During our discussion with José Maria De Fonte Ferreira from the University of Aveiro, we covered topics like bioactive glasses, bone defects, 3D-printing, and even ChatGPT. And when asked, ChatGPT told us that alkali-free bioactive glasses are currently a hotly debated topic! Why? Let’s find out!
Since you are a retired professor, I would like to know: what are you doing here in Warsaw? You are supposed to be lying on the sandy beach, somewhere in Portugal, I guess.
Yes, you are right: I am a retired professor from University of Aveiro. But I like to continue doing research. And we had a bilateral project between University of Aveiro and your institute with the aim to produce bioactive glass fibres. The leading idea was to produce a kind of cotton-like glass fibres.
What does it mean?
Glass fibres that could be flexible enough to fill bone defects that need to be repaired. Because flexible fibres will be able to easily adapt to any bone defect, independently of its shape. In this way, the doctors will be able to fill the entire bone defect with something that can be easily moulded to the shape, and accommodated according to the density the doctor requests, thus, constituting a versatile scaffold material for bone regeneration.
But how is it possible to fill a hole in a bone with a glass fibre? It sounds so weird. Aren’t there any other things that you can use?
Sure, other biomaterials can be used as well. But bioactive glasses are among the most promising synthetic bone grafts options. The first developments in this field were conducted by professor Larry Hench at about 54 years ago. Larry Hench was challenged by a U.S. army colonel, serving as medical supply officer in Vietnam, to invent a material that could form a living bond with tissues in the body. The soldiers suffered from serious injuries in legs, arms, and so on. There was an urgent need in the Vietnam war, for example, they would end up without arms, legs, and so on. The plastics and metals attempted as implant materials were presenting high rejection rates.
Plastic is not good and metal neither, because if you wanna go to the airport, you need to pass all of these gates and then you make some noise.
Yeah, it might also happen. So, the researchers wanted to find out something, an alternative repair material, able to establish bonds with living tissues. Soon after they developed the first bioactive glass, which revealed that capacity after being implanted in the tibia bones of rats. That material became known as “BioglassÒ 45S5.”
But we can break it.
Yes, glasses are usually fragile materials, therefore, prone to break. But this is not a much concerning problem when it is intended to repair small bone defects. In such cases, the bone graft materials are often applied as particles, or granulates, for example. Ideally, the intergranular spaces, and the room left by the gradual resorption of the bioactive glass particles, will be gradually occupied by the new bone tissue formed. But the relatively high packing density of the particles means long resorption times. Our intention here was to use high performing bioactive glasses fibres in order to form a kind of scaffold with a more open structure to fasten the repair of small bone defects.
How small? I mean: is it even visible?
Uh, yes, of course. However, if the bone defects are too small, they don’t even need anything, any material inside, because the bones have self-healing capacity, but above a certain size, which is considered the critical size…
About one centimetre or even more in diameter. Bone defects larger than the critical size, take too long for self-repair, or even are not able to heal without being filled with some material that establishes bridges between the borders of the defect. So, people are developing materials to apply in this kind of bone repair surgeries.
But, first of all, in my opinion, the BioglassÒ 45S5 developed by Larry Hench is a poorly performing material. It has huge content (22.5 wt%) of sodium oxide, which then may leach out inside of the body, generating a high local pH and killing the cells. For in vitro testing, for example, people use some tricky procedures like, preconditioning steps in suitable aqueous media to first leach out most of the sodium, and only afterwards putting the material in contact with cells. This is something that we shouldn’t trust much.
So, my group has developed a new concept of alkali-free bioactive glasses, and we have a significant number of publications reporting about their superior performances either in vitro and in vivo. They are receiving much attention now. According to artificial intelligence – ChatGPT, the alkali-free bioactive glasses and devices constitute a hotly-debated topic now.
What’s the name of your glasses?
Due to its fast biomineralization rate (formation of a Hydroxyapatite surface layer after immersing the material for 1 h in Simulated Body Fluid), the name of “Fast’Os” was given, suggesting its ability to establish fast bonds with living hard tissues.
So, what I am trying to say, is that our Alkali-free bioactive glasses are much friendlier, and better suited for this kind of biomedical applications (bone repair surgeries).
In this project we are making some changes because one important thing for producing bioactive glass fibres is a good thermal stability to enable fibres drawing while preventing the occurrence of crystallization events. So, the composition needs to be suitably adjusted to allow a large processing temperature window and enable us getting high quality fine and amorphous fibres.
