (iv) gold wires used by Phoenicians to bind loose teeth together, (v) dental amalgam to repair decayed teeth (in the year 659 AD) [2].

(3) A black leg grafted to a white patient - this famous miracle of Sts. Cosmas and Damien was described by multiple paintings and works and it was considered as stating for an early vision on regenerative medicine. Among theses works, the painting "The Healing of Justinian by Saint Cosmas and Saint Damian" by Fra Angelico (ca. 1395 – 1455) is well-known.

(4) Furthermore, the Greek mythology also rings a bell on the regenerative medicine through the idea of regenerating liver in the myth of Prometheus sentenced to eternal suffering "as an eagle ate his liver for eternity while the liver regenerated" [4].

(5) Impressively, in addition to amputation practiced by Egyptians, trepanation appears as another radical surgical procedure performed by the Incas 500 years ago to treat cranial trauma including intra-cranial pressure. Peru is reported to be the richest area in terms of prehistoric trepanned skulls. However, Verano and Andrushko, anthropologists at Tulane University and Southern Connecticut State University, respectively, reported that little evidence has been found on the assertion that cranioplasty was a common procedure in Peru. Furthermore, the well-known Kanamarca skull is the first "unequivocal case in the Prehispanic Americas where a bone plug is removed and replaced in the trepanation opening" [5]. Another example of old prehistoric trephination procedure is the Crichel Down skull, excavated in England in 1938.

The Time of Modern Biomaterials.

Regeneration of body parts became even a more critical issue in the modern times we are living, when longevity and life quality are tremendously increasing in connection with the rapid evolution of science and technology. Higher life standards also involved a turnover in the type of pathologies the medicine is facing and, in the last 30 years, more than 60% of all deaths are considered to be due to lifestyle diseases (when compared to influenza, heart disease, pneumonia, and cancer - the main causes of death before 1990's). However, it can be considered that the main success of the second half of the 20th century consists in increasing life expectancy: from 58 years at the beginning of the 20th century to 78 years today. And this statistic is expected to still improve in the near future.

The regeneration capacity of body parts in humans is limited, while most of the "gold standard" therapies remain still the autologous grafts of limited availability and associated to additional morbidity, pain, or even infection at the donor site. In addition to autograft, transplantation is another important and effective long-term treatment for a wide range of health problems. However, recent statistics indicate the reduced availability of donors when compared to the number of suffering patients needing a transplant (i.e. liver donors are available for only 20% of the patients needing liver transplants). Therefore, in the last 20 years, scientists joined their efforts in a multi- and interdisciplinary approach devoted to finding alternatives for transplants.

In this context, one may schematically divide the evolution of implantable materials into four main stages as follows:

The 1st generation of biomaterials (approximately 60 years ago) was represented by already available materials, suitable for implantation. With such materials the success was strictly accidental rather than rationally designed. Examples: gold fillings, bone cements, breast implants, glass eyes, even parachute cloth for vascular implants...

The 2nd generation of biomaterials was designed using common materials with initial non-medical applications. Scientists developed new biomaterials through joint efforts of physicians and engineers. This was the moment when the idea of multidisciplinarity appeared.

The 3rd generation of biomaterials consisted in bioengineered materials developed by the joint effort of biologists, chemists, physicians and engineers. These biomaterials are provided with bioactivity and, very important, with biomimetic properties.

The 4th generation of biomaterials consists in tissue engineered materials. This is the most recent and modern approach. Its development started somewhere about 20 years ago.

Tissue Engineering – Definition and Dimension of the Field.

"When nature finishes producing its own species, man begins, using natural things in harmony with this very nature, to create an infinity of species." This quotation of Leonardo da Vinci was used by Jean-Marie Lehn, a Nobel laureate, when referring to the present and future of Supramolecular Chemistry. In our opinion, from this epitome just two terms were missed - Tissue Engineering.

"Since the early 1990s, the research in biomedical field has been dominated by a paradigm shift whereby the concept of replacing damaged tissues and organs with biomedical devices has been overcome by the goal of their partial or complete regeneration." With this phrase, Professor Matteo Santin (School of Pharmacy and Biomolecular Biosciences from University of Brighton), one of the most respected contemporaneous scientists starts his chapter on high-performance and industrially sustainable tissue engineering, in his book Strategies in Regenerative Medicine edited in 2009 [4]. Further, in this section, different key scientific moments and approaches will be mentioned, in order to briefly refer to the birth of the tissue engineering as a tool to achieve tissue regeneration.

