Everybody hopes for better health and restoration of impaired bodily function, and now that hope is illuminated by the promise of powerful biological tools that make human cells grow and replace human tissue. ASTM Committee F04 on Medical and Surgical Materials and Devices is taking the lead by defining some of those tools as standards that can be used for the development, production, testing, and regulatory approval of medical products.(1)

Medical implants fall into several categories, grouped as synthetic prostheses, artificial organs, transplants, and others. Historically, medical devices have been regulated by the U.S. Food and Drug Administration’s Center for Devices and Radiological Health, in some cases using consensus standards and guidelines. Some of these documents have been developed and/or adopted by FDA. Several other categories of products, i.e., those containing biologicals or drugs, are regulated by the FDA’s Center for Biologics Evaluation and Research and the Center for Drug Evaluation and Research. Products that combine cells and biomaterials and biomolecules fall within the areas of responsibility of several centers, and thus are regulated by interdisciplinary teams. These centers typically utilize “Guidance for Industry” documents to describe the regulatory framework used to regulate their products. Many standards exist for synthetic prostheses and have been developed with the help of standards- setting organizations, both national and international.

Advances in the basic and applied sciences over the past decade have led to an explosion in the research and development of medical applications for restoring the functions of human tissue/organ systems. Growth in spending has doubled from an estimated $246 million in 1995 to $504 million in mid 1999 (Lysaght, 1995, 1998). These efforts have led to promising therapies for tissue/organ defects and an increase in the number of regulatory applications for tissue engineered medical products. Thus there is a great need for standards for use by stakeholders in the field of tissue engineering.

What Is Tissue Engineering?

Tissue engineering is used in the new science of regenerative biology and medicine. Regenerative biology seeks to understand how some body tissues are replaced naturally, through the study of a variety of animal models, including lower vertebrates such as amphibians, which are strong regenerators. Regenerative medicine seeks to apply this understanding to restore the structure and function of damaged human tissues that do not naturally regenerate. Therapeutics for regenerative medicine include three approaches. The first is the transplantation of replacement cells into sites of injury or disease (cell therapy). The second is to induce regeneration in situ by the pharmacological suppression of inhibitory factors and/or supplying factors that stimulate regeneration (drug/ gene therapy). Tissue engineering is the third approach, and involves the construction in vitro of bioartificial tissues by seeding cells into scaffolds of artificial or natural materials, then using them as extracorporeal devices or implanting them into the body (combination device and biologic therapy) (Stocum, 1999).

Regenerative biology and medicine, including tissue engineering, is not a single discipline. Rather, it requires the multidisciplinary integration of biology, chemistry, physics, engineering, and clinical science. Regenerative medicine is currently limited in what it can do. Cell transplants have had some success in reversing the symptoms of Parkinson’s disease, Huntington’s disease, and diabetes. Such transplants are also useful in repairing cartilage and reconstituting the immune systems of cancer patients who have undergone extensive chemotherapy. Dermis has been successfully regenerated in burns or venous ulcers by the use of growth factors or artificial dermal matrices. However, these procedures and their results are far from ideal. Regeneration is not perfect and cell sources are in short supply. There is still a need for immunosuppression when foreign cells are used, and the long-term outcomes of the procedures have not yet been adequately assessed. Several products have reached various stages in the pre-market and investigational drug approval process (see the sidebar on previous page). The future, however, is promising. Within the next 20 years, we should be able to restore many more tissues via the transplantation, or stimulation in vivo, of regeneration-competent cells derived from a variety of sources.

Groundwork for the development of an effort in standards for tissue engineering proceeded within the U.S. FDA, via the FDA InterCenter Tissue Engineering Working Group, starting in 1994. Advances in research and development of tissue engineered products resulted in increases in product submissions to the FDA. The need for standardized terminology and test methods was therefore identified to facilitate product reviews.

Challenges in the Field of Tissue Engineering

Currently, the big roadblocks for the field of tissue engineering are:

• Inadequate understanding of the basic biology of regenerative processes (Stocum, 1999);
• Lack of adequate biomimetic materials to act as scaffolds for either induction of regeneration in vivo, or to build bioartificial tissues in vitro;
• Inadequate cell sources for transplantation or building bioartificial tissues;
• The problem of immunosuppressive regimens introduced by allogeneic and xenogeneic (foreign) cells; and
• Bioethical issues associated with the use of fetal and embryonic stem cells as sources. Recently published National Institutes of Health (NIH) Stem Cell guidelines will permit federal funding of human pluripotent embryonic stem cell research, with certain stipulations (Brower, 2000).

