Hana Richter, Kristina Fuerst, Alex Howard, Colin Oakley

Nanofibers and Tissue Engineering

Tissue engineering, also known as regenerative medicine, utilizes various methods from engineering and science in order to help restore or improve biological functions.  Tissue engineering is also viewed as a way for "understanding the principles of tissue growth, and applying this to produce functional replacement tissue for clinical use" [1].  Nanofiber technology plays a key role in the progress of research in the field of tissue engineering, and is one of the most rapidly growing fields in the area of regenerative medicine.

In order for a particular material to be considered a "nanomaterial," at least one dimension of the material must be equal to or less than 100nm.  Nanofibers, with a diameter of equal to or less than 100nm, allow engineers to work on a scale invisible to the human eye.  This small size, along with the flexibility and strength of nanofibers make them very beneficial to the progress of technology in today's society, as they can be incorporated into many kinds of materials or structures, ranging from clothing to HVAC system filters, and even to artificial organ components.  The vast array of applications of nanofibers, in addition to the multiple available processes of production, make nanofibers a focal point in modern, cutting-edge tissue engineering.

A Necessity for Nanotechnology

The advantages that nanotechnology offers to society necessitate the need for advances within the field.  Many prevalent diseases often cause organ failures or tissue damage.  As older technologies are becoming realized as less ideal for treatment, nanotechnology is enabling tissues and organs to be maintained or improved, in a less invasive manner, which could open up endless opportunities for patients with no other alternatives.  Using nanofiber technology for scaffolding, which is elaborated on later, provides opportunities to treat congential heart defects,  regulate cell behavior, provide intervertebral disc repair and regeneration, provide joint repair and regeneration, regenerate cartilage, in addition to many more possibilities.   Organ donations, for example, cannot keep pace with rising demand, and mechanical devices are not always pratical (or even possible) because they are often incompatible with the living system.  These mechanical devices lack the similarities to the organ that would allow them to entirely replace native tissue.  Nanotechnology can provide organs and tissues for patients that last longer, thus reducing medical costs and medical attention incurred by individual patient, which is where it becomes obvious that applicable use of nanofiber technology in humans is essential for progress.

Funding for Nanotechnology

In order for the benefits of tissue engineering to match the demand of society, exploration in the field of nanofiber technology needs the confidence, and funding, from the scientific world.  Programs, such as the National Nanotechnology Initiative (NNI) have been established in order to fund projects that are developing Federal world-class research within the nanotechnology realm.  Many scientists see that it is crucial for the United States to be leading in nanotechnology research, and the NNI is just one of the efforts to make this possible.  According to the 2009 NNI budget, $1.5 billion dollars have been provided in order to 'support nanoscale science and engineering research and development at thirteen federal or national agencies' [2].  In 2001, the NNI budget spent a total of $464 million dollars, compared to a $1.4 billion dollars in 2006.  By viewing these statistics provided by the NNI website, it is apparent that as progress is made through funded research, more evidence arises as to how beneficial nanotechnology can be.  

History of Tissue Engineering

Tissue engineering, first conceived in the 1980’s is a relatively new biomedical technique.  Dr. Joseph Vacanti at the Harvard Stem Cell Institute essentially founded the field when he began researching the possibilities of using synthetic scaffolds for cell implantation.  Initial work centered primarily around investigating functional artificial tissue replacements composed of degradable polymers.  The public’s first introduction to Tissue Engineering was a BBC report on a creature created by Charles Vacanti, dubbed the “Auriculosaurus,” (or mouse with human ear) in 1997.  Despite its inception a decade prior, the term “tissue engineering” did not have the same meaning until a 1991 article in Surgical article,Technology International. Previously to the article, Tissue Engineering was used when describing prosthetics and the integration of biomaterials and non-organic materials concerning the human body. 

Previous Methods

Older methods of tissue engineering were far from perfect, but they represented a monumental step forward in medical research and an expansion in medical resources.  An initial method of tissue engineering was administration of a growth factor in vitro/in vivo.  Another venture was the growth of replacement tissues and organs for patients lacking specific functions. The goal of this approach was to provide a viable alternative treatment to patients waiting on long waiting lists for organ donations.  If synthetic replacements succeeded, more people could be treated at much lower costs.  A third technique was also explored – implantation of a natural or synthetic scaffold (which is where nanofibers are a promising future.  For example, in order to truly see the effect of a treatment in a patient, human trials are required.  Unfortunately for tissue engineers, there are numerous laws and regulations obstructing experimentation on humans (for good reason!).  Thus, without the ability to run in vivo trials in humans, researchers are forced to temporarily deal with animal body chemistries that are not identical to humans – the effects of tissue engineering may impact intended patients differently than they impact research animals.

Failures of Previous Methods

Groundbreaking innovations never come easily, and tissue engineering has encountered many setbacks over the course of its evolution.  For example, engineered tissues have been known to cause and immune response and be rejected by the host’s body – a serious problem posing extreme dangers to recipients of such tissues.  The implantation process has also occasionally caused massive cell die-offs and vascular disruptions in research subjects.  Additionally, the high level of cell replication creates potential for oncogene expression leading to (potentially malignant) tumor formation.  In addition to these health risks, there have been issues in “scaling up” tissue engineering practices from laboratory animals to actual human patients – techniques must be modified to remain safe, effective, and ethical for human trials, and modifications can cause loss of function. 