A lot has been told on the first experiments to generate tissues. Generating tissues is equivalent to rebuilding lives and such a pro-life aim deserves all efforts. Approximately 40 years ago, in a pioneering experiment by Dr. W.T. Green, chondrocytes were cultured on bone fragments and the resulting construct was implanted in mice to generate cartilage [6]. But this assumption did not lead to the expected result. However, this experiment was recognized by tissue engineering professionals [4] as one of the stepping stones of the development in this field; despite being unsuccessful, it allowed a very important and forthcoming vision on the importance played by the cell culture substrate in providing an appropriate environment for the desired cellular events to occur.

Some years later, tissue engineering officially emerged with the pioneering vision of Langer and Vacanti who announced in Science [7] that "a new field, tissue engineering, applies the principles of biology and engineering to the development of functional substitutes for damaged tissue" as a response to the fact that "the loss or failure of an organ or tissue is one of the most frequent, devastating, and costly problems in human health care". And this was the first definition ever. The terms Tissue Engineering and Regenerative Medicine are often used with the same meaning. Therefore in many published materials they do appear as Tissue Engineering/Regenerative Medicine. It is however well established that Tissue Engineering develops the biomaterials required by the Regenerative Medicine.

Furthermore, the introduction of both concept and new research field had a strong impact over the scientific community who received this event with huge enthusiasm. Numerous research groups devoted their efforts to this promising field. An indication on the increasing dimension of this research field is given by the fact that more than 46288 scientific research articles were published from 1993 to date. Among them, more than 199 are review articles on different tissue engineering related-aspects, with an interesting subject-evolution (information extracted from PubMed at http://www.ncbi.nlm.nih.gov/pubmed):

1995-2000: bone regeneration

2001-2006: protein- and gene-based tissue engineering, vascular grafts, bone repair, stem cells, growth factors, angiogenesis

2007-2012: bone engineering using genetically modified cells, bone regeneration, growth factors and endothelial cells in therapeutic angiogenesis, nanotechnology, gene therapy, stem cells, osteoblasts in bone tissue engineering, hypoxia inducible factors and mimicking agents in guided bone regeneration, osteogenesis and angiogenesis

Scheme 1. Number of Tissue Engineering Reviews published in the interval 1995-2012, according to PubMed [8]

Another indication on the development and importance of this research field is that 10 journals are core-devoted to Tissue Engineering: Tissue Engineering, Parts A, B, & C; Journal of Tissue Engineering, Journal of Tissue Engineering and Regenerative Medicine, Journal of Biomaterials and Tissue Engineering, Journal of Tissue Science & Engineering, Current Tissue Engineering, Journal of Biomimetics, Biomaterials, and Tissue Engineering.

Moreover, medical technology companies appeared into the industrial landscape, having as core or satellite activities the development, manufacturing and marketing of tissue engineered products. Among them: Integra LifeSciences, LifeCell Corporation, Organogenesis, Smith and Nephew, Advanced Tissue Sciences, Genzyme Biosurgery, Cook Biotech, Forticell Bioscience, Zimmer. In 2003 the European Commission identified 113 tissue engineering companies in EU [9]. A recent list with biomaterial-related companies was published [10].

The increasing interest this field received is also reflected by the fact that Societies for Tissue Engineering were funded over time: (1) TERMIS-EU (formerly European Tissue Engineering Society); (2) The Tissue and Cell Engineering Society (TCES), UK; (3) Korean Tissue Engineering and Regenerative Medicine Society; (4) The Japanese Society for Tissue Engineering, (5) Tissue Engineering and Regenerative Medicine International Society (TERMIS) (2005).

Nowadays, TERMIS covers under its umbrella three regional chapters as follows [11]:

TERMIS-EU (Europe);

TERMIS-AM (Americas);

TERMIS-AP (Asia-Pacific).

Among other tasks, such organizations are involved in promoting the development of this field in both academia and industry. Another institution activating in this young and powerful research field, the National Tissue Engineering Center (NTEC) was established by the U.S. Congress, in 2002 [12]. All these data state for the explosive evolution of a research field of maximum importance.

In this light, starting with the above mentioned definition given by the parents of the field, and considering the most recent advances, we recall two more recent definitions:

(i) The NIH definition of Tissue Engineering/Regenerative Medicine [13]:

"Tissue engineering / regenerative medicine is an emerging multidisciplinary field involving biology, medicine, and engineering that is likely to revolutionize the ways we improve the health and quality of life for millions of people worldwide by restoring, maintaining, or enhancing tissue and organ function. In addition to having a therapeutic application, where the tissue is either grown in a patient or outside the patient and transplanted, tissue engineering can have diagnostic applications where the tissue is made in vitro and used for testing drug metabolism and uptake, toxicity, and pathogenicity. The foundation of tissue engineering/regenerative medicine for either therapeutic or diagnostic applications is the ability to exploit living cells in a variety of ways. Tissue engineering research includes biomaterials, cells, biomolecules, engineering design aspects, biomechanics, informatics to support tissue engineering and stem cell research."

