ACS PRF | ACS
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43336-G7
Synthesis of Intelligent, Bio-Inspired Hydrogel Microlens Arrays
Variable-focus microlens arrays are of particular interest for optical signal processing, MEMS devices and sensors. Previously, we have demonstrated tunable microlens by means of electrowetting and fabrication of biomimetic microlens arrays that mimic the structure and function of biological lens arrays discovered in brittlestar. In the past year, we fabricated tunable microfluidic optical device, which integrated microlens arrays with a microfluidic channel. By actuating fluids with different refractive indices and varying the concentration of dye molecules, we demonstrate the change of the focal length by 10 times and tunable transmission of the microlens, respectively.
The use of soft materials, such as responsive hydrogels will allow us to change the lens volume and shape up to several hundred percent in response to the external stimuli. However, the soft nature of hydrogels in water makes it questionable of their structural integrity during actuation. As shown in our previous study of biomimetic hydrogel lens arrays from photoacid crosslinkable copolymers of poly(2-hydroxyethyl methacrylate), the hydrogel lens tended to collapse in water. In light of this, we turned our focus to study the mechanical behavior of structured hydrogels and the ability when exposed to water. We copolymerized methyl methacrylates with various hydrogel monomers, including 2-hydroxyethyl methacrylate, N-isopropryl acrylamide, methacrylic acid, and ethyleneglycol dimethacrylate, and vary their chemical compositions to tailor the stability of patterned hydrogels both in the air and in contact with water.
In parallel to optimization of the composition to tune the hydrogel response to pH and temperature, we began a new study to fabricate a single-component, strain responsive, variable-focus microlens array (both concave and convex) with real-time tunability. Most of the reported tunable microlens arrays are multi-component systems, and require complex fabrication and assembly processes. Often times, the lens focal length cannot be tuned continuously in real-time. Based on confined buckling of a soft elastomer, poly(dimethylsiloxane) (PDMS), coated with a patterned thin layer of hard oxide, we fabricated concave microlens array by biaxial mechanical strain. The convex microlens array was fabricated by replica molding from the concave one. It offers several advantages over other tuning mechanisms, including (1) reversible tuning of lens shape in real time, and (2) the timing and amount of strain applied to the lens can be separately controlled, resulting in smooth and continuous change of the focal length on demand. An 83% and 158% increase of focal length in concave lenses was observed experimentally at 12% and 17% applied strains, respectively. Further, the strain-responsive behavior of the concave microlens arrays can be tuned by simply changing the pre-strain. The new lens will be very useful in MEMS devices, optical devices, and sensors.
Learning from nature and developing new synthetic strategies to mimic the unique hierarchical design and function found in bio-organisms have been the major focus of the PI's research. The PRF fund has provided a jump start of her career at University of Pennsylvania to carry out multi-disciplinary research ranging from material synthesis to device fabrication in her lab. During the 2-year grant period, the PI and her students have published 3 papers (1 as invited review) and 1 conference proceedings, 1 submitted for publication, and 1 more manuscript in preparation. The PI has given 8 invited lectures related to the hydrogel patterning and microlens fabrication. To broaden the impact of bio-inspired research, the PI has lead-organized 3 well-received international meeting at the American Chemical Society (ACS), and Materials Research Society (MRS), respectively, which aimed to bring together a diverse group of researchers from biology, chemistry, physics, bioengineering, chemical engineering, and material science to discuss the latest advancements in biological and bio-inspired materials synthesis and assembly.
The PI continues to emphasize the importance of education and training of students. She has supervised 1 undergraduate student for independent study, 1 summer research fellow through NSF Research Experiences for Undergraduates (REU) program, and 1 high school teacher through NSF Research Experiences for teachers (RET) program. On the graduate level, 1 master student has completed his master degree on research in tunable microlens arrays integrated with microfluidic devices and currently is a PhD student at University of Wisconsin. 1 PhD student is studying the mechanical properties of different hydrogels in air and in water, and how to manipulate their responsiveness to pH and temperature without sacrificing their structure integrity. Together with the postdoc supported on this grant, they have developed a novel strain responsive concave and convex microlens arrays, and the results were submitted to Appl. Phys. Lett. for consideration of publication. Through involvement in research related to PRF project, another PhD student and postdoc received extensive training in the chemistry and patterning of hydrogels.
