Regenerating leather waste for flexible pressure sensing applications
- 59 Downloads
Pressure sensor can be applied in a wide range of fields, such as voice recognition, human motions detection and artificial electronic skin, the sensing of which is greatly influenced by the flexibility and stretchability of substrate materials. Here, based on the piezoresistive effect, new kinds of flexible pressure sensors have been realized from a pair of flexible and biocompatible collagen films: one is coated by silver nanowires (Ag NWs) and the other by interdigital electrode, respectively. The collagen films are regenerated from leather waste and could bring economic benefits to society. The prepared pressure sensors are applied for voice recognition and human motion detection.
KeywordsLeather waste Collagen films Pressure sensor Flexible Human motion detection
- Ag NWs
Electron donor-electron acceptor
The initial current
Poly (ethylene naphthalate)
Poly (ethylene terephthalate)
Scanning electron microscope
Thermo gravimetric analyzer
The relative change in the current
The change in applied pressure
Driven by the development of intelligent manufacturing, intelligent robotics, human-machine interaction and biomedical diagnosis have received extensive attention [1, 2, 3, 4, 5, 6, 7, 8], and they have high requirments for flexible bending performances. However, many traditional electronic devices are based on rigid semiconductor silicon materials. Thus the bending and elongation characteristics of the devices are significantly limited. The development of flexible sensing techonology could solve this problem effectively, such as new types of flexible pressure sensors, which have shown part of characteristics of human skin [4, 5, 6, 7, 8, 9, 10]. Moreover, a flexible sensor with the property of external forces perception can be used for bionic electronic skin and various wearable electronic devices [9, 10].
There are some key components which determine the basic performances of the electronics, such as active materilas, substrate materials and electrodes. Among these considerations, the substrate materials play an important role [5, 7, 8, 9, 10, 11, 12]. The substrate materials with ideal flexibility and robust mechanical strength can provide a sensitive response to the external pressure. Generally, industrial fabricated polymers and some natural macromolecular materials can both serve as flexible substrates [10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27]. Comparing the two kinds of materials, the latter one has prominent advantages due to the biocompatibility, biodegradability and sustainability, thus serving as desirable substrate for the next-generation flexible electronic devices .
Normally, many types industrial fabricated polymers, such as polydimethylsiloxane (PDMS) [10, 14, 15], poly (ethylene terephthalate) (PET) [16, 17], poly (ethylene naphthalate) (PEN)  and poly (imide) (PI) [15, 18] have been used as substrates for flexible electronic devices. Bao’s group fabricated a flexible and sensitive organic thin film transistor based on PDMS, which can be used for pulse monitoring in cardiovascular surveillance . Though these polymers have prominent advantages in flexibility, stretchability and even excellent fit between the device and tissue, their manufacturing process and fabricated devices are not eco-friendly in the long term. However, natural macromolecular materials, such as collagen , silk fibroin [20, 21, 22], cellulose [23, 24, 25, 26], chitin  and so on, will overcome those difficulties and be the ideal choices in the development of electronic devices due to their flexibility, biodegradability and biocompatibility. Roger’s group integrated thin-film silicon with transient circuits based on silk films . Their device can be used for versatile applications due to the tunable degradation time of silk-fibroin films. However, the fabrication of their devices are still in high-cost and undesireable.
Considering that approximately 200 kg of leather would be manufactured from 1 ton of wet-salted skin/hide, resulting in the generation of more than 600 kg leather waste . Collagen is an important component in leather waste. The usage of the above collagen can not only reduce the leather waste for the environment, but also promote the development of functional materials. In recent years, there have been many researches on the applications of collagen fibers from leather, such as using the collagen fibers to fabricate microwave absorption materials, anode materials, conductive leather and magnetic composites [29, 30, 31, 32]. However, research on using collagen films as substrates from leather waste for flexible electronics has not been reported.