(ii) The Pittsburgh Tissue Engineering Initiative definition:

"Tissue engineering is the development and manipulation of laboratory-grown molecules, cells, tissues, or organs to replace or support the function of defective or injured body parts. Although cells have been cultured, or grown, outside the body for many years, the possibility of growing complex, three-dimensional tissues - literally replicating the design and function of human tissue - is a recent development. The intricacies of this process require input from many types of scientists, including the problem solving expertise of engineers, hence the name tissue engineering. Tissue engineering crosses numerous medical and technical specialties: cell biologists, molecular biologists, biomaterial engineers, computer-assisted designers, microscopic imaging specialists, robotics engineers, and developers of equipment such as bioreactors, where tissues are grown and nurtured." [14]

1.2. Tissue Engineering approaches

As a general logo, it can be said that regenerating tissues are rebuilding lives. To succeed in this goal, two tissue engineering approaches were rapidly developed:

in vivo tissue engineering - based on scaffolds able to recruit endogenous cells for tissue repair and

in vitro tissue engineering - using implantable tissue-like constructs obtained from microenvironments generated ex vivo.

Generally speaking, tissue-engineered substitutes fall in one of the two categories:

acellular or

cellular.

To understand better the difference between the two classes, we will further refer to skin substitutes. According to the Current Procedural Terminology of the American Medical Association [15], skin regeneration acellular products such as cadaveric dermis without the cellular material "contain a matrix or scaffold composed of materials such as collagen, hyaluronic acid, and fibronectin". On the other hand, cellular tissue-engineered skin substitutes are products containing living cells such as fibroblasts and keratinocytes within a matrix [15]. The acellular products are easier to bring on the market since they do not contain cells. Good examples in this sense are AlloDerm® (LifeCell Corporation) and the two-layer INTEGRA® Dermal Regeneration system.

Furthermore, in an in vitro tissue engineering approach there are several actors that join their efforts in order to ensure the success:

the scientists (i) engineer biomaterial scaffolds able to template and support tissue formation, (ii) cultivate cells into the matrix and (iii) place the construct in a bioreactor to ensure the environment needed for cellular activity;

the cells, the real "tissue engineers", start engineering the tissue in vitro; at a certain development level, the incubation is stopped and the first generation of tissular constructs is obtained;

the surgeon implants the tissular first generation construct into the host, where maturation and integration are expected to occur leading to tissue ingrowth/regeneration [16].

Once this intimate collaboration between scientists and cells elucidated, it is time to move to another important issue, the applications of tissue engineering products:

* Obtaining functional grafts is the first and most well-known aim of this domain.

* The second field of application is the better understanding of cell behaviour (especially stem cells) in three-dimensional (3D) systems.

* Last but not least, tissue engineered constructs and tissues may be used as model system to understand both physiological as well as pathological phenomena. [16]

Despite tremendous research supported by both public and private investment, clinical advances in tissue engineering are considered to be still too slow than initially expected and required. Furthermore, the regulatory bodies are responsible for protecting the human health and therefore they are very exigent with respect to the approval of tissue engineered products. A good example in this sense is the fact that FDA (U.S. Food and Drug Administration), the leader in safety regulation, proposed on February 28th 1997 an Approach to the Regulation of Cellular and Tissue-based Products [17]. In this document it was mentioned that "cellular and tissue-based products and their potential uses are too diverse for a single set of regulatory requirements to be appropriate for all. In an effort to develop a comprehensive scheme that would treat like products alike, but that would establish appropriate regulatory distinctions among cellular and tissue-based products in areas where there were differences, the agency identified the principal public health concerns and attendant regulatory issues associated with the use of these products." Among the "Public health and regulatory concerns associated with cellular and tissue-based products" the following are listed: "A) How can the transmission of communicable disease be prevented? B) What processing controls are necessary, e.g., to prevent contamination that could result in an unsafe or ineffective product, and to preserve integrity and function so that products will work as they are intended? C) How can clinical safety and effectiveness be assured? D) What labelling is necessary, and what kind of promotion is permissible, for proper use of the product? E) How can the FDA best monitor and communicate with the cell and tissue industry?" [17]. The document further concludes: "With these concerns in mind, the FDA differentiated cells and tissues and their uses by their risk relative to each concern, so as to enable the agency to provide only that level of oversight relevant to each of the individual areas of concern" [17]. On the other hand, in 2001, the European Commission’s Scientific Committee on Medicinal Products and Medical Devices "came to the conclusion that human tissue-engineered products are not appropriately covered by any European regulatory framework" and therefore "A European level regulation was considered essential to guarantee safety and quality of tissue-engineered products applied and traded within Europe or being imported from overseas." [9].

