A simpler punch biopsy tool

Engineering a device that helps doctors make faster diagnoses for skin conditions

The punch biopsy

The punch biopsy is a popular skin biopsy procedure that doctors use to obtain full-thickness samples of skin. The technique is very effective for diagnosing suspicious skin lesions. The procedure for obtaining a skin sample via a punch biopsy takes little time, has a low risk of complications, and involves the use of a few simple tools.

Punch biopsies are one of two common techniques that dermatologists use to obtain skin samples. The other method, called a shave biopsy, involves “shaving” a small piece of skin off of a suspicious lesion. While the shave biopsy is less invasive, it can only capture skin samples from the very upper layers of the skin (specifically, the epidermis and the top half of the dermis).

That can be a problem, because different skin diseases live in different places. Some skin diseases like lichen planus. a little-understood autoimmune disease, live in the topmost layers of the skin. Other diseases like melanoma, a truly dreadful form of skin cancer, can spread through all parts of the skin. Because a punch biopsy can extract a sample of all skin layers, the procedure is an optimal choice for dermatologists who are unsure about what a skin lesion might be. It’s also great for doctors—like primary care physicians and family physicians—who have little experience with diagnosing skin conditions.

So what’s the problem?

The simplicity of the punch biopsy technique is also it’s biggest shortcoming. It’s easy for a young doctor to look at all the tools that are needed to perform a punch biopsy and to understand how a punch biopsy should be performed.

The doctor first uses a circular blade to create a cylindrical “punch” into the skin. Then she uses tweezers to pinch the skin sample, and scissors to cut the the skin sample away from the fatty connective tissue underneath the skin. Once the doctor has isolated the skin sample, she can transfer it to a sterile container and send it off to the pathology lab for analysis. You can watch a punch biopsy being performed here (careful, it’s a little gross… avoid it if you’re really squeamish).

But the simplicity of this procedure doesn’t mean that a punch biopsy can be easily performed. There’s a lot that can go wrong. An inexperienced doctor could punch too deep or not deep enough into the skin, or could crush the sample when pulling on it with the tweezers, or could drop the sample on the floor when moving it to a sterile container.

So there’s a lot of effort and dexterity that goes into performing a good punch biopsy. And because of this, many primary care physicians will feel more comfortable referring a patient to a dermatologist instead of performing the procedure themselves.

This is where the problem really begins. Wait times for dermatologists are among the highest in the country for any clinical specialty. Patients in Atlanta can expect to wait an average of 13 days to see a dermatologist. Patients in Seattle wait an average of 42 days. And patients in Philadelphia wait a whopping 78 days, on average, to see a dermatologist. Nationwide, the new patient wait time for a dermatologist is 33 days.

These numbers don’t include the time it takes to send a skin sample off to the lab to have it analyzed, or the time it takes for a patient to see their primary care doctor in the first place. What if patients aren’t getting the diagnoses they need in time to start effective treatment?

An opportunity for improvement

We reasoned that if we could make the punch biopsy easier to perform, then primary care physicians would be more comfortable with doing the procedure themselves instead of sending their patients to a dermatologist. That would substantially reduce the time it takes for patients to receive diagnoses for skin conditions, possibly leading to better treatment outcomes. So we set out to design a punch biopsy procedure that would take less practice to master, that has less room for error, and that would result in high-quality skin sample every time.

We met with primary care physicians to hear about their the challenges they face with the current procedure. We talked to pathologists to learn about their process for analyzing skin samples. We visited dermatologists in their clinics to see punch biopsies in action, and we watched patients as they had their skin sampled.

We broke the procedure down into its most basic steps, and mapped the tools needed for each step of the process. This allowed us to identify the parts of the procedure that had the greatest room for improvement.

Blue sky ideas

After learning as much as we could about the punch biopsy procedure, we started to develop solutions for the problems we had identified. The most delicate and error-prone parts of the procedure are the “pull” and “cut” steps, during which the doctor needs to grip the sample with tweezers, pull on it to access the underlying fatty connective tissue, and cut the sample free with tweezers. If we could simplify these steps, we might be able to make the entire procedure more appealing to primary care physicians.

We searched for inspiration far and wide. We took apart strawberry corers, we studied how woodpeckers dig for insects in tree trunks (side note: it’s extraordinary), we looked at designs for subway tunnel drills, and we explored the physics of cutting cheese with wires. We thought about how we might incorporate all of these pulling and cutting and drilling mechanisms into a handheld device for taking skin samples.

To get some physical intuition for our ideas, we built prototypes out of everything from old syringes to 3D-printed plastic parts, from pipe cleaners to machined aluminum.

After a week of playing around in the design studio, we weighed all of our ideas against one another. We considered factors like ease of use, ease of manufacture, risks of damaging the skin sample, and the ability of a design to fit seamlessly into the methods that busy doctors already use.

Having our cake

One problem kept nagging at us as we explored a wide range of designs for a simpler punch biopsy device: How can we cut the skin sample away from the underlying fatty tissue while the sample is still in place? If we could do this, we would eliminate the need to pull on the sample to cut it away with scissors. This would make he whole procedure a lot less tricky, and it would substantially reduce the risk of damaging the sample.

We thought about this problem for a long time. We consulted with more doctors and medical device designers to learn about tools that might already have solved this problem. We had few leads. Time was running out. We started to get frustrated. And then, more than half-joking from exasperation, we decided to try something else:

One of the more interesting things you can do with floss (in addition to, well, flossing) is to use it as a sponge cake cutting tool. If we could scale this motion down to the size of a skin sample, we might be able to engineer a device that cuts a cross-section of skin without the need to pull on the sample to expose fatty tissue.

Building and testing

We came up with several different strategies for miniaturizing the cake-cutting motion that we were so excited to try. One of the designs we decided to build features a sharp wire looped between two concentric circular blades. A spring allows the inner blade to retract into the outer blade, and couples the motion of the inner blade with the closure of the loop of wire that runs between the two blades.

When a doctor releases the spring on the device, the sharp wire constricts around the bottom of the skin sample, rapidly tearing it from the underlying fatty tissue with 30 pounds of force. The retracting inner blade acts to guide the motion of the wire, preventing it from cutting too to high into the skin.

We built a prototype of the device out of some aluminum, a few healthy dabs of super glue, and a length of wire studded with diamond dust. If you look closely, you can see the inner blade and loop of wire retracting into the shaft of the device when we trip the spring:

Once we had a device that consistently behaved the way we wanted it to, we tested it on pigs’ feet that we got from a local butcher shop to see if it could cut and extract skin samples.

Turns out that the cake cutting mechanism works fairly well! We took our device prototypes back to the clinic to get some feedback from experienced dermatologists and novice primary care physicians. We were especially excited to see the family doctors and their residents-in-training play with the device. They tried it on bananas, oranges, styrofoam boxes—anything they could get their hands on (except their own skin. We had to draw the line somewhere). Using our device, an inexperienced doctor can take a clean punch biopsy with one swift motion. We hope this could one day lead to faster diagnoses and more effective treatments for patients.