2 Main text
2.1 Preparation and Properities of the flexible collagen film
The collagen fibers and regenerated collagen films are firstly studied by using Fourier-transform infrared (FT-IR) analysis (Fig. 1b). The peak of Amide A (3308 cm− 1) represents N-H stretching vibration absorption, which relates to the hydrogen bonds between the collagen chains. Amide I (1659 cm− 1), Amide II (1550 cm− 1) and the Amide III (1450 cm− 1, 1241 cm− 1) represent C=O stretching vibration adsorbtion, N-H bending vibration adsorbtion, C-N stretching vibration absorption and N-H bending vibration adsorbtion, respectively [42, 43]. The profile of the collagen film is similar to the collagen fibers, and this means they have similar chemical functions (Fig. 1b). However, there are also some differences which are clearly identified from the spectra. The peak of Amide A (3393 cm− 1) in collagen film shifts to lower frequency and narrows down, indicating some hydrogen bonds have ruptured between the collagen chains, which is also consistent with the EDA theory. The collagen fibers possess an intensive band at 1659 cm− 1 corresponding to Amide I in helical form. However, the peak of Amide I (1650 cm− 1) in collagen film shifts to higher frequency, indicating that there are some changes with the triple helical structure of the collagen film.
To investigate the thermal stability of the collagen fibers and regenerated collagen film, we obtained the Thermo Gravimetric Analyzer (TGA) profiles of the collagen fibers and film (10 °C min− 1 ramp, N2 atmosphere). Normally, The thermal stability of a large molecule depends on its molecular weight, spatial structure and intermolecular forces. Here, there are two main stages of the weight loss of the collagen fibers and films: one is at 40~150 °C, representing the loss of absorbed and bound water of collagen fibers and films; the other is at 200~500 °C, representing the thermal decomposition of polypeptide chains in collagen . The decomposition temperature of the films (254 °C) is lower than that of the fibers (276 °C) (as shown in Fig. 1c). That is because the ILs dissolves collagen fibers mainly by breaking the forces of the hydrogen bonds and ionic bonds between the collagen molecules, and in the regeneration process, the location and quantity of the hydrogen bonds have changed and even some hydrogen bonds have been disrupted due to the high dissolution temperature.
2.2 Preparation and performances of the flexible pressure sensor
2.3 Human physical motions sensing
2.4 Voice recognition sensing
Apart from detecting some human physical motions, the pressure sensor is able to recognize different voices of human because of its high sensitivity to applied pressure and the piezoresistive characteristics (Fig. 5d, Additional file 1: Figure S5). A fabricated pressure sensor has been attached to the neck of the volunteer to detect the muscle motions. When the volunteer pronounces different words, such as “bee”, “apple”, “tomato” and “watermelon”, different current signals of the sensor are collected. Every graph in Additional file 1: Figure S5 represents one word. The current signal patterns increase corresponding to the increasing syllables of the word. When the volunteer repeats the same word every few seconds, the current signals keep consistent, which indicates the stability of the sensor and easy recovery from the deformation caused by the pressure of muscle motions. Also, when the volunteer pronounces different words, the current signal changes because different words give rise to different muscle motions. The striking differences between these current signals indicate the good performance of the pressure sensor to act as a voice recognition device.
In conclusion, we have demonstrated the use of regenerated collagen films from leather waste as the substrate for flexible pressure sensors. The prepared pressure sensor has a high sensitivity of 13.33 KPa− 1 in a range of 64–1909 Pa and 1.27 KPa− 1 in a range of 2545–6364 Pa. And the sensor exhibits a response time of 349 ms and relaxation time of 147 ms under 636 Pa. Moreover, the sensor has good stability and repeatability from the measurment of current changes responding to applied pressure for repeated loading/unloading operation under 3182 Pa. In addition, the sensor can monitor different ranges of human motions including voice recognition, finger and wrist bending-releasing, which demonstrates its potential applications in speech recognition and synchronously monitoring some physical activities of patients. This work provide a new thought for the development and application of bio-waste materials, such as leather waste in pressure sensing technology, biomedical diagnosis, human-machine interaction and so on.
