The Inside Scoop (NIDCR)

When placing a white composite filling into a decayed tooth, the devil is always in the polymerization process. That’s when dentists shine a high-energy light onto the dough-like filling packed into the cavity, prompting small unbound molecules, or monomers, within the material to link into polymer chains. But as the chains assemble and produce the cross-linked matrix of what will be in a matter of seconds a hard white filling, dentists also confront a basic law of chemistry. The chemical reaction that enjoins the monomers also causes them to shrink slightly.

If too much shrinkage occurs during the polymerization process, the chances increase that the composite will fail prematurely. The Inside Scoop recently talked to materials scientists and NIDCR grantees, Drs. Christopher Bowman and Jeffrey Stansbury of the University of Colorado about their research and the ongoing quest for a shrink-proof dental composite. Here’s what they had to say. How long have materials scientists sought a dental composite that doesn't shrink during the polymerization process? Bowman: From day one.

Dr. Rafael Bowen introduced the first effective resin-based dental composite in the 1960s. It consisted of a methacrylate monomer system and silica powder as the filler material that bonds to and stabilizes the polymer. Methacrylate is a resin that is derived from methacrylic acid. Today, although the filler materials have evolved considerably, most dentists still use commercial dental composites that utilize Bowen’s original methacrylate monomer system or a close derivative thereof.

There certainly are other fine monomer systems on the market or under development. But the problem of matrix shrinkage remains unsolved. What causes the shrinkage? Stansbury: The laws of chemistry. As the monomers link, they become less frenetic and fit together more densely. This process is known as “double bond conversion,” or just “conversion” for short. What happens biochemically is the monomer’s reactive vinyl groups form covalent bonds, meaning they share one or more pairs of electrons and enjoin like links in a chain. This biochemical transformation causes the monomers to lose volume and shrink slightly. How much does a composite shrink? Bowman:

That’s a little difficult to answer, and here’s why. Nearly all dentists today aim a light source onto the soft composite to initiate the polymerization process inside the tooth. We call this “curing,” or hardening, the composite. Some of the most debated questions in the field today are: What is the ideal length of the cure? Or what is the ideal depth of cure and how much polymerization, or double-bond conversion, is ideal. So there are many technique-driven variables in play that influence the degree of shrinkage. The size of the tooth and the cavity obviously would be other obvious variables.

Bowman: Absolutely. But to answer your question, the overall shrinkage is proportional to the degree of polymerization. In other words, curing a composite is a bit of a balancing act. You need enough polymerization to produce the needed toughness and durability of a hard filling. That usually occurs in 20 to 40 seconds of cure with a light source, yielding about a 50 to 70 percent rate of polymerization and about 2 to 5 percent shrinkage. But the conversion rate slows considerably thereafter, and you must take into account the additional shrinkage that will result from extending the cure to add a relatively small percentage of polymerization to increase the toughness and durability of the filling.

It's kind of like baking a cake? Bowman: Broadly speaking, yes. You don’t want to stop baking the cake too quickly, neither do you want to leave it baking too long. It’s the same with curing a composite, and that’s why I say it’s a balancing act. Two percent shrinkage isn't much. Why is even a two-percent loss of volume still of concern? Stansbury: The composite has no room to move inside the tooth. Not only is the filling sealed tightly to the enamel and dentin, the cavity itself is a finite space. If the matrix shrinks at all, the fixed composite must absorb and/or transfer the pressure somewhere else. And if the stress reaches a critical level, something must give.

Bowman: That’s right. It’s like squeezing a balloon. If the compression surpasses a certain threshold, the balloon will rupture. In the case of a composite, the stress might crack the tooth, break the composite-enamel seal, or otherwise shorten the functional life of the filling. Your groups now are actually watching the polymerization process in real time. How do you monitor the process?

Bowman: By integrating two measurement techniques. First, we’re visualizing and measuring the polymerization process itself, the conversion of the monomers to polymers. Second, we’re evaluating stress development. If any dimensional change occurs as the liquid monomers form into solid polymers, there is the potential for stress in the system. We’re evaluating both in real time. Do these techniques give you a window into how to avoid creating stress during the polymerization process, or the concept of stress relaxation? Stansbury: Well, that depends on how you mean stress relaxation. Some use the term to mean that you can slowly initiate the polymerization process and minimize the stress. Or, they mean you can start the polymerization process, then pause and give the polymer time to relax. That’s not the case. Why?

Stansbury: We’ve shown in our research that the pace and magnitude of any potential stress relaxation decrease as the conversion increases. Therefore, the best prospects for stress relaxation occur early in the polymerization. The problem is very little stress occurs early on. It’s near the end of polymerization process when the greatest stress occurs. The irony is, stress relaxation is extremely slow at this point and limited in its breadth. What about the related idea of rearranging bonds early in the polymerization process to optimize them? I know you've published on this idea recently.

Bowman: That’s right. Let me first give you an analogy to try to explain the concept. Let’s say you and I pull on a rubber band. There are all sorts of covalent bonds in there that link the two ends of that rubber band. That’s why, when we let go of it, the rubber band returns to its original shape. The bonds want to return to their original dimensions and structure. But when we pull the rubber band, we put those bonds under stress. Now, we can take those bonds and effectively break them to reduce the stress. In short, we have the ability to cause that material to effectively adapt and alleviate that stress. And can you do the same during the polymerization process?

Bowman: That’s what we’re trying to do. When you cure a composite, like pulling on the rubber band, stress is generated. But if I can break those chemical bonds that are under stress and reform them with bonds that aren’t under stress, the material can adapt to and alleviate the stress from the polymerization. Is that a chemistry or engineering problem? Or is it both? Stansbury: It’s absolutely at the interface of both. Learning how to allow the material to adapt is an engineering concept. But let me say, discovering the chemistry to do it is by no means a trivial matter. Where are you in discovering the right chemistry?

Stansbury: With the work underway in Chris’s lab, we’re certainly making strides, as are other groups around the world employing a variety of approaches. We’ve developed materials that upon exposure to light alleviate more than 90 percent of the stress that’s applied to that system. Is that the article that your group published in the journal Science last year? Bowman: Yes, that’s the Science paper and also an Advanced Materials paper that is in press. These articles demonstrate the proof of principle. They show that we indeed can create networks in which stress relaxation not only is possible but possible to a very large extent. We’re now taking the concept to the next level.

We are studying how to integrate these types of bonds into a practical dental composite system. That means we must develop the right mechanical properties, the right glass-transition temperatures, and the right chemistry. So there are a lot of practical issues on the table, and they will take some time to solve.

In addition to stress relaxing polymers, what are some of the other monomer systems under development in your laboratories? Stansbury: We’re also looking at some traditional methacrylate-based systems that have much lower shrinkage. I think they can easily be adapted into new products. Which types of modifications have you made? Stansbury: Well, our primary effort has been to use the polymerization-induced phase separation to compensate for the shrinkage that would normally be expected in these systems.

Would you explain that? Stansbury: Sure. Let me define what I mean by “polymerization-induced phase separation.” You probably recall from chemistry class, a “phase” is any distinct part of a mixture that can be physically separated. Hydrogen and oxygen, for example, are the phases of water. But I mean phase separation a little differently. In this sense, as the polymerization progresses, the polymer forms with separate “phases” of filler and monomer.

For example, there will be nanoscale regions that differ in copolymer composition. These interfaces between the different phases reduce the polymer chain packing density and thus lower the shrinkage without a reduction in conversion. But the polymer must be well bonded to the filler to achieve this reinforcement effect. So rather than engineer around the shrinkage problem and essentially drop in something new to the mix, you're directly influencing what's already there to maximize volume? Stansbury: That’s right, and we’ve gotten a significant reduction in the final stress that develops.

A company already has agreed to license the technology and put it into a new dental restorative material in the very near future. What about filler? Would the modifications of the methacrylate system lead to changes in filler? Stansbury: It can be used with conventional fillers, which is one of the attractive advantages. It can basically be dropped right in and formed very nicely. Are there other monomer systems on the drawing board? Bowman: Well, we’re looking at the thiolenes.

Their polymerization involves the same free-radical initiation process as methacrylates. But thiolenes have the potential advantage of lower shrinkage than methacrylates but with higher conversion and crosslink density. So, these are all things that, in some cases, address short term needs. In other cases, they address midterm expectations. Then we’ve got things that are a little farther down the line. I sense the chemistry is a real challenge. Is that correct?

Stansbury: That’s correct. You can’t just pull something off the shelf that will work better than the current system. The current system has withstood many challenges over the years. Though it still has its problems, the system has held up to them. Sounds like Bowen really knew what he was doing. Stansbury: He really did, in all honesty. I worked with him for a long time earlier in my career, so I know firsthand that he has some special talents in this area.

Bowman: Jeff’s right. You can’t just pull something off the shelf. You must create the monomers and evaluate them for an array of properties. These range from biocompatibility and mechanical qualities to shrinkage properties and, in some of these applications, either polymerization mechanisms or phase separation aspects. It takes a lot of time, and inevitably you need to make a large number of candidate monomers before you get to the right one. So, it takes a lot of trial and error? Bowman: Well, I hesitate to call it trial and error. I say this because it really can’t just be trial and error.

There are too many possibilities and permutations to think in terms of absolute success or failure. There’s a lot meaningful information in between. You really have to let it guide you further down the road to what you’re looking to find. Is it a great time in composite research? Stansbury: It really is. There are number of people doing great things in the United States and around the world in terms of making new monomers, looking at new polymerization strategies, developing new fillers, and even curing strategies. All are being studied extensively, and some great things are definitely in the works. I think one of the neat things here is that our work is actually finding its way into broader markets.

What about those broader markets? Stansbury: We’re at a point now where dentistry is not just following but is able to lead the way in some of these areas. How so? Stansbury: Take the curing process, known more formally as photopolymerization. It’s not just a way to turn your monomer from a liquid state to a solid polymer. You can also engineer the structure of your material of interest and control its shrinkage based on how you perform the photopolymerization.

For example, I can take the same material, polymerize it two different ways, and get two completely different stress and shrinkage results. These are things that are now being picked up on in areas such as stereolithography, a commonly used manufacturing technique to rapidly fabricate plastic products.

We’re already working with a company, trying to take some of the things that we’ve developed for dental applications and convert them into an improved material for stereolithography. Would you have ever suspected that you'd be doing that? Stansbury: No, I didn’t. But polymerization stress and shrinkage are problems in all sorts of polymer applications. The broader applications actually make a lot of sense. Thanks for talking.

Bowman and Stansbury: Thanks.