Regenerative Medicine and 3D Bioprinting: Polymers Sow the Seed of Life

Regenerative medicine and tissue engineering are mainstream areas of scientific research. In the USA alone at least US$200 mn is given each year to research institutions in public funding. Specialist polymers play a crucial role as materials for synthetically manufactured organs like the skin, trachea, bladder and liver, as well as artificial blood vessels and nerve guides. Resorbable polymers are used to make the cell scaffolds on which the organs are grown.

Bioprinting

Sirris, Belgium’s national research institute, has provided information on bioprinting—the ability to print various biological materials and cells along with tissue scaffold materials. It credits the Wake Forest Institute for Regenerative Medicine in Winston Salem, North Carolina, USA, as a leading research institute on the subject. Sirris is a leading centre for additive manufacturing—the ability to print layers of various materials such as polymers, ceramics and metals into intricate three dimensional objects.

Bioprinting is an arm of additive manufacturing. In general, resorbable polymer scaffolds are printed using stereolithography and then injected, or seeded, with stem cells from the target organ. Additive manufacturing is crucial to the process because it allows an individual product to be made from a single data file. The datafile is taken from a CT scan of the patient’s organ, providing the required geometry to give a perfect fit.

The goal of printing biological materials is to produce functional cells, tissues and organs to repair, replace or enhance biological function that has been lost by disease or injury. The method is seen by many as the most promising solution to meeting high demand for suitable organs for transplantation. To illustrate the gap between demand and supply, the number of available kidney donors in the USA stood at around 5,000 in 2003 while the number of patients needing a kidney transplant was around 55,000. The average cost of a single kidney was US$30,000 (source Sirris). In terms of livers, there were 114,300 people waiting for a transplant in 2012.

To give you an idea of how new this technology is, the method used for bioprinting, stereolithography, was invented in 1984. But it was not until 2004 that the first technology to engineer 3D tissues with cells and without the help of a scaffold was developed. In 2006 trials were undergone to implant the world’s first “3D printed” artificial bladders into seven patients. Medical Plastics News has not been able to confirm whether these trials were successful. To follow were the first 3D printed nerve guides in 2009 and blood vessels without the use of a scaffold in 2009 and 2010 respectively. And in 2011 and 2012 scientists built the first patches of cardiac and lung tissues as well as the first artificial trachea to treat cancer.

Looking forward, Sirris expects bioprinting to be capable of producing simple tissues for implant in a couple of years, albeit for clinical research purposes. Lobs or pieces of organs should be possible by 2030 or possibly longer.

To demonstrate the 3D printing procedure, Sirris quotes the artificial bladder produced and implanted in 2006 from the Wake Forest Institute for Regenerative Medicine. The process was called the Orthotropic Neobladder Procedure. Transplantation was successfully done that year on seven patients without any rejection of the organ.

Other successful examples include the development of an artificial liver using a 3D printed collagen skeleton seeded with human liver cells. The scaffold is then placed in a bioreactor to give it nutrients and oxygen and stimulate cell growth.

The world’s first artificial trachea was built using additive manufacturing at the University of Karolinska in Sweden. The trachea can be used to replace cancerous trachea. The material used is a synthetic nanocomposite polymer.

A portable skin printing system was successfully tested on mice in 2011. It uses living cells to create tissue-engineered skin grafts to cover burn wounds and is expected to be used to treat injured soldiers in the battlefield to stop bleeding without having to wait for an ambulance. Fibroblasts and keratinocytes are printed directly on to skin. Suspensions with cells are mixed with fibrinogen, type 1 collagen and thrombin at the moment of application.

In terms of bioprinting equipment, modified 3D printing machines are used, including those from inkjet printing companies Canon, Epson, HP and Lexmark, as well as USA-based companies Sciperio, Therics, Envisiontec, Neatco and Sandia NL.

Researchers expect there to be ethical and moral arguments against organ printing, particularly from the perspective of cell cloning.

