David Kaplan, Stern Family Professor of Engineering at Tufts University’s College of Engineering, and his team have developed a unique and practical method for processing silk into solid form for products such as rods and plates for medical implants.
Tech Briefs: What first gave you the idea that you could do solid-state processing of silk?
Professor David Kaplan: Back about five years ago, along with a colleague in physics at Tufts University, Peggy Cebe, we published a paper on using ultra-fast calorimetry — laser-based heating — heating to 2000°K in seconds. We showed that you can take a silk fiber and melt it and then resolidify it. So, that gave us hope that there was a processing window that might work. That’s the only thing we had to go on. There was not a single paper out there that talked about thermal processing of silk. Then we just started working from there to play with water content and formulations to work this out.
Tech Briefs: Why did you think it would be a good idea?
Professor Kaplan: I’ve been working on silk for a long time with success for many kinds of technologies in medical products. But the barrier to really wholesale acceptance is material processing and sourcing. The issue is that when you get silk into water solution, it wants to reassemble. So, you’re fighting a time-dependent conundrum, where you only have a certain window when the solution is good and then you have to make a fresh batch again. This gets to be manpower intensive and costly, not to mention the costs for shipping water vs just the protein. We felt there really had to be a way to overcome that problem. The answer we came up with is essentially: make silk pellets, which are shelf-stable yet soluble in water on-demand. That was the real driver — to solve the sourcing problem — to endow any kind of new product manufacturing with this capability.
Tech Briefs: What’s so good about silk-based products as materials as opposed to other materials?
Professor Kaplan: I could go on for days, but I’ll try to be very concise. It’s not a single feature, it’s a combination. It is a biocompatible, yet fully degradable material in the body, on the body, or in the environment. It’s completely sustainable — it comes from worms that grow on leaves that are grown by sunlight. And everything in terms of the processing can be performed in water. So, essentially, it’s an attractive, sustainable polymer that’s very compatible in medical and environmental products. It’s also unusual because we can add in bioactive components like drugs, cytokines or cells, and they maintain their bioactive functions in the material. Silk has a very stabilizing effect because it’s an amphiphilic [both water-loving as well as oil-loving] high molecular weight protein polymer, it’s mostly hydrophobic [can’t mix with water], but it has some hydrophilic [can mix with water] domains as well to permit solubility in water.
Last, is polymorphism — we’ve learned how to control the structure of silk — which affects the mechanical properties and the lifetime for degradation. So, it’s versatility, sustainability, robust mechanical properties, and compatibility that drives much of the interest in silk.
Tech Briefs: It never occurred to me when I was reading about this that being a biological material to start with is what makes it biologically compatible. So, you “tune” its properties by changing the temperature and pressure you make the pellets with?
Professor Kaplan: In this particular technology: yes. You control it based on temperature and pressure.
Tech Briefs: Can you design it, do you have a menu that says: if you want X property, this is how you do it?
Professor Kaplan: Yes, pretty much. I’ll give you one example that is quite unique. Suppose you make a solid part, like a rod or a plate, and put into the human body. It might take, hypothetically, two years to degrade away completely. In some cases, that might be fine, but in other cases, for example, when we’re making orthopedic screws and plates for use in children to repair bones out of this material, then you want it to degrade away in three months or so because the bones are growing fast. So, we found a clever way to do it. You take an enzyme — a protease — that digests silk and we include it in the thermal processing. Because the parts the we make are essentially dry when they’re manufactured, the enzyme stays inactive. That means you can leave the medical devices on the shelf for years and they remain stable. But as soon as you implant the device in an animal, then the part hydrates enough and the water activates the enzyme that you’ve encoded into the part. Depending on the dosing of the particular enzyme, you can then tailor the degradation rate to a week, a month, or a year — whatever you want. It’s a really cool way to degrade the device from the inside out with good process control.
Tech Briefs: Using your process, do you mold the parts or machine them?
Professor Kaplan: We do both. You can press-fit mold or machine after you mold. You can make a blank that you can then machine. But basically, you mold it to whatever you want.
Tech Briefs: How far along are you in terms of commercializing this?
Professor Kaplan: We’re trying to find a commercial partner as we speak. We’ve talked to a few folks and are trying to find the best path forward.
Tech Briefs: Are your processes now readily adaptable to large scale manufacturing?
Professor Kaplan: That’s exactly the point — it becomes a traditional plastics manufacturing operation.
Tech Briefs: What excites you about this project?
Professor Kaplan: I’ve worked on the material for 30 years. This is the first time I can say we’ve really solved the sourcing problem — that’s why to me, it’s really exciting.
An edited version of this interview appeared in the March Issue of Tech Briefs.