'Like uprooting tree stumps' – a simpler method to manufacture biosensors

Publishing in Science Advances, researchers from KTH Royal Institute of Technology liken their technique to uprooting a tree stump with rope. 

The work aims at solving a bottleneck in fabricating nano-scale holes in thin membranes, a key component in a range of sensing and analysis technologies used in medical research and, increasingly, in specialized clinics where sequencing systems are used for outbreak surveillance, cancer genomics or infectious disease identification, among other things. Nanopores are valuable tools in these settings because their extremely small openings in ultra-thin materials can be used to detect and analyze individual molecules such as DNA and proteins.  

Frank Niklaus, professor of micro and nanosystems at KTH Royal Institute of Technology, says the study demonstrates the unique approach of using built-in mechanical stress to create nanoholes.  

A typical sensor device membrane has a single nanopore, which acts as a single molecule detector. These openings are produced one-at-a-time, using slow, expensive or difficult-to-scale nanofabrication techniques, such as electron-beam drilling and Transmission Electron Microscope (TEM) sculpting, says Xinxin Liu, a doctoral student at KTH Royal Institute of Technology and first author of the study. 

But the study demonstrated that this technique can produce large arrays of nanopores in parallel by harnessing fracture mechanics generating openings as small as about  
6-10 nm.  

The process begins with a stack of layers placed on a silicon wafer. The top layer is deliberately stretched under tension. Underneath is a second layer of material that will be etched away until a small pre-defined shape remains, which is meant to define the shape of the nanopore in the third layer of material underneath. This third layer is the sensor membrane in which the nanopore will be formed.

“The beam pulls on until it tears a nanoscale fragment from the membrane, creating the hole,” Liu says. “Much like a removing a tree stump with a rope – the beam attaches to the nanoscale anchor, pulling it away from the membrane, which serves as the ground.” 

The technique was tested successfully in multiple materials, including dielectric, metallic and semiconductor sensor membranes. The study showed these nanopores were effective at DNA analysis and molecule detection.  

Liu says the researchers have patented the technique and formed a startup to progress it toward commercial implementation. “We have already shipped samples to a collaborator for testing, so it has moved beyond the lab stage.”