Reprogrammable, Sustainable, and 3D‐Printable Cellulose Hydroplastic

Abstract Modern human societies are highly dependent on plastic materials, however, the bulk of them are non‐renewable commodity plastics that cause pollution problems and consume large amounts of energy for their thermal processing activities. In this article, a sustainable cellulose hydroplastic material and its composites, that can be shaped repeatedly into various 2D/3D geometries using just water are introduced. In the wet state, their high flexibility and ductility make it conducive for the shaping to take place. In the ambient environment, the wet hydroplastic transits spontaneously into rigid materials with its intended shape in a short time of <30 min despite a thickness of hundreds of microns. They also possess humidity resistance and are structurally stable in highly humid environments. Given their excellent mechanical properties, geometry reprogrammability, bio‐based, and biodegradable nature, cellulose hydroplastic poses as a sustainable alternative to traditional plastic materials and even “green” thermoplastics. This article also demonstrates the possibility of 3D‐printing these hydroplastics and the potential of employing them in electronics applications. The demonstrated hydroshapable structural electronic components show capability in performing electronic functions, load‐bearing ability and geometry versatility, which are attractive features for lightweight, customizable and geometry‐unique electronic devices.


Figure S1. ATR-FTIR spectroscopy of cellulose hydroplastic, cellulose acetate and commercial α-cellulose.
Cellulose hydroplastics have chemical structure similar as pristine α-cellulose upon successful deacetylation of cellulose acetate sheets, with the disappearance of the three distinct peaks at 1735 cm -1 , 1370 cm -1 and 1220 cm -1 , corresponding to the carbonyl bond stretching (νC=O), methyl bending (δC-CH3) and alkoxy group stretching (νC-O-C) vibrations of the acetate group, respectively.The cellulosic hydrogel was synthesized using a conventional method crosslinking carboxymethyl cellulose (CMC) with epichlorohydrin (ECH). [1,2]Briefly, 3wt % CMC was dissolved in alkaline solution (6 wt% NaOH in water).30 g of the CMC solution reacted with 3ml of epicholorohydrin at 60 o C for 12 h.The hydrogel was then washed with DI water until pH reaches around 7. Due to the large amount of water present in the hydrogel, it exhibits a very high softness unsuitable for the hydroshaping process.The high water content also leads to a lengthy drying time along with an immense shrinkage upon drying.These features set hydrogels apart from hydroplastics.Top images depict various water clusters differentiated by their color, with each dot representing a water molecule in cubic simulation box of equilibrated systems.The cluster analysis was performed by DBSCAN method [3] implemented on the 3D coordinates of oxygens of water in the simulation box.For cluster analysis, the optimum cut-off distance was set as 0.35 nm obtained by K-distance analysis and a minimum threshold of 3 connected water molecules was set to define a cluster core point.The histograms of shows the water cluster size distribution in respective systems.The results shown are the time-average values from last 10ns of simulation trajectory.Transiting from the dry cellulose hydroplastic to the wet hydroplastic, a red-shift can be observed from the intensity increase at smaller wavenumber between 3100-3300 cm -1 , and intensity reduction at larger wavenumber between 3300-3500 cm -1 (Figure S8A).The O-H stretching band can be deconvoluted into 4 peaks centered at wavenumber 3230, 3420, 3557 and 3630 cm -1 , representing tetrahedrally-bonded water, partially-bonded water (both 3432 and 3557 cm -1 ) and non-bonded water molecules, respectively.The dry state has areal proportion of 21.2%, 77.0% and 1.8%, for tetrahedrally-bonded water, partially-bonded water and nonbonded water molecules, respectively (Figure S8B).The wet state has areal proportion of 32.2%, 65.0% and 2.8%, for tetrahedrally-bonded water, partially-bonded water and nonbonded water molecules, respectively (Figure S8C).This corresponds to a significant increase in tetrahedrally-bonded water molecules and a reduction in partially-bonded water molecules upon transiting from dry to wet state.For the hydroshaped strip sensor, the body impedance (  ) at kHz frequencies is largely capacitive and can be modelled as a 100 pF capacitor (  ) in Fig. S13A.This impedance is connected to the hydroplastic sensor (total resistance 2 ℎ ) at the touch point, which divides the hydroplastic sensor into two resistors with values  ℎ +  and  ℎ − . is proportional to the displacement of the touch point from the centre of the hydroplastic sensor, and  = 0 would correspond to a touch exactly in the middle of the linear sensor.
The op-amps apply an AC voltage of   =   cos  to the sensor, and when a touch happens, the body capacitance causes some AC current ( 1 +  2 ) to flow into the sensor.Importantly, any asymmetry due to the touch point ( ≠ 0) results in a slightly different AC current through the two op amp feedback resistors.The currents through the feedback resistors are: Here,   =   ( 1 +  2 ) is the AC voltage at the touch point.current difference can be solved, To first order in  (this is justified because   ≫  ℎ and ), the output AC voltage measured by the lock in amplifier is Thus, the voltage measured by the lock in amplifier is proportional to the displacement .In particular, since the body impedance is largely capacitive, the imaginary part of   dominates, and we can define In the actual implementation of the circuit, a 100 kΩ resistor is used for   , and the hydroplastic sensor is AC coupled to the op-amps with 22 μF capacitors.This is to eliminate the effect of unequal input offset voltages of the op-amps.  is a 1 Vpp signal at 1 kHz supplied by a SR830, and the signals from the two op amp outputs goes into the SR830 in differential input mode which directly measures the complex phasor   .Because the body impedance is largely capacitive, the quadrature output of the lock in amplifier,  − ≡ (  ) is obtained by using the Y-output of the SR830 and setting the internal phase shift of the lock-in amplifier to 0. Figure S13B-C shows this lock in amplifier reading which varies as different points on the hydroplastic sensor is touched.Figure S14 shows how two independent amplifier channels connected to a single hydroplastic sensor can enable two-dimensional touch positional sensing. [4]One SR830 lock-in amplifier measures the x-position, while the other measures the y-position.By reading these values in real-time simultaneously, the 2D position of the touch point can be measured.The system consists of the 3D-printed hydroplastic electric Xun sensor, Arduino Nano chipset and a speaker.The Xun possesses an array of six touch-pads, each of which comprises an electrode contact pair.Within each electrode pair, the first electrode (EC1 -6) is connected to +5V in common with those of the others, while the second electrode (E1 -6) is weakly pulled to ground via a high ohmic (22 MΩ) resistor.Each of the second electrode is also coupled to digital GPIOs (D2 -D7) of Arduino Nano chipset configured as input pins for detecting the digital states.When the touch pads are not pressed, the electrode pair contacts are opened by default such that the interfacing GPIO pins normally assume digital low states.As the player presses a touch pad, the electrode pair contact is electrically closed by the fingers' skin impedance.This pulls up the electrical potential at the interfacing GPIO pin inducing a digital high state.Based on the combination of the input states detected, the microcontroller is programmed to generate pulse signal of the specified frequencies at the digital output pin, D9, thus driving a low-power audio speaker to produce the desired musical note in response to the user's fingering.The musical notes and acoustic signal frequencies generated in response to each touch combinations are presented in Table S2.

