Floral Cosmonauts: Self-assembling silver dendrite networks in microgravity

Harpreet Sareen, Anna Garbier

Biological neuronal networks are highly complex and interconnected with superior information processing capabilities. Such networks have previously served as a model for creating inorganic synapses [1] through silver compounds. However, efficiently generating such complex networks through conventional fabrication remains a challenge. Our mission objective was to grow nanometer/sub-micrometer scale silver dendrite networks in microgravity and characterize them for nanoscale manufacturing on Earth. 

Convective forces due to gravity on Earth introduce defects during crystallization process. However, removing the gravity vector during the process of self-assembly of such networks can potentially lead to thin and long (high aspect ratio) one dimensional nanowire networks, with controllable growth parameters. Our payload is a self-contained experiment (4.0" x 4.0" x 7.0", 1.1lbm) housing growth media and electronics powering this growth over the microgravity window.

The chemical media comprises of Silver Nitrate media with three different molar concentrations  (0.1mM, 1mM, 10mM) and starting seeds (28Ga Ag wire in each chamber) correspondingly. When power is supplied, reduction at the cathode causes dendritic silver crystal networks to form in real time. Each macro structure is produced in one minute. In three minutes, we produced three different networks: lateral division (horizontal wire placement), clustered formation (vertical wire placement), and longitudinal outgrowth (vertical wire placement). This time-based growth and output had earlier been verified in our petri-dish experiments, with matched quantity/concentration/voltage to be used within the Blue Origin Nanolab.

The above tests were replicated in the NanoLab, with each growth also being video recorded from the triple containment experiment chambers. The electronics in the NanoLab listen to incoming commands from the Blue Origin rocket to trigger the growth and perform lab maneuvers at precise intervals. No two crystallization chambers or their respective cameras run concurrently. These recordings are then also saved on file. 

Post flight, each chamber's crystal growth showed approximately 2x volume and less dense  clusters than on Earth. The dendritic crystals obtained by such a method should have more perfect microcrystalline structures. The rendered samples will be observed for macro, micro and nanoscale growth in comparison to control samples on Earth. These silver nanowires are currently being characterized through SEM (Scanning Electronic Microscopy), TEM (Tunneling Electron Microscopy), and electron diffraction. Such self-assembly of nanowires, with control over growth parameters and without use of any templates could have applications to produce atomic switch networks. Given high aspect ratio and large synthesis, these networks could potentially be a hardware alternative to neuronal networks, as opposed to currently pre-dominant software only techniques.

Our current manufacturing techniques on Earth utilize 'clean rooms' for fabrication of extremely precise electronic networks. Current techniques need us building fabrication systems many floors under the ground to reduce the noise floor of systems. If silent space -- the microgravity -- is 'clean', we hypothesize the fabrication of future nano-electronics would not be on Earth but in Space. The goal of this project was to move a step forward in that direction


[1] Ohno, Takeo, et al. "Short-term plasticity and long-term potentiation mimicked in single inorganic synapses."  10.8 (2011): 591.Nature materials