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Organs by Inkjet

Inkjet printers can already print living cells.

Inkjet printers can already print living cells.

By Cameron Ferris

The development of a new biological ink takes us one step closer to the goal of printing living cells in three dimensions to create whole organs.

Advances in medical devices and our ability to transplant tissues and organs from one patient to another have significantly improved the quality and longevity of life, but these approaches don’t provide all the answers. Artificial devices can be short-lived and don’t function exactly like a living tissue, while organ transplants are complicated by issues with rejection and a critical shortage of donors.

One radical solution is to create replacement body parts on-demand using a patient’s own cells – and the field of tissue engineering aims to do just that. By combining the cells with synthetic or natural biomaterials, tissue engineering creates living substitutes that are intended to look and behave like the real thing.

The prospect of tissue engineering is extremely exciting, yet the science is equally challenging. The human body is complex: multiple cell types, biomaterials and other biomolecules are positioned within a tissue in specific three-dimensional arrangements that are critical to its function.

To enable the fabrication of structures that mimic the complexity of a real tissue, some tissue engineers are turning to a familiar technology – printers. On a daily basis we reproduce text, graphics and highly detailed photos using printers that accurately place inks on paper. In a very similar way, printers can accurately place cells and biomaterials to fabricate new tissues.

To explain this technology further, let’s look at three important components required to make cell printing happen: the bio-printer, the bio-ink and the bio-paper.

The Bio-Printer

Many different types of printers are able to print living cells. Some squeeze cells out of a needle, just like toothpaste. Others use a laser to blast cells off a ribbon and onto a target. The most common printers, though, and the ones we use in our research, are inkjet printers.

This is likely to be the type of printer that is sitting on your desk at home. Open the cover and you will find print heads containing different coloured inks. Very small nozzles on the base of these print heads fire tiny droplets of ink onto the page precisely when they are required, all coordinated and controlled by computer-fed data.

Inkjet printing technology is well-developed, inexpensive and capable of very high resolution patterning. It can produce features that are close to the size of individual cells – perfectly sized building blocks to construct a tissue drop-by-drop. This makes it very attractive as a cell printer.

However, carrying living cells through the printer and the tiny nozzles of the print head is tricky, and requires special ink. Getting this bio-ink right is one of the biggest challenges in cell printing, and has been the main focus of our research.

The Bio-Ink

The simplest bio-inks are essentially salty water (cell culture medium) in which cells won’t shrivel or swell. The problem is that cells sink in these solutions. If all the cells sink to the bottom of a print head, the nozzles quickly clog and printing fails. This is made even worse by the fact that cells often become clumped together.

So an ideal bio-ink would prevent cells from sinking and clumping. At the same time, though, it has to remain printable – it can’t be viscous like honey or it would be impossible to fire through the nozzles of the print head. What is needed is something not too dissimilar from salad dressings that keep herbs and spices suspended while sitting in the fridge but pour as easily as water onto your salad.

We developed a cell-suspending, printable bio-ink in a unique way. Using a common food additive called gellan gum, we created tiny gel particles in cell culture medium. These microgels interact in a way that gives structure to the liquid when at rest, so that cells stay perfectly suspended. But as soon as the microgels are disturbed with any significant force, the liquid flows again like water. In this way the bio-ink remains easily printable.

In addition to the cell sinking problem, there is another aspect to getting the bio-ink right that involves the phenomenon known as surface tension. Water molecules like to stick to each other, and the cohesion at the surface is particularly strong because there are fewer neighbouring water molecules with which to stick. It is this surface tension that allows some small insects to sit comfortably on the surface of a pond. Unfortunately, it also stops the proper formation of the tiny droplets ejected from an inkjet printer.

The good news is that the surface tension of water can be reduced using molecules called surfactants. The bad news is that most surfactants also fatally damage the delicate membranes that surround living cells – in other words, they are cytotoxic. So, we embarked on a search for non-cytotoxic surfactants for our bio-ink.

Our search led us to a couple of solutions. The first was a fluorosurfactant – a special class of surfactants that contain some fluorine in the place of the usual hydrogen atoms. These do a great job of reducing surface tension, but are not so good at disrupting cell membranes – an excellent combination for a bio-ink. The fluorosurfactant was able to give us the low surface tension we needed without any observable cytotoxicity after exposing cells to the bio-ink for 2 hours.

The second was a surfactant known as Poloxamer 188 (P188). Researchers have known for a while that this surfactant is different to others. Instead of causing damage to cell membranes, it is able to patch damaged cells until the membrane heals itself – almost like a cellular band-aid. When we included P188 in our bio-ink, printed muscle cells were healthier than when P188 wasn’t included.

Our bio-ink allowed us to print muscle and nerve cells from standard inkjet print heads. The printing was very reproducible, and the printed cells survived and behaved like normal cells.

The cells used in this work were from mice, but our current work is moving towards printing human stem cells. Stem cells, given the right signals, can form any cell type in the body – muscle, bone, or whatever is needed.

New technologies are being developed to readily obtain stem cells from an individual patient. Printing a patient’s own stem cells avoids complications with immune rejection, which is a critical issue in organ transplantation.

So the bio-ink is a significant development, but an ink is useless without the paper! Printing high-quality photos is only possible with high-quality paper, and an even more sophisticated bio-paper is required to print living cells.

The Bio-Paper

Cells must stay hydrated to survive – if they were printed onto normal paper they would quickly dry out and die. A bio-paper therefore needs to be wet. It also needs to be soft to cushion the impact of the delicate cells that are fired onto its surface at high speed.

In our work, we printed cells onto collagen gels. Collagen is part of the normal matrix for living cells. The gels keep the cells hydrated and provide biological attachment points for the cells.

We printed nerve and muscle cells simultaneously from two different print heads, much like printing two differently coloured inks, to create patterns on the collagen surface. Even simple two-dimensional printed cell structures like this can provide important information about how cells like nerve and muscle interact with each other.

To achieve the goal of printing functioning tissues and organs, though, we need to print in three dimensions. This can be achieved by using another print head that delivers a supporting gel material.

By printing cells and gels in a layer-by-layer fashion, it is possible to build three-dimensional structures. Because they have been fabricated using the fine control provided by inkjet printing, these structures can contain multiple cell types, biomaterials and other important biomolecules positioned in specific locations to mimic a real tissue.

Printing Organs

There is still a lot of work to be done before printers are stationed in every hospital and churning out new organs on demand. But research teams around the world are working to make this seemingly science-fiction prospect a reality. Some success in printing simple tissues like skin has already been achieved, while even hard tissue like bone is on the printing agenda.

But to print complex organs like the heart and lungs will require continued advancement of cell printing technologies and materials. We hope that the bio-ink described here moves us one small step closer to this goal.

Cameron Ferris is an Associate Research Fellow at the ARC Centre of Excellence for Electromaterials Science, University of Wollongong (http://www.electromaterials.edu.au).