Spinning Some Silken Science

Spiders and silkworms make silk by the yard. Why can’t we copy them? Silk is strong, light and flexible and is being examined for use in everything from medical sutures to advanced electronics. Silk researcher David Kaplan explains the challenges in bioengineering silk.

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JOE PALCA, host:

From NPR, this is SCIENCE FRIDAY. I'm Joe Palca.

It's tough, it's light, it makes clothing feel soft and smooth, but we really don't understand it fully. It's silk, a fiber that spiders and silkworms make, among others, but that humans have a hard time duplicating.

And researchers are working to bring silk far beyond the realm of fancy stockings and finding ways to use it for medical applications and even in electronics.

Joining me now is David Kaplan. He's the Stern Family Professor of Engineering and a professor and chair in the Department of Biomedical Engineering at Tufts University and one of the authors of a review article this week in the journal Science about the wonders of silk. Dr. Kaplan, welcome to the program.

Dr. DAVID KAPLAN (Tufts University): Thank you, Joe, a pleasure to be on.

PALCA: So first of all, you know, I was realizing and I started thinking about this, silk is not - there's not just one silk. There's a lot of silks.

Dr. KAPLAN: Correct. There's a whole family of silks, from - everything from silkworms to spiders that encompass a wide range of properties.

PALCA: So is there a class of chemicals or a class of compounds that people say, oh well, if it has these properties, it's silk? I mean, what do you have to have to be able to be called a silk?

Dr. KAPLAN: Well, by definition most silks are defined as externally spun protein fibers that come from nature, from various animals in nature. Most synthetic materials in fiber form really only emulate parts of the properties of silks.

Silks are quite unique in a combination of both high strength and toughness that really to date is quite difficult to match with synthetic approaches.

PALCA: I see. So everybody knows about spiders and silkworms, but there's other animals that make silk, right?

Dr. KAPLAN: Well, there's quite a range, everything from underwater insects that spin their housing, if you will, to stay underwater and survive and reproduce, as well as to many other organisms that spin silks.

PALCA: So why has silk been hard to re-create in the laboratory?

Dr. KAPLAN: The silk overall is relatively simple in its chemistry and sequence, but it's a very high-molecular-weight polymer, which makes it difficult to reproduce, and we're only just learning about some of the key domains or regions of the protein in terms of their chemistry and how that relates to the self-assembly and functional material properties that are generated from the protein.

And so I think now that we're armed with that kind of information, we're finally in a position to start to better mimic some of these features with synthetic approaches.

PALCA: By the way, if you would like to talk with Dr. Kaplan about silk, or if you have a silk question or a silk experience or some silk product that you've always wondered about, you can call us at 800-989-8255. That's 800-989-TALK.

So the question I - that I also wonder about is, if you have silkworms, which are, you know, commercially raised and produce plenty of silk for the fashion industry, why isn't that enough? Why can't you just use the silk they make?

Dr. KAPLAN: To date, for most of the studies and also most of the newer applications, that is sufficient, and you know, there is quite a large supply from around the world for raw silkworm fiber for these uses.

But if we look ahead to some of the coming applications and the expanded material properties we can now generate from this same protein, we envision that there's going to be a need down the road for expanded supplies. And this is where new modes to produce the silk will be required.

PALCA: Well, give me a for-instance. What in the medical world, for example, might be using silk in a way that it is not being used today?

Dr. KAPLAN: So a good example is historically silks have been used as suture material, still used today, for literally thousands of years, mostly still used in certain kinds of facial surgery and eye surgeries.

But if you look just at the use of silk as a biomaterial for a wide range of growing new tissues, new ligaments and bone tissues, many other orthopedic devices, soft tissue repair, and the list goes on, we envision, you know, those demands alone will drive the need for additional supplies of silk.

PALCA: And I mentioned in the introduction electronics. Was I speaking out of my hat then, or are there electronic ideas as well?

Dr. KAPLAN: No, some of the more recent advances are, you know, quite remarkable, even from my own perspective, where we can start to use silk as a degradable platform to transfer electronic components for use in and on the human body.

And early example of that have been demonstrated now in the last year, where you can improve, for example, recordings of brain signals just by using silk as the conduit for better conformal codings.

In terms of optical devices and platforms, it turns out silks also serve the same function as a fiber-optic cable. So you can conduct light through silk fibers in a way that you can do with synthetic materials as well.

And these are all very unexpected findings in the last few years that'll drive additional applications where you could use, for example, implantable but hull-degradable optical devices to diagnose and provide input to the human body during repairs.

PALCA: I really like this - optical silk properties. That sounds wild.

Dr. KAPLAN: Yeah.

PALCA: Yeah, okay. Well, let's take a call now and go to Derek(ph) in Reading, California. Derek, you're on the air with SCIENCE FRIDAY.

DEREK (Caller): Hey, how's it going?