Soon, Sirris aims to be able to use a bioprinter to print living tissues.

Lorenzo Moroni, assistant professor at the University of Twente’s tissue regeneration department in the Netherlands, has kindly provided the following account of additive manufacturing of resorbable scaffolds.

Three dimensional tissue scaffolds are porous materials which are able to interact with cells to let them proliferate and form a targeted tissue or organ. The choice of the most optimal additive manufacturing (AM) technology depends often on the desired resolution of the scaffold’s struts, the production speed, the versatility in working with different biomaterials, and the surface properties of the scaffolds.

As distinguished from all the other AM technologies, extrusion-based rapid prototyping (RP) systems offer a control over these parameters at the micron scale, without being limited to the roughness of the initial powder material used in other platforms like 3D printing and selective laser sintering, or the photosensitivity of the material required in stereolithography and two-photon polymerisation.

Extrusion-based RP systems have been extensively used to fabricate custom-made scaffolds and modulate their mechanical properties with encouraging results. Several thermoplastic polymers are, in this respect, today available for processing. These comprise poly e-caprolactone (PCL), polylactic acid (PLA), polytrimethylcarbonate (PTMC), polyethylene oxide terephthalate-co-poly butylene terephthalate, polymethylmethacrylate-co-butyl methacrylate, and copolymers thereof, among others.

This printing technology has progressed from single-head to multi-dispensing systems which allow depositing different materials at the same time to produce constructs with locally differing physico-chemical properties, thereby offering a further improvement to biomimicry strategies aiming at matching the natural milieu of the tissue to be regenerated when designing a scaffold. Most recently, a few open source systems have been also developed, promising to abate the initial investment costs.

Outlook: Despite these successes, a number of challenges are posed to the biofabrication community to empower extrusion based RP platforms.

A better control over the heating and extrusion mechanism is needed, so that also thermoplastic polymers with a lower thermal stability like PLA and copolymers can be easily processed without concerns over degradation during fabrication. New biomaterials are also needed, able to display tuneable surface properties for a more dynamic interaction with cells.

When mimicking the native architectures of tissues and organs is considered, a better control over the layer-by-layer process should be achieved, allowing the deposition of materials on curved surfaces without the need of supporting structures.

Production speed is also of concern when envisioning a large production of off-the-shelf scaffolds or of large scaffolds with anatomical shapes for personalised implants. In this respect, the continuous development of open source systems will facilitate these improvements, as it will be easier to access mechanical and electronic components, as well as the software governing the technology than currently available commercial systems.

References available on request.

Resorbable Polyurethanes

Medical Plastics News has learned that in 2010 USA-based Biomerix was looking into the use of bioresrobable polyurethanes for dual functional dressings to improve wound healing, particularly for diabetic ulcers and pressure sores. The researchers hope to use the polyurethane technology to develop dressings which can be used in negative pressure wound therapy (NPWT) while also acting as a scaffold for tissue regeneration.

The broader impacts of this research are in a variety of applications in tissue regeneration and repair for general, cardiothoracic, and plastic surgery; trauma, sportsmedicine, and fracture healing. The novel scaffold technology will be developed within the framework of large scale foam manufacturing methods using industrial foaming and thermal reticulation techniques. This will also reduce the cost of the biomaterial and substantially impact healthcare spending across a broad range of clinical application areas in the US.

The Biomerix team was awarded a grant of US$200,000 by the USA’s Small Business Innovation Research (SBIR) award in 2010.

In September 2012, Biomerix established a strategic distribution agreement with US cell culture technology supplier Synthecon for Biomerix’s three dimensional cell culture scaffolds.

In February 2013, US thermoplastic polyurethane manufacturer Lubrizol announced that it had developed bioresorbable polymers based on a polyurethane chemistry. When asked whether the announcement was linked to the biomerix project or something similar Lubrizol declined to comment.