The Council of the European Communities ensures also a high level of human health and safety, after the last amendment of Council Directives 90/385/EEC relating to active implantable medical devices and 93/42/EEC concerning medical devices for the benefit of patients, consumers and healthcare professionals [18].

All these emerged in complicated rules the scientists and companies developing tissue engineered materials should respect. However, the fact that tissue engineered products are targeted to save lives makes these concerns reasonable and emphasizes another dimension of the complexity characterizing this novel and extraordinary research field.

1.3. Tissue Engineering Classification

Tissue engineering can be classified using different criteria as described in the following table:

Table 1. Tissue Engineering - classification.

Criterium

Classes

Tissues to be regenerated

Soft Tissue Engineering: tissue-engineered nervous system, tissue-engineered skin substitutes, cardiovascular tissue-engineering, tissue-engineered ligaments and articular cartilage;

Hard Tissue Engineering: Bone tissue engineering

Aim the products are used for

Therapeutic Tissue Engineering

Diagnostic-devoted Tissue Engineering

Use of cells

Cellular Tissue Engineering:

(i) Cells are cultivated on a biomaterial scaffold in a bioreactor before implantation

(ii) Cells and/or bioactive factors are delivered

Acellular Tissue Engineering:

(iii) a bioactive scaffold is used as an instructive environment to in vivo recruit and host cells to regenerate tissues. [16]

Type of cells

Tissue Engineering based on cells from exogenous sources

Tissue Engineering based on autologous cells

In vitro / In vivo

In vivo approaches: (i) and (ii) above

In vitro tissue engineering: (iii) above [16]

Scaffold involvement

Tissue-engineered constructs: scaffold with or without cells

Scaffold-free tissue engineering [16]

1.4. Biomedical applications requiring porous scaffolds

Regenerative medicine needs tissue engineered materials for multiple conventional and unconventional pathologies: trauma, wound healing, ophthalmologic injuries, respiratory and cardiovascular diseases, nervous system, ligament and tendons, hard tissues repair (bone, joints and teeth). Nature provided the human body with extremely complicated and complex structure. For such a brave and challenging aim as generating tissues, scientists should start from understanding the composition, structure, morphology, architecture, regulation mechanisms, physiology and pathology of the target tissue. View the complexity of most tissues, porous scaffolds become crucial for a wide array of applications ranging from hard tissue grafts to soft tissue substitutes. Each of these application fields includes a wide array of sub-classes devoted to the specific needs of the target tissue or organ. Accordingly, soft tissue engineering is concerned with the development of artificial skin, biological substitutes for adipose tissue, for cardiovascular and nervous system, and for artificial lungs and kidneys; hard tissue engineering focuses on bone and teeth regeneration.

Under the form of membranes, sintered or self-assembling particles, bead-based structures, entangled fibrous bodies, meshes, foams, sponges or solid porous blocks, porous matrices should be designed to provide appropriate biomechanical and microarchitectural features in addition to specific chemical structure and bioactivity to optimally assist the cascade of ordered cellular events associated to tissular regeneration.

The porosity is defined as the ratio of void space in a solid and it is a morphological property of a material. The following characteristics are essential in allowing and stimulating cell collonization, tissue ingrowth and, nevertheless, angiogenesis occurrence:

- size,

- distribution,

- connectivity of the pores.

In the design of biomaterials, macroporosity is usually characterized by pores superior to 50 µm, while microporosity is associated with pores of approximately 10 µm. A wide range of porosity inducing methods have been developed, ranging from classical techniques such as particulate leaching, particles sintering, assembling fibers, multilayer deposition, thermal treatments (thermally induced phase separation (TIPS) and melt co-continuous polymer blending (MCPB)), to unconventional tools such as laser ablation, computer assisted 3D-printing, electrospinning, or even combinations of the above. However, obtaining scaffolds with predefined porosity remains a challenging task. For example, for bone regeneration, providing an open porosity with pores between 100 to 500 µm is essential. On the other hand, the architectural strict requirements should be fulfilled in addition to providing biocompatibility, appropriate stability or degradability, fluid and gas permeability, and, extremely important, mechanical strength for load bearing applications. Furthermore, characterization methods have been developed to qualitatively and quantitatively assess porosity.