PALCA: Good.

DEREK: Yeah, I had a friend who always liked to look up weird stuff, and he was telling me about a goat, where it was engineered to have silk come out of its udders. I don't know how correct that is. It seems pretty far-fetched to me. But I was wondering if you could talk about that, and I'll take my answer off the air.

PALCA: Okay, thanks. Thanks, Derek.

Dr. KAPLAN: Yeah, thanks, Derek. So there were some commercial ventures over the past years. One of them was to generate transgenic goats. This was a company up in Montreal, Canada, no longer an active program at the company, but they had reported successful transgenic goat production of silk.

So it sort of is a proof of concept that, yes, you can carry out the process genetically and produce silk in the milk of the goats. There still, however, are many challenges to that process, one being increasing the molecular weight of the silk so it mimics better the real material, and number two, how do you process that silk from the milk into usable materials. And those are challenges that are starting to be looked at by other labs.

PALCA: You also talk in your paper about using, growing silk in plants. Is that being seriously considered?

Dr. KAPLAN: Yes, so there's a couple of labs around the world that are pursuing transgenic plants as a long-term solution as well to production levels of silk for a wide range of materials.

And this may ultimately be one of the better options. If you think about the fact that silk is already produced in fiber form in nature, it would be in essence a logical source of the material if it can be cropped along with a plant and then harvested from there.

PALCA: All right. Let's take another call now and go to Caroline(ph) in Chapel Hill, North Carolina. Caroline, you're on the air with SCIENCE FRIDAY.

CAROLINE (Caller): Hi, I was in China recently and saw a silk floss-making quilt factory, and we were instructed in the whole process of the silkworm and mulberry leaves and all that. And when I got home, I was talking to some people, and they said that they had been to some other country in Southeast Asia where the silk was being produced by the silkworms were in colors.

And I just wanted to find out a little bit more about that because from my understanding in China, they said, you know, it was white, and that was it. So I was just wanting to see what's real.

PALCA: Good question, Caroline. Dr. Kaplan - all silk white? Some silk colored? How do we know?

Dr. KAPLAN: So it's very interesting. It appears that some of the colors you see depend on the food source for the feeding, you know, moth -insect that's going to become the moth, and colors vary. For example, in Thailand, the silk is much more golden in color than in Japan and China, where you see much whiter silks.

So there's a huge variation, depending on the race of the particular moth species, and you can also alter what's in the diet, if you will, which affects the color of the silk as well.

So there are some ways to modulate the color, depending on, you know, what's being fed.

PALCA: And I presume there are some after-production ways of dying the silk.

Dr. KAPLAN: Oh, that's historic, yes. Clearly, silk takes up dyes very, very well. So it's a really nice textile material because of that for color dying.

PALCA: Yeah, I'm sure. Okay, let's take another call and go to Paul in Ypsilanti, Michigan. I think that's right. Welcome to the show, Paul.

PAUL (Caller): Hi, thank you guys for taking my call.

PALCA: You bet.

PAUL: First, you know, I work in law enforcement here in Washtenaw County in Michigan, and I heard some rumors, and I was reading some information a while ago, on the application of silk in projectile-resistant technology.

The idea was that silk would be lighter than Kevlar but at the same time more effective if it could be utilized and weaved into, you know, body, you know, like shirts or vest materials.

PALCA: Kind of a lightweight armor kind of thing.

PAUL: Right, exactly.

PALCA: Yeah, yeah, okay. Well, Dr. Kaplan, what about that?

Dr. KAPLAN: So historically, there's been quite a bit of interest in how to exploit silk in a new kind of protective material, if you will, like along the lines of Kevlar. And certainly there are labs that have strong interest in that. You know, the comment that it would be more comfortable and lighter weight is certainly the case, because you could, in principle, use less material and achieve similar protective goals.

What really is important is the fact that silk is very, very strong if you pull on it, but it's also very, very strong and durable if you kind of squish it or compress it. And that's where it's quite distinguished from the Kevlar, which is strong when you pull on it, but if you squish it, it kind of doesn't perform as well.

And so these combinations of properties give rise to the interest in making things like protective vests and other equipment. And down the road, I anticipate we'll see much of this.

Historically, if you look back in museums in Japan and China, you'll find armor was made of silk, you know, thousands of years ago for the same reasons because of the real toughness of this fiber.

PALCA: Wow. And I'd like to get you to describe, because it was so remarkable. You know, we tend to think of - or at least I do - when, you know, there's something coming out of an animal like a spider or a silkworm, you think of it extruding this silk, but that's not quite how they make it. There's a mechanical process going on.

Dr. KAPLAN: That's right. Mechanics play a big role. If it's the silkworm, the silkworm actually spins out of modified salivary glands in its head or in its mouth, and the silk is really pulled out because there's a figure-eight motion of the head as the worm tries to spin its cocoon, essentially, from the outside in to encase itself as it's going to molt into the moth.