When commenting on Lubrizol’s resorbable polyurethane, Tilak M Shah, CEO and CTO of USA-based processor of medical polymers and bioresorbable materials Polyzen said: “I am somewhat familiar with Lubrizol’s bioresorbable material on a polyurethane platform. They were working on it for a while very quietly until now, and I guess are now ready for the commercial launch.” 

Regarding Biomerix’s technology, he said: “I am not familiar with Bimerix’s grant work. Biomerix’s current wound dressing is based on bio-stable polycarbonate urea polyurethane. The grant is specifically for bioresorbable dressing, so it’s a good possibility they are working with Lubrizol’s bioresobable on urethane platform, which would create a similar foam scaffold.”

Spider Silk Garners Growth for Cells

According to a report on Qmed.com, researchers from Tufts University in Boston, USA, and the CNRS <i>Institut de Physiques de Rennes</i> in France have written about how implantable devices made from spider silk can spur regrowth of human tissue as they degrade.

The report states: “The researchers created the optical devices by pouring purified silk protein solution into moulds for microprism arrays. The speed at which the implants dissolve can be controlled by regulating the water content of the protein solution. Upon drying, the solution dries and forms a material resembling reflective tape.”

It goes on to say: “The researchers experimented with embedding the material with gold nanoparticles. When implanted in mice, these implants were illuminated with green laser light, thus heating the implants for use in thermal therapy to combat bacterial infections or destroy malignant cells. At the same time, the implant's optical properties enabled the scientists to monitor the process.”

And: “The researchers also experimented with embedding the material with the cancer drug doxorubicin. The drug remained stabile even when the material was heated to 60°C.”

Polycarbonates in Bone Repair

Scientists at the University of Southampton, UK, have created a new method to generate bone cells which could lead to revolutionary bone repair therapies for people with bone fractures or those who need hip replacement surgery due to osteoporosis and osteoarthritis.

The research, carried out by Dr Emmajayne Kingham at the University of Southampton in collaboration with the University of Glasgow, also in the UK, and published in the journal Small, cultured human embryonic stem cells on to the surface of plastic materials and assessed their ability to change.

Scientists were able to use the nanotopographical patterns on the biomedical plastic to manipulate human embryonic stem cells towards bone cells. This was done without any chemical enhancement.

The materials, including a biomedical implantable polycarbonate, offer an accessible and cheaper way of culturing human embryonic stem cells and presents new opportunities for future medical research in this area.


Professor Richard Oreffo, who led the University of Southampton team, explains: “To generate bone cells for regenerative medicine and further medical research remains a significant challenge. However we have found that by harnessing surface technologies that allow the generation and ultimately scale up of human embryonic stem cells to skeletal cells, we can aid the tissue engineering process. This is very exciting.” He added: “Our research may offer a whole new approach to skeletal regenerative medicine. The use of nanotopographical patterns could enable new cell culture designs, new device designs, and could herald the development of new bone repair therapies as well as further human stem cell research.”

The study was funded by the UK’s Biotechnology and Biological Sciences Research Council (BBSRC). This latest discovery expands on the close collaborative work previously undertaken by the University of Southampton and the University of Glasgow. As previously reported in Medical Plastics News, in 2011 the team successfully used plastic with embossed nanopatterns to grow and spread adult stem cells while keeping their stem cell characteristics; a process which is cheaper and easier to manufacture than previous ways of working.


Dr Nikolaj Gadegaard, Institute of Molecular, Cell and Systems Biology at the University of Glasgow, said: "Our previous collaborative research showed exciting new ways to control mesenchymal stem cell—stem cells from the bone marrow of adults—growth and differentiation on nanoscale patterns. “This new Southampton-led discovery shows a totally different stem cell source, embryonic, also responds in a similar manner and this really starts to open this new field of discovery up. With more research impetus, it gives us the hope that we can go on to target a wider variety of degenerative conditions than we originally aspired to. This result is of fundamental significance."