Coming back to the development of porous scaffolds for tissue-engineered products, the ever-increasing dimension of this research field can be suggested through mentioning here that an extensive body of literature reports on different aspects related to the importance of porosity for tissues regeneration. A simple search on PubMed using the key words "porous scaffold" generates 2028 articles [19]; the key words "porous hydroxyapatite" are found in not less than 1860 works [20], while the terms "porous tissue engineering" are identified in 2913 publications [21].

2. Tissue engineering constructs

As previously stated, nature provides genetically engineered living bodies with hierarchical architecture up to nanoscale, self-assembling and self-repair ability (musculo-skeletal defects under critical level of damage have the ability to self-repair, while hard tissues constantly remodel).

The current and future trends in the design parameters of biomaterials for tissue engineering are based on composition-structure-properties relationship, moving from the acellular approach to a rational combination of biomimetic scaffolds (polymers, ceramics, metals, composites), biomolecules, cells, and even engineering problems. The scientists developing tissue engineered products have learnt to accept and respect the intimate relationship between function, form and structure. Some key aspects are further discussed.

2.1. Constitutive elements

The definition of tissue engineered products is a structure oriented, as previously stated in the NIH definition of tissue engineering [13]. Accordingly, a tissue engineered construct is built up from the following constitutive elements:

"1) Biomaterials: including novel biomaterials that are designed to direct the organization, growth, and differentiation of cells in the process of forming functional tissue by providing both physical and chemical cues. 

2) Cells: including enabling methodologies for the proliferation and differentiation of cells, acquiring the appropriate source of cells such as autologous cells, allogeneic cells, xenogeneic cells, stem cells, genetically engineered cells, and immunological manipulation.

3) Biomolecules: including angiogenic factors, growth factors, differentiation factors and bone morphogenetic proteins.

4) Engineering Design Aspects: including 2-D cell expansion, 3-D tissue growth, bioreactors, vascularization, cell and tissue storage and shipping (biological packaging).

5) Biomechanical Aspects of Design: including properties of native tissues, identification of minimum properties required of engineered tissues, mechanical signals regulating engineered tissues, and efficacy and safety of engineered tissues.

6) Informatics to support tissue engineering: gene and protein sequencing, gene expression analysis, protein expression and interaction analysis, quantitative cellular image analysis, quantitative tissue analysis, in silico tissue and cell modeling, digital tissue manufacturing, automated quality assurance systems, data mining tools, and clinical informatics interfaces.

Stem cell research - Includes research that involves stem cells, whether from embryonic, fetal, or adult sources, human and non-human. It should include research in which stem cells are isolated, derived or cultured for purposes such as developing cell or tissue therapies, studying cellular differentiation, research to understand the factors necessary to direct cell specialization to specific pathways, and other developmental studies. It should not include transgenic studies, gene knock-out studies nor the generation of chimeric animals." [13]

Tissue engineering constructs are complex 3D structures specific to both product type and intended use, designed to be active and to remodel after implantation. They are usually intended to be produced in small lot sizes (even one) and present heterogeneous composition. The construct specification from the in vitro studies is accepted that may not be predictive about clinical safety and efficacy. [22]

View the scientific background of the authors, further in this work, the interest will be exclusively devoted to the development of scaffolds for tissue engineering applications. The topics will include the selection of the materials and elements of design, in particular the control of biodegradability and porosity. We will share some of our experience in engineering polymer biomaterials for tissue engineering uses.

2.2. Polymer scaffolds for tissue engineering

Since the emergence of Tissue Engineering field, different techniques and materials were used to produce scaffolds: natural, ceramics, polymers, composites or multicomponent complex structures. Each application requires one or another of the above, and the selection is usually based on the correlation between the biological and functional requirements and the composition, structure and characteristics of the material. The interaction of the scaffold with cells, fluids and tissues is strongly dependent on the chemistry of the material (surface and bulk) but also on physico-chemical features that can decisively contribute or affect protein adhesion, cell adherence.

Polymers represent interesting and versatile compounds displaying a large panel of properties (chemical structure, surface properties, bulk physico-mechanical properties and modifications in different environments and as a function of time) that make them suitable for a wide range of tissue engineering applications. These compounds can be processed through various methods to produce films, blocks, fibers or particles with compact or porous architecture. Furthermore, their structural and properties resemblance with the constituents of the extracellular matrix (ECM) renders them useful when addressing scaffold design.

Depending on their origin, polymer molecules fall in one of the categories: (i) natural (i.e. collagen), (ii) semi-synthetic/artificial, (iii) synthetic. Table 2 synthetically presented some of the polymers used to develop tissue engineering scaffolds.