Hallway Talk

Sometimes hallway talk is perceived as a waste of time – deadlines, deliverables, grant writing, and the inevitable battles with font sizes and embedded movies have become very serious matters from which we cannot be distracted. But I have a particular affection for hallway interaction because it led to a decisive shift in my research. It is an understated, powerful catalyst for interaction – just like lunches, coffee breaks, or having a beer. After talking in the hallway, some go back in their office and close the door, while others leave their’s open.  

As an applied physicist working on femtosecond lasers and nonlinear optics for the better part of a decade, joining a Biomedical Engineering Department was a step into unfamiliar territory.  A conversation with a colleague that was largely about Boston/New York Sports rivalries (no comment) ended up with me being handed a small plastic-looking disc made of silk. This was to be used as a tissue engineering scaffold to generate a cornea and it needed small holes for cells to grow into so that, eventually, the cornea would grow in a dish.  We intended to add the holes with femtosecond machining but when doing so, the laser spot was nowhere to be found on the surface of the disc. We quickly realized that the silk surface was so smooth that there was no scattering whatsoever. This opened up an unlikely marriage of optics and silk that neither of us would have independently thought of and that quickly expanded into a materials transformation effort that continues to grow today. 

Silk proteins are a unique family of biopolymers because of their structural and biological properties. From a materials science perspective, silks spun by spiders and silkworms are the strongest and toughest natural fibers known. Native silkworm silk can be simply boiled to extract the constituent protein.  The resulting water-based silk solution can be used as the building block for a multitude of material formats, ranging from fibers, blocks, gels, mats, or plastic-like stable films with control of thicknesses down to below ten nanometers and patterned to form diffractive optics and 3D photonic crystals.  These films are formed by simple casting of purified silk solution which crystallizes upon exposure to air, without the need for exogenous cross-linking reactions or post processing.

Most importantly, silk is processed in an all-water-based, room temperature, neutral pH environment.  Further, silk materials are edible and biocompatible, along with the mechanical toughness and stability described above. They have precisely tunable mechanical properties, and can have programmable lifetime. Lastly when forming materials, any dopant that is added to the solution is stabilized and its function maintained in the material forms described above.

This is a particularly rich context for materials, heralding a “silk renaissance” that brings together form (through the multiple material formats) and function (through material sustainability, biocompatibility, easy doping and biological activation).  This coexistence of form and function transforms the context for materials, enabling unusual devices or adding value to existing ones.  Among these are edible metamaterials, implantable optics, and resorbable electronics. Analytical devices can be designed out of a biocompatible and implantable material, with the chemistries needed for detection stabilized safely in the silk matrix.

This principle was used, for example, to stabilize blood within a diffraction grating that “sensed itself”: when the device is formed (by casting a blood-silk solution on a diffraction grating master, allowing the solution to dry and then lifting off the free-standing doped film), the hemoglobin remains active in the silk film and continues to respond to the external environment. This is one of the many possibilities that can be imagined when form and function come together. 

Lots of exciting interdisciplinary possibilities exist as new material properties enable familiar techniques and devices to operate in contexts where they usually do not. 

At the root of this is the human connection between people that have the pleasure of reimagining things through their disciplinary experience. It would not have happened but for that dialogue where experiences were shared and came together seamlessly – in a hallway.