The industrial challenge lies mainly in the manufacture of biomaterials (Picciolo, 1997), cell culture media and expansion protocols, drug delivery systems, and gene vectors that are of reproducibly high quality. The nature of biomaterials varies widely according to their use, but there are common features all must have:
• Reproducible geometry and chemical composition;
• The incorporation of enough biological signals and cues to induce cell migration, proliferation, and differentiation;
• A rate of biodegradability that matches the rate of regeneration;
• Sterilizability;
• Toxicity;
• Non-induction of an inflammatory or immune reaction;
• Ease of surgical manipulation;
• Ease of manufacture from readily available materials; and
• Low cost.

The challenge for academic scientists is to understand the basic biology of tissue embryogenesis and regeneration. What are the physical and chemical signals that instruct or permit cell proliferation and differentiation? How widespread are regeneration-competent cells in the body and what are their characteristics? What is the differentiative capability of these cells? How can we evade the immune system when transplanting foreign cells? What distinguishes regeneration from the formation of scar tissue? What is the effect of tissue mass and age on regeneration? What can strong regenerators such as frogs, salamanders, and newts tell us about mechanisms of regeneration?

The major issues for government are maintaining or expanding the funding base for regenerative biology and medicine and the development of workable policies on stem cell research. The latter is difficult, and will require finding a way to pursue the potential benefits of this research within current moral and ethical codes. Programs such as the NIST Advanced Technology Program, which is directed to bring technology to the marketplace, will be instrumental in identifying promising approaches for expanding funding.

To achieve consistent evaluation across the various FDA centers, a team approach to product reviews is being used when products contain components that are appropriate to several centers. Getting potential therapies into clinical trials and setting reasonable time frames for successful therapies to be put into clinical practice is the challenge for the regulatory authorities. Evaluating clinical efficacy is difficult for these types of products because the use of “sham” controls (implantation of an inactive material) is ethically precluded by many situations. Surrogate endpoints with validated correlations to positive clinical performance must be established.

The patient needs to understand the nature, risks, and potential outcomes of a procedure when considering whether or not to be part of a clinical trial or to undergo a clinically approved procedure, particularly those involving the new technologies with their great promise.

Major issues for the physician are to thoroughly understand the science and technology behind a regenerative procedure so as to be able to recommend or not recommend a procedure, and to inform patients realistically of potential risks and benefits if recommended. In addition, physicians should play a major role in the development of clinical materials and procedures. In summary, the promise may be bright, however, the path is long and needs careful negotiating to preserve the Hippocratic oath to “do no harm.”

Role of Standards in Meeting the Challenges

Standards for tissue engineered medical products (TEMPs) will assist all of the stakeholders of ASTM Committee F04 in pursuing the development of safe and effective medical products that restore tissue function. Industry will benefit by having standardized procedures for producing and testing the products and the components that are used in the TEMPs. Academic scientists will be able to use the standards developed for various components that are used in their products.

Uniformity in regulatory review and approvals will be improved and expedited, as the opportunity for standards recognition by the FDA and self certification by the manufacturer are provided (Marlowe, et al., 1998). This will improve the quality of the products and also affords the patient and the doctor information on the expected performance of the products.

Strategy for Developing Standards

Advances in the field of medical applications of tissue engineering have come about due in part to understanding the role that the extracellular matrix plays in supporting cell survival, proliferation, and differentiation in vivo. Interaction with the extracellular matrix is an important mechanism for control of phenotypic structure and function of cells in a bioartificial tissue, resulting in the desired performance of the product to achieve tissue restoration. The substitute matrix supplied to the cells enables their maintenance and proliferation. These substitute materials act as scaffolds for the support of the cells and deposit of biomolecules, which signal the cells to migrate throughout the material and replace the tissue. Scaffolds also can be molded to the exact shape desired for the regrown tissue. The interplay of each of these components is necessary for functional tissue restoration. Thus, the development of standards for characterization, processing, and functionality of the components of bioartificial tissues will contribute to the validation of the performance of the product. Each of these, in turn, will be useful when these same components are used for other products.

Standards for interactions among the components and with the host tissue are critical to the validation also. The interactive effects of the product will have to be evaluated and test methods developed to measure the impact on performance in the long term.

Due to the fact that final products differ from one another, standards specific to each would be difficult to develop and keep up-to-date, however, common elements in these products could be standardized. Thus many products would benefit from a few standards.

Formation of Committee F04 Division IV on Tissue Engineered Medical Products

Because medical device regulation has been greatly facilitated by ASTM Committee F04 standards in the past, the marriage of the tissue engineering effort with that of Committee F04 was sought. The ASTM process for standards development and publication is useful and provides for consensus development by all important constituencies as well as an arm into the international standards arena. Thus the FDA approached ASTM to host the standards development effort for TEMPs. Dr. Peter Johnson, formerly of the Pittsburgh Tissue Engineering Initiative, was known for his effective efforts in formulating the initiative and accepted the challenge of the position as chair of what was to become Division IV of Committee F04.