4 Experimental section
4.1 Preparation of the collagen films
[BMIM] Cl (3.0 g) was placed into a 50 mL dried round flask with magnetic stirrer. Pickled skin powders (180 mg) were added in potions of 3 wt% of [BMIM] Cl each time and then microcrystalline cellulose (30 mg) were added as a cross-linker, the process of which was controlled by an oil bath at 100 °C for 5 h. When the solution is like syrup, the mixture solution was well spreaded on a glass sheet and then immersed into deionized water which served as a precipitator. Then the film was washed several times to ensure the ionic liquid washed away completely. Then a regenerated collagen film was obtained and then dried in a vaccum drier to a constant weight.
4.2 Characterization of the collagen films
The morphologies of blank collagen films and Ag NWs/collagen film were characterized by scanning electron microscopy (SEM) (JSM-7800F, Japan) with an accelerating voltage of 3 kV. FT-IR spectra were recorded with a FTIR 460 plus (JASCO, Japan). The thermal properties of collagen films were detected using TGA (TG209-F3, Netzsch) with a ramp of 10 °C min− 1 under N2 atmosphere. X-ray diffraction (XRD) patterns were recorded by Advance diffractometer (AXS D8, Bruker) using Nickel-filtered Cu Kα radiation (λ = 1.5406 Å). UV-vis spectra were measured using Shimadzu UV-1750.
4.3 Fabrication of the flexible pressure sensor
Interdigital electrodes of silver paints were painted on a flexible collagen film. Ag NWs were dissolved in ethanol with a concentration of 5 mg mL− 1 and the solution was dropped on the other collagen film. It was then spin-coated at a rate of 2000 rpm for 30 s for three times to obtain a mean sheet resistance of 37 Ω sq.− 1. Then the two films coated with Ag NWs and silver paints were pressed together with edges and adhered by scotch tape.
4.4 Performances of the flexible pressure sensor
The current-voltage (C-V) curves were collected using an electrochemical workstation (CHI 660E) between 0 and 0.5 V at a scan rate of 0.01V s-1. The other current measurements, including instant current-time curves recorded, human motion detection and voice recognition, were characterized by Keithley 4200 semiconductor characterization system. A 1.1×1.4 cm2 slight glass was covered on the pressure sensor to ensure homogeneous application of external pressure when the sensitivity and stability curves of the sensor were collected. And the pressure in experiments was applied by loading standard weights on the sensor and the size of the effective area is 1.1×1.4 cm2. To measure the performance of the sensor while it was bent, the sensor was attached to the joint of an index finger of the volunteer with adhesive tapes. When the volunteer performed finger bending-releasing motions, the current responses corresponding to bending and releasing motions can be recorded. And when the sensor was attached to the wrist of the volunteer to detect some large range of motions, apparent and repeatable current signals can also be recorded during the wrist bending. The voice recognition tests were also conducted. The sensor was attached to the neck of the volunteer to detect the muscle motions. When the volunteer pronounced different words, such as “bee”, “apple”, “tomato” and “watermelon”, current signals were collected. The applied voltage between the interdigital electrodes was 0.5 V, and the above current signals recorded were current-time (I-t) curves in the measurements.
The authors acknowledge all volunteers who participated in the project.
JL and BZ contributed equally to this work. JL and BZ designed experiments, performed, analyzed the results, and drafted the manuscript. RZ, KZ and RX were responsible for preparation of some experiment materials and some characterizations. JW and WZ helped to revise the manuscript. FH, SL and BZ supervised the project, helped design the experiments, and revised the manuscript. All authors read and approved the final manuscript.
The project was supported by the National Key R&D Program of China (Grant No. 2017YFA0207201), National Natural Science Foundation of China (51702155, 21574065, 21604038, 21504043, 21604040), National Science Foundation for Distinguished Young Scholars (21625401), the Jiangsu Provincial Founds for Natural Science Foundation (BK20170975, BK20160975, BK20160981) and the Natural Science Fund for Colleges and Universities in Jiangsu Province (17KJB480007).
The authors declare that they have no competing interests.
- 12.Zou B, Chen Y, Liu