Figure S3 .
Figure S3.Water content measurement of cellulose hydroplastic in wet state and ambient (60%RH) dry state by thermogravimetric analysis.

Figure S4 .
Figure S4.Cellulosic hydrogel's water content, drying time and shrinkage.(A) Thermogravimetric curve indicating a very high water content of around 99.9 wt%.(B) Image shows a large deformation with small pressure applied on the hydrogel, demonstrating its softness.(C) Images shows the drying process of the hydrogel.

Figure S8 .
Figure S8.Deconvolution of Raman scattering spectroscopy O-H bond stretching band of cellulose hydroplastic.(A) Raman spectrum of dry state and wet state.(B) Deconvoluted dry state O-H bond stretching band.(C) Deconvoluted wet state O-H bond stretching band.

Figure S10 .
Figure S10.Biodegradation of cellulose-carbon hydroplastic composites.(A) Images showing the process of cellulose-carbon hydroplastic composites enzymatic biodegradation at different time intervals.(B) Total sugar content as a function of time in the enzymolysis solution at which the cellulose-carbon hydroplastic composites undergo enzymatic biodegradation.

Figure S13 .
Figure S13.Surface capacitive sensor system of strip hydroplastic sensors.(A) Schematic of the signal conditioning circuit for touch position sensing.(B) Labelling of touch positions on "inverted-V-shaped" hydroplastic sensor.(C) Lock-in amplifier reading when various positions are touched as a function of time.

Figure S14 .
Figure S14.Surface capacitive sensor system of four quadrant dome-shaped hydroplastic sensor.(A) Schematic of the signal conditioning circuit for 2D touch position sensing.Two independent lock-in amplifier channels allow for 2D sensing of the touch position.(B) Labelling of touch positions on dome-shaped 2D hydroplastic sensor.(C) Lock-in amplifier readings when various positions are touched as a function of time.The same principle described for Figure S13 can be extended to the dome-shaped touch sensor.FigureS14shows how two independent amplifier channels connected to a single hydroplastic sensor can enable two-dimensional touch positional sensing.[4]One SR830 lock-in amplifier measures the x-position, while the other measures the y-position.By reading these values in real-time simultaneously, the 2D position of the touch point can be measured.

Figure S16 .
Figure S16.3D-printed PCB with PEDOT:PSS.(A) LED placed between two well-printed PEDOT:PSS conductive tracks on cellulose acetate.(B) Lighting up of the LED when current is passed through the two ends of the printed PEDOT:PSS tracks.(C) Delamination of uncovered PEDOT:PSS tracks from cellulose hydroplastic in water.(D) Defects on the printed PEDOT:PSS after the hydroshaping process.(E) Optical microscope image showing microcracks along a printed PEDOT:PSS track after hydroshaping.(F) Optical microscope image showing the partial removal of some PEDOT:PSS from its original location after hydroshaping.

Figure S17 .
Figure S17.Schematic circuit of Electric Xun system.(A) Schematic circuit diagram of hydroshaped electric Xun.(B) Illustration of Acoustic Xun versus electric Xun and demonstration of playing the electric Xun.