Subcommittees and Task Groups

The organization of the standards effort began with the application of the Hoisin process to identify the important drivers of this technology. A group of experts were asked to describe the important features of TEMPs. These lists were then grouped and condensed into 10 categories. These categories became the Subcommittees for Division IV TEMPs of ASTM Committee F04 on Medical and Surgical Materials and Devices. To date, 40 task groups have initiated work on drafting standards. Information on the status of the efforts of each of these task groups can be obtained from the F04 home page.

Areas that are being developed encompass characterization and testing of the components used to prepare TEMPs, including the biomaterials used, the cells seeded on the scaffold biomaterials, and the other biomolecules needed. Our first approved standards are a guide to substrates for TEMPs and a guide to characterization of alginate, a natural material used in TEMPs. Other areas under draft are a guide to cells and cell processing, enumeration methods for cell counting, and bone and bone morphogenetic test methods.

Future Plans of Committee F04 Division IV

Efforts within Division IV will emphasize the recruiting of knowledgeable persons within the specific areas where additional standards are needed, the identification of which will help Division IV keep pace with the developments in the field. When more research describes the interactions among the various components of TEMPs, the ability to specify combinations that lead to success will enable development of this level of standard. The goal is to work toward performance standards for various product types that can be applied to incarnations of various component combinations (Omstead, et al., 1998; Christenson, 1997). As with any such efforts, the proof will be in the clinical manifestations of performance. Mapping of the standards against regulatory needs will be the basis for selecting areas for standards development.

The importance of the global economy will be driving the need for standards at the international level. Thus the ASTM efforts will preferably lead to partnering with the international community for the development of standards that would then be enabled for acceptance at the international level. This strategy will enhance the significance of the standards in varying scientific cultures as well as “stretching” the available workforce for this volunteer effort. Interactions with the International Organization for Standardization (ISO) as well as the European Commission for Standardization (CEN) have produced important decisions for the undertaking of standards development.

On a technical level, the comparative molecular analysis of regenerating vs. non-regenerating tissues in both mammals and lower vertebrates will be useful in identifying the differences in gene activity that distinguish regeneration from scar tissue formation. There may be novel genes in action that distinguishes these processes, or the same genes might be deployed in different spatial or temporal patterns of activity, or both. The human genome database can be culled for the orthologues of genes involved in the regeneration of animal tissues and the proteins they encode identified and tested for their therapeutic properties.

Understanding the biology of stem cells will be of crucial importance for the replacement of tissues through cell transplantation or for bioartificial tissue construction. We need to know what signals and cues are necessary to direct the differentiation of stem cells into specific phenotypes, the nature and range of plasticity of stem cells found in adult tissues, and how to avoid immunorejection of cells, either as transplants, or as part of bioartificial tissues. Advances are being made on all these fronts, and we can anticipate substantial breakthroughs in the near future.

With these future possibilities, the importance of the ASTM TEMPs standards work is accentuated. Persons with appropriate expertise are encouraged to join us in this exciting and rewarding activity. //

Footnote
(1) G. L. Picciolo, K. B. Hellman and P. C. Johnson, 1998. “Tissue Engineered Medical Products, Standards: The time is ripe!” Tissue Eng. Vol. 4, No. 1, pp. 5-7.

References
V. Brower, 2000. “NIH Stem Cell Guidelines” Gen. Engin. News, Vol. 20, No. 16, Sept. 15, 2000.
M. S. Chapekar, 2000. “Tissue Engineering: Challenges and Opportunities” Applied Biomaterials, in press.
L. Christenson, 1997. “Outcomes Research Is Crucial for Tissue-Engineered Products” Gen. Engin. News, Jan. 15, 1997; Vol. 17, No. 2, p. 1.
M. J. Lysaght, 1995. “Product Development in Tissue Engineering” Tissue Eng., Mary Ann Liebert, Inc. Larchmont, N.Y., Vol. 1, No. 2, pp. 221-228.
M. J. Lysaght, N. A. Nguy, K. Sullivan, 1998. “An economic survey of the emerging tissue engineering industry.” Tissue Eng., 1998, Fall; 4(3): 231-238.
D. E. Marlowe, P. J. Phillips, 1998. “FDA Recognition of Consensus Standards in the Premarket Notification Program” Biomedical Instrumentation & Technology, Hanley & Belfus, Inc., pp. 301-304.
D. R. Omstead, L. G. Baird, L. Christenson, G. Du Moulin, R. Tubo, D.D. Maxted, J. Davis, F.T. Gentile, 1998. “Voluntary guidance for the development of tissue-engineered products” Tissue Eng. 1998 Fall; 4(3): 239-66. Review.
G.L. Picciolo, 1997. “Enabling biomaterials technology for tissue engineering: Introduction” Tissue Eng., 1997, 3/1 (67-70).
D.L. Stocum, 1999. “Regenerative biology: A millennial revolution.” Seminars in Cell and Developmental Biology 10: 433-440.
Also see footnote above.

Copyright 2001, ASTM