Why can’t it be crystallized?
The crystallization negatively affects the flow of the melt during the fibres drawing process. On the other hand, having a larger processing temperature window is also likely to facilitate the extrusion of the melt through fine nozzles to build, in the future, tailor-made scaffolds by 3D printing, directly from the melt. Scaffolds, in this case, mean porous structures to support the tissue engineering and guided bone ingrowth. This is very challenging! It has not been considered in the planned tasks of the project, but we intend to explore our interaction and cooperation with a start-up here in Warsaw – Sygnis to perform some preliminary experiments towards setting a procedure to allow this kind of fabrication in the future. The Sygnis has expertise in 3D printing through a variety of printing techniques. The aim is to develop a future procedure that enables usage of our bioactive glasses for 3D printing directly from the glass melts.
Okay, so basically, let’s say that with your glasses, you can create scaffolds thanks to 3D printing and also these special things that fill gaps in bones, right?
So, there are two separate things we’re talking about. I got lost.
No, we are talking about two complimentary things. One is the fabrication of cotton-like bioactive glass fibres, the target we intend to achieve in the frame of this project. The 3D printing of bioactive glass scaffolds from the melt is even more challenging and is targeted for a near future. What we intend to preliminary explore is the synergies between the Institute and the start-up company, Sygnis, while thinking in future prospects.
Scaffolds with a given fixed structure and shape can also be used for similar purposes. Imagine that we have a bone defect with a given shape and we want to build something “tailor-made”, let’s say, to fulfil such defect.
Then, you can produce the scaffold system. You can plan this shape and then start producing with 3D printing. But the purpose is the same.
The purpose might be the same, similar, or complementary. Imagine specific situations in which the shape of the body in the implantation region is to be kept (e.g., a maxillofacial surgery). In this case, a rigid scaffold might be preferred to a flexible cotton-like ball of fibres.
Correct me if I’m wrong, because previously you said that sodium glasses are bad because of their pH, right?
Then what pH is expected?
The pH in our body is neutral – around 7.4. The bioactive glass from Larry Hench develops very high pH values, like 10, 11, for example, becoming cytotoxic. Our bioactive glasses will be essentially alkali-free and non-cytotoxic. They do not kill the cells. We have already used our alkali-free bioactive glasses for producing scaffolds, not from the melts, but from powders dispersed in liquids to form extrudable inks, for example. In this process, they also behave much better in comparison to the 45S5 composition of Larry Hench. By the way, my research group played a key role in the fabrication of scaffolds from 45S5 by robocasting, for the first time, at the world level. Nobody has been able to achieve and report this before. Many attempts might have been made, but without any success.
Anyway, there is another process to fabricate porous scaffolds – the replication of a polymeric sponge method. Imagine that you have a polymeric sponge. You can impregnate it with a slurry of a glass powder, squeeze the excess of slurry and let the impregnated sponge to dry. If you then heat it up slowly to enable the polymer to burn out without destroying the structure, you will be able to get a porous scaffold. This traditional process of replicating the structure of the polymeric sponge is applicable to any kind of glass or ceramic material. I can apply it to my bioactive glasses very easily.
However, people who report on the preparation of porous scaffolds from 45S5 by this method did not show macroscopic pictures of the so obtained scaffolds, but instead, microscopic views that can be taken from any small broken piece. When it comes to large parts, they are unable to produce them because of the poor sintering ability of the material. The sintered scaffolds are brittle and too weak from the mechanical point of view. Such poor processing difficulties do not enable scaffolds to be satisfactorily produced from 45S5.
Oppositely, using the polymeric sponge method or the 3D printing by robocasting, I can easily produce what I want from our alkali-free bioactive glasses.
So, you were the first person in the world who was capable of doing this.
We were the first group in the world producing scaffolds from 45S5 by robocasting, a 3D printing technique that uses a suitable extrudable ink.
I am wondering why you took the 45S5, a so difficult material to process, if you do not appreciate it at all?
Well, this is an interesting question and I can easily respond to it: a friend of mine from Spain, who I met in 2010 at a conference in Madrid, was trying for a year and a half (without success) to fabricate scaffolds from the BioglassÒ 45S5 developed by Larry Hench. This material is still in “fashion” among less demanding several researchers.
Then I told him: “oh, you can throw it away because we will not get there.” This guy was from Badajoz University in Spain. He had received a master student of mine for 3D printing calcium phosphate scaffolds, because, at that time, there was no robocasting machine at the University of Aveiro. We prepared the extrudable inks for robocasting, in our lab, at the University of Aveiro, Portugal. Then, my student brought the inks ready to be printed to Spain and, in one week, they printed all the scaffolds we needed for the thesis.
You know, he got amazed with the solid concentrations (greater that 50% vol.), which were significantly higher than those he used to achieve in his lab. So, he became very well impressed with our processing skills. Then, he asked me to receive 2 PhD students from his group to develop extrudable inks from glasses 45S5 to produce the scaffolds in his lab.
But you don’t like these glasses.
Right! But I could not say “no” to him because he did me a great favour receiving in his lab my master student to 3D-print calcium phosphate scaffolds. So, okay – I said – send me two PhD students for one month, and we will do our best to get there. They was a couple from Iran. After one month we had a mountain of results but not yet any satisfactory solution at all. So, they had to extend their stay in my lab for one more month. After the end of first month, I told them: “well, let’s think outside the box. We will never get there following the traditional approach in robocasting.”
The traditional approach involves dispersing powders in a liquid, usually in an aqueous media, by adding a dispersing agent to enable us to concentrate as much as possible in solids. The first objective is to get a fluid suspension with maximised solids loading. However, a fluid suspension is not suitable for extruding because it would spread and the extruded filaments could not keep the shape. So, we need to do something else to thicken the system in order to confer it suitable viscoelastic properties and confer shape retention capacity to the extruded filaments. This is usually achieved by adding a coagulating agent.
If you use an anionic dispersing agent, the coagulating agent should exhibit a cationic character to be effective. Imagine dissociated polyelectrolyte chains with some ionic groups charged negatively. When adsorbed at the surface of the particles, such groups tend to repeal each other keep the dispersed particles apart. This will allow you adding more particles in the system. Then, if you want to transform the fluid system into an extrudable paste, you need to counteract and disturb the stabilizing effect of the dispersant by adding a coagulating agent that thickens it towards getting an extrudable paste.
This can be done and is usually done by adding an additive with an opposite nature, let’s say – cationic. Cations bridge the anionic groups and make the stabilizing structures to collapse. Then, this coagulated system can be extruded.
Well, the interaction between anionic and cationic species is stronger in the neutral region of pH. If the pH of the suspension is too high (10 – 11), such as in the case of the BioglassÒ 45S5 particles, the cationic agent is doing nothing there because at such high pH values it is even in a non-disassociated form.
So, how can an additive that is not dissociated in water act as a coagulating agent? It’s impossible. So, I explain to them: “we are doing nothing here. Cationic agent is not acting.” To demonstrate this, we add some acid to the suspension. And, in that case, it became thicker.
However, acidifying means stimulating further dissolution of the bioactive glass and increasing pH again, so nullifying the temporary coagulating effect. Again, we were doing nothing there. That’s why we had to think outside the box. We had to use a completely different approach. With a single and multifunctional processing additive, we solved the problem and were able to produce scaffolds from 45S5.
But what kind of single additive? What is it?
It was exactly sodium carboxymethyl cellulose (CMC) of high molecular weight. It’s, let’s say, a long polymeric chain with a few functional groups, which dissociate, releasing a few ions of sodium to the solution. The functional groups become negatively charged and might be slightly adsorbed at the surface of the particles through small segments, extending laces and tails to the solution, while the entanglement of the long chains thickens the liquid media, increasing the viscosity of the system. So, when we load the CMC solution with particles, we have a double thickening effect, one due to the entanglement of the long polymeric chains, and the other due to the increasing charge of solids. We reach a solid volume fraction of 45% vol., which is much higher than we attempt before using other dispersing agents, hardly achieving much above 30% vol. So, using CMC we went up to 45% vol. Thanks to that, the elastic properties of the paste were good enough to allow extruding the filaments and keeping their shape.
(Dear readers, we’re still referring to the poor processing ability of the BioglassÒ 45S5, namely by robocasting.)
Using CMC as a single and multifunctional processing additive, we got a paste-like system that could be put into syringes. The syringes were then squeezed by the robocasting machine and the extruded filaments could be deposited to build scaffolds by 3D printing.
Okay. But how did you come up with carboxymethyl? That it can make it solid?
Being an organic additive, the polymeric additive will be burned off during the sintering step. If CMC is burned slowly there will not be leftovers of it. Only bioglass will remain there.
But are there other possible options? Let’s say other alternative polymers?
Yes! Other interesting copolymeric systems can be used with similar purposes. They consist of hydrophobic and hydrophilic segments, the interactions of which are strongly dependent on temperature, changing from a liquid-like to a solid-like flow behaviour.
So, can I ask a question about the cotton-like fibres, cotton-like glass, right? That’s the thing that you are developing here, right?
Right! This is what we intend to develop here with professor Buczyński. Imagine cotton fibres, flexible and soft. This is the kind of texture we are interested to develop from our bioactive glasses in the frame of this project.
But it’s not the thing you’re working on along with Sygnis company?
No, no. What we intend to do with Sygnis, is to undertake discussions and preliminary experiments towards guiding them to set a 3D printing equipment aiming at extruding, in a near future, our bioactive glasses directly from the glass melts. It’s another more challenging and straightforward way for producing bioactive scaffolds.
As you’re showing me this extrude, I would like to make a remark that it reminds me of my trip to Murano, Italy, where I saw how they made sculptures out of glass. That’s exactly what you do when working with glass?
Well, these guys at Murano also take profit from the flow properties of the glass melts to build their sculptures, using a traditional approach from which we can learn. But for our particular purpose, the glass compositions will be melted in an adapted crucible, a container with a hole at the bottom, through which a glass drop comes out and is then stretched by mechanical and centrifugal forces to acquire the desired fibrous structure.
Yes, and later it becomes solid. But how quickly?
All of this is very complex and needs to be kept under strict control, including the environmental temperature of the melt and around the tip from where the glass flows, the temperature gradient, and the cooling rate, etc.. A set of processing parameters needs to be kept under specific and very well controlled conditions.
And it must be extremely hot, right?
Yeah. We might be talking about 800-900 ºC. There is the temperature range we are looking for. It means that the temperature range should be strictly controlled and should be checked quickly, and even some pressure over the glass probably will be necessary to force the flow to come out.
The idea is to deposit the glass filaments and enable them to cool without undergoing crystallization. This is important for the filaments do not stick to each other, because during cooling, there is some shrinkage and sticking to each other will lead to mechanical stresses and loss of flexibility.
And it can break.
Yes, especially if there is a drastic thermal shock, it’ll break. If you deposit something fluid over something that is already hardened, it means that the shrinkage of what you are just depositing will be higher because the other remaining part has already shrunk and strengthened something. It means exactly that temperature of this environment should be kept under control to prevent, let’s say, mechanical stresses, to develop above a certain level that put in danger the integrity of the produced fibres.
Okay, so let’s get back to cotton-like glass fibres. There is a step aiming at the development of a device for producing this kind of cotton-like fibres, but you also say: “glass wool”.
Glass wool is used to thermally isolate houses. We want a structure similar to that of this glass wool, but made of our bioactive glasses. Not for isolating but for fulfilling bone defects. Large bone defects can be originated, for example, when cancer tumours are removed from the bones and we want to fulfil that part. Scaffolds and cotton-like fibres could be used for this purpose.
When there is, let’s say, an accident and one might lose part of a broken bone, which also needs to be regenerated. There are many situations that justify the use of regenerative materials to help healing bone defects.
Great idea! I have a question about Portugal. What could you recommend to see, where would you go on a holiday in Portugal?
Portugal has many different landscapes when one moves from the North to South, or from the littoral towards the interior parts. We have extensive sandy beaches and beautiful mountains to see, depending on what you most appreciate.
I like the mixture, this combination when there is a lake or sea and mountains, everything. I want everything, I want it all (laugh).
I think in that case, if you visit the North, near Braga, for example, there is a national park where you can find amazing things: waterfalls. It’s wonderful. And when it comes to beach, you have a long coast. From the North to the South, you can just select what you prefer in terms of beach. The South is warmer and might be preferred for sea lovers. Let’s say, Algarve contacts with the Mediterranean Sea, so the water is warmer there.
Have you already tried the Baltic Sea? Because it’s so cold.
So, do you like Poland?
Yes, especially this time. I don’t like much winter time. It’s too cold for me.
And how do you find Warsaw?
I like it pretty much, especially the ancient part of the city.
I thank you very much for talking with me.