Letter pubs.acs.org/macroletters
Aligned Bacterial Cellulose Arrays as “Green” Nanofibers for Composite Materials Muhammad M. Rahman and Anil N. Netravali* Department of Fiber Science & Apparel Design, Cornell University, 37 Forest Home Drive, Ithaca, New York 14853, United States ABSTRACT: Conventional bacterial cellulose (BC) membranes are formed by randomly aligned nanofibrils stacked in a reticulated fashion. While such membranes are used in many applications, full utilization of the mechanical properties of nanofibrils has not been achieved in composite materials because of their random alignment. In the present research, aligned BC, in the form of arrays, has been developed through an optimally designed polydimethylsiloxane grating substrate placed at the air−liquid interface of the culture medium. The substrate prepared with the help of a 3-D printed mold served as an alignment template providing bacteria a preferred direction for BC growth. Additional alignment in BC arrays was obtained by stretching the hydrogel at a controlled strain rate and strain ratio. The degree of orientation, Young’s modulus, and tensile strength of aligned BC arrays increased by approximately 184, 409, and 256%, respectively, compared to the conventional BC pellicle. Such aligned BC arrays could open up future opportunities to design lightweight advanced “green” composites as alternatives to petroleum-based composites for a range of technical applications.
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all directions without any restriction during fermentation, alignment of nanofibrils cannot be easily obtained. However, some intrinsic characteristics such as crystallinity and degree of polymerization have been manipulated to improve the mechanical properties by adjusting chemicals in the culture medium.16 Here we report the development of a facile method for onestep growth of aligned BC for composite materials through an optimally designed polydimethylsiloxane (PDMS) grating substrate placed at the air−liquid interface of the culture medium. The grating substrate serves as a template that provides the bacteria a preferred direction for growth of BC. The resulting BC, in the form of arrays, possesses excellent alignment and uniformity. Further enhancement of the alignment was obtained on the BC arrays with optimized stretching at a strain rate of 0.1% min−1 and strain ratio of 1.20. Stretched and dried BC arrays show promising mechanical properties in comparison to the conventional BC pellicle as a result of the preferential alignment. Importantly, this simple design produces aligned BC arrays that provide a new way to develop high-performance multifunctional “green” materials for various applications. Aligned BC arrays were produced by placing an optimally designed PDMS grating substrate at the air−liquid interface of the culture medium, while the surface with gratings (fins) was placed in contact with the culture medium. Figure 1(a,b,c) illustrates the procedure used to create the grating substrate,
ncreased environmental awareness and the alarming rate of petroleum depletion have triggered a paradigm shift toward the development of eco-friendly and biodegradable materials. Cellulose is the most abundantly available naturally occurring, eco-friendly, and biodegradable polymer produced by plants, sea animals, and some species of fungi, algae, and bacteria.1 Significant research has been conducted to produce cellulosebased eco-friendly structures that result in comparably performing materials to many petroleum-based materials.2−5 Bacterial cellulose (BC) produced by some aerobic bacteria has a native cellulose I crystal structure with hydrogen-bonded semicrystalline polymer chains in an extended chain conformation.6 It is synthesized as a multilayer hydrogel at the air− liquid interface of a culture medium containing carbon and nitrogen sources. Although having an identical chemical structure to plant-based cellulose, BC is extremely pure and does not contain hemicellulose, lignin, pectin, or wax.7 It is preferred over plant cellulose due to higher crystallinity, degree of polymerization, and specific surface area.7 Also, it has some unique functional properties such as excellent specific mechanical properties,6 high optical transparency,8 biocompatibility, and low thermal expansion.9 Hence, BC is considered as a promising multifunctional “green” building block in a variety of applications.10 A conventional BC microstructure is formed by randomly aligned ribbon-shaped nanofibrils stacked in a reticulated fashion.11 Earlier, the conventional BC has been utilized as reinforcement in polymer-based composites.12−15 Because of the random alignment of these BC nanofibrils, full utilization of their mechanical properties is yet to be achieved as reinforcement in composite materials. Since the bacteria move freely in © XXXX American Chemical Society
Received: August 14, 2016 Accepted: September 7, 2016
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DOI: 10.1021/acsmacrolett.6b00621 ACS Macro Lett. 2016, 5, 1070−1074
Letter
ACS Macro Letters
Figure 1. Description of experimental set-up. (a) An Objet VeroClear RGD810 mold fabricated using a 3-D printer, (b) PDMS poured onto Objet VeroClear RGD810 mold to get negative replica of the mold, (c) PDMS grating substrate peeled off from the mold, (d) PDMS substrate placed at the air−liquid interface of the culture medium with the grating surface placed in contact with the culture medium and cultivated for 14 days, (e) photograph of BC arrays produced after 14 days, and (f) stress−strain plot of a BC array at a strain rate of 0.1% min−1. Inset photograph shows the BC specimen being stretched.
hydrogen bonding. BC nanofibrils try to straighten themselves in the stretching direction with increased stretching unless they are fully constrained. Further stretching beyond the optimum strain ratio led to fracture in the fibrous network due to entanglements. During the stretching process, a continuous squeezing out of interstitial water from the fibrous network occurs which results in irreversible hydrogen bond formation between BC nanofibrils. Hence, a lateral shrinkage with maximum alignment in the stretching direction can be observed. To utilize the potential of alignment along the stretching direction, establishing maximum stretch without damage of the fibrous network is most important. Maximum stretch of BC without damage can be obtained by optimizing two limiting factors, i.e., strain rate and strain ratio. It has been shown that high strain rate and strain ratio typically lead to nanofibril fracture.18 Therefore, some initial experiments were performed to determine the optimum strain ratio and strain rate for the largest inelastic deformation in the stretching direction for maximum alignment. Figure 1(f) shows typical stress vs strain plot of as-grown BC hydrogel arrays at a strain rate of 0.1% min−1, and the inset photograph shows the BC specimen being stretched. The optimum stretching of arrays was observed at a strain ratio of 1.20 without any initiation of nanofibrillar fracture. Stretching to higher strain ratios (>1.20) displayed initiation of fracture, through stress drop, in the weakest zone of the hydrogel arrays in nearly all cases. As can be expected, very low strain rate was found to be better to obtain the maximum strain ratio. Low strain rate provides sufficient time for the maximum reorganization and reduces the stress on polymeric fibrils. Based on the preliminary experiments, the strain rate was set to 0.1% min−1. Stretching at extremely low strain rate (