New Production Methods in Nanofiber Technology

The production of nanofibers to act as scaffolds for the generation of engineered tissues has surmounted the use of stem cells.  Stem cells were widely used to restore defective tissue due to their pluripotent capabilities and their ability to self-assemble.  Even with extensive research in the area of stem cells, issues pertaining to the maintenance and expansion of the undifferentiated population of cells and control of cell differentiation are faced.  Nanofibers, on the other hand, are ideal to use in tissue engineering because they are produced to mimic the extracellular matrix (ECM), are biodegradable over time, have a large surface area to volume ratio (accounting for the porous ECM), and are nonimmunogenic.  The ECM is a natural scaffold for cell, tissue, and organ growth.  It provides specific ligands for cell adhesion, regulates cell proliferation, function, and migration by providing different growth factors, and interacts with its environment in three dimensions.  The topology of the ECM plays an important role in cell behavior because its proteins (consisting of collagen fibers and elastin fibers, hyaluronic acid, proteoglycans, and other nanofibers such as fibronectin and laminin) provide binding sites for cells.  Nanofibers have the capability to mimic the intricate three-dimensional structure of the ECM and carry out its functions. 

Nanofiber Production

New processes to generate a matrix of nanofibers include those of phase separation, self-assembly, and electrospinning.  Self assembly is the process by which disordered components can naturally and spontaneously combine to form patterns.  In nanotechnology, engineers can create fibers with the ability to self-assemble into a desired structure.   This process is briefly portrayed in the video "Nanotechnology Self-Assembled Materials" (www.youtube.com/watch). In phase separation, there is a thermodynamic separation of the polymer into two different layers: polymer rich solution and polymer poor solution.  The short video titled "Simple Phase Separation" (www.youtube.com/watch) shows the very simplified idea behind this method of nanofiber production.

Electrospinning Process

Electrospinning is the most highly used method of nanofiber production due to its high production rate, simplicity of set up, and low production costs.  Electrospinning has the ability to produce many different types of nanofibers in the same matrix and can easily construct it to be however porous, rigid, or elastic as necessary.  In electrospinning, an electric field is used to draw a polymer stream out of solution.  The process involves a polymer solution that passes through a spinneret.  To create the extrusion force, a high voltage generates the electric field, such that the particles within the solution become charged, creating a repulsive force.  At a specific voltage, the repulsive force overcomes the surface tension of the solution and the solution ejects out of the spinneret in a jet stream.  The nanofibers form when the solvent evaporates after the solution ejects from the spinneret.  Entanglements of the polymer chains (nanofibers) prevent the stream from breaking apart.  The fibers are collected and patterned on a grounded plate. The diagram to the left labels each part of the electrospinning mechanism.  "Electrospinning" at www.youtube.com/watch shows exactly how these fibers are formed.

Citations

[1] MacArthur, B. D. & Oreffo, R.O.C. (2005). "Bridging the gap". Nature 433, 19.

[2] http://www.nano.gov/NNI_FY09_budget_summary.pdf

References

Electrospinning. October 30, 2008. Video. <http://www.youtube.com/watch?v=E1zuQEYGMJ0>

Nanotechnology Self-Assembled Materials. April 10, 2008.  Video. <http://www.youtube.com/watch?v=nJZU_EjK5JQ>

Simple Phase Separation. May 31, 2007. Video. <http://www.youtube.com/watch?v=nJZU_EjK5JQ> 

Fujihara, Kazutoshi; Ma, Zuwei; Ramakrishna, Seeram; Ramaseshan, Ramakrishna; Teo, Wee-Eong; Yong, Thomas. “Electrospun Nanofibers: solving global issues.” MaterialsToday. Elsevier Ltd. March 2006. <http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6X1J-4J95TSY-P&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&

version=1&_urlVersion=0&_userid=10&md5=7ad1af5de24c49df8240d64a25d00d60>

Hwang, Mark. “Nanofiber Technology: Designing the Next Generation of Tissue Engineering Scaffolds.” Department of Biomedical Engineering, Department of Anatomy and Neurobiology. VirginiaCommonwealthUniversity.  <http://imaging.bioen.uiuc.edu/yingxiao_wang/classes/BioE598_2008/MH%20Review%20Presentation.ppt>   

Mahato, Ram; Ye, Zhaoyang. “Role of Nanomedicines in cell-based Therapeutics.” Futuremedicine. February 2008. 

<http://www.futuremedicine.com/doi/abs/10.2217/17435889.3.1.5>

Paralysis 'Cure' Promised." BBC News July 14, 2006. December 13, 2008.  <http://news.bbc.co.uk/2/hi/science/natures/394454.stm.

Williams, D.F. "Biomaterials in Tissue Engineering." Sadhana Vol. 28, Parts 3 &4. June/August 2003. 563-574.

Vacanti, Charles. "History of Tissue Engineering and a Glimpse into its Future." Tissue Engineering. Vol. 2. No. 5. Mary Ann Liebert, Inc. May 2006. 1137-1142

http://www.medicalengineer.co.uk/pages/tissue-engineering/advantages-and-disadvantages-of-tissue-engineering.html

Title Image: <http://snsnano.com>