FEM simulations of multi-responsive hybrid nanorods

Flexible polymeric substrates with multi-responsive nanorods capable of pressure, temperature and humidity sensing, provide a smart material for artificial skin applications. The smart material consists of humidity and temperature responsive p-NIPAAm hydrogel core with defined Lower Critical Solution Temperature (LCST). The outer shell is build up by piezoelectric ZnO. The piezoelectric response of the Zinc Oxide shell is created from either external applied pressure and/or swelling of the inner hydrogel core. The hydrogel swells upon changes in humidity or temperature in the environment.
Based on the work of Muralter et al., p-NIPAAM with different LCST is obtainable by Initiated Chemical Vapor Deposition (iCVD). The process allows solvent-free preparation and deposition of highly uniform and conformal organic polymers. Additionally, iCVD allows cross linked polymerization of NIPAAm to obtain different swelling % and LCST [1].
Piezoelectricity in Zinc Oxide is dependent on the preferential orientation of the film grown. The work of Pilz et al. demonstrates the usage of Plasma Enhanced Atomic Layer Deposition (PE-ALD) to obtain thin Zinc Oxide films with controlled preferential orientation. For substrate temperatures below 100 °C, (100) preferential orientation has been obtained whereas the preferential orientation switches to (002) for temperature above 100 °C [2].
Preparation of nanorods is done by patterning a template material, where nanotrenches are patterned using Ultraviolet-Nanoimprint Lithography (UV-NIL) or Thermal-Nanoimprint Lithography (T-NIL). The nanopatterning step is followed by deposition of a Zinc Oxide layer followed by a hydrogel layer to fill the trenches. In the course of the nanopatterning process, different template materials will be investigated regarding their elastomeric properties. Investigated materials include PMMA, PUA and PDMS.
Moreover, the influence of the nanorod aspect ratio, Zinc Oxide layer thickness and template material on the swelling of the hydrogel core and the nanorod output piezoelectric potential is investigated using COMSOL Multiphysics.