In the case of the spider, the legs of the spider generally do the work of pulling the silk out. And the only exception, obviously, is if you flick a spider off the ceiling, it'll have its safety line, its drag line that it will pull out during that process, as well.

PALCA: I see. So it can take gravity to do the pulling, as well?

Dr. KAPLAN: Yes. That's right.

PALCA: So why is it that - I mean, when you're talking about these various uses - I mean, what's the hurdle here? I mean, is it more labs thinking about ways to use this? Is it more ways to produce it? Where are the scientific barriers to some of these really interesting applications?

Dr. KAPLAN: So I would say it's a kind of phased level of advancement. It's, you know, only been the last 10, 15 years where there's been what I would call seriously renewed interest in the fundamentals of how silk achieves - silk's achieved the remarkable material properties that they are able to achieve. And with that has come in the last, you know, decade significant advances in new materials derived from this same protein that's been around in textiles for 5,000 years. That brings us to one level of new material performance from adhesives, electronics, optics, the things I've mentioned in the medical field.

But now, there's sort of that next generation we want to get to, which is - now that we more fully understand all the chemistry and the genes, et cetera, can we, you know, directly emulate what nature has achieved, which are these remarkably strong fibers? And in doing so, we'll then be able to open up additional material applications that we can't quite yet achieve today.

PALCA: We're talking with David Kaplan. He's the chair of the Department of Biomedical Engineering at Tufts University, and we're talking about silk.

I'm Joe Palca, and this is SCIENCE FRIDAY, from NPR.

Let's take another call now and go to, let's see, Siri(ph) in Cleveland, Ohio. Siri, welcome to SCIENCE FRIDAY. You're on the air.

SIRI (Caller): Hi, gentlemen. Thanks for taking my call.

PALCA: Sure.

SIRI: Well, I have a very short question, I guess. I have a lot of vegan friends, and they refuse to use silk because it's a, you know, a kind of an animal product or comes out from insects. My question was, is there any way to extract silk naturally without actually killing off the insects? Or do the insects actually have to die in the process?

PALCA: Interesting. Okay.

Dr. KAPLAN: Siri, it's a great question. In the case of the silkworm, it's, in essence, an inevitable state of their molting process. So I don't know that you can actually retrieve the silk without the insect going on to the moth state and going to its normal cycle. I don't have a good answer there.

For the spider, however, you can certainly collect silk, and the spider does fine. And we and others have used that technique in the lab for many, many years, and this is just called a controlled silking process. So there's no real harm to the spider in that process, and the spider continues to live further and produce more silk.

PALCA: Okay. I think we time for one more quick question, so let's go to Carol in Louisville, Kentucky. Carol, you're on SCIENCE FRIDAY.

CAROL (Caller): Hi. Thank you for taking my call.

PALCA: Sure.

CAROL: I'm a surgeon, and I use silk, and as you guys discussed earlier, in many areas of my field of endeavor. And one thing I've noticed is that silk is, relatively speaking, a permanent suture. That's why - that - we teach residents, as we're teaching them how to do surgery.

But on occasion, we'll have - patients will come in weeks, months, sometimes even years later with little silks popping through their skin. And we call them spit fistulas, where this kind of opening to the outside comes, and the silk suture works its way to the surface because the patient has rejected something about the silk, and it comes through.

Has there been anything done, or are you thinking about trying to develop a silk that can be an absorbable suture? Because some of those absorbable sutures are so irritating, and silk, for whatever reason, seems to be one of the least irritating substances that I can use on skin or soft tissue.

PALCA: Okay, Carol, it's a good question. Dr. Kaplan?

Dr. KAPLAN: Carol, great question. And so what you're referring to is the silk that's been around forever, and as you probably know, it's generally termed things like black-braided silk. And that's because the silk is coated with waxes and dyes just for the reason you bring up, because if you don't do it, it would cause irritation in the human body. And so the way you avoid that but still use the good mechanical and knotting properties of silk is to coat it with waxes that prevent it from irritating the body, and therefore it becomes non-degradable. And that's how it's defined.

The big breakthrough and why we're even talking today is because around 10 - a little more than 10 years ago, it was realized that if you properly purify the silk, extract out some of the components that cause that irritation, then silk doesn't need to be protected with waxes and dyes. It's perfectly compatible, and it fully degrades in the human body. And so these are the kinds of earlier studies that have propelled this new generation of biomedical materials based on silk that are really the same core protein that you're talking about, but a very different process that now leads to materials that are fully degradable in definable lifetime.

PALCA: OK. Dr. Kaplan, we have to leave it there. Carol, thank you for the question. That was Dr. David Kaplan. He's Stern Family Professor of Engineering and a professor and chair in the Department of Biomedical Engineering at Tufts University.

When we come back, what evils lurk in the genomes of vertebrates? We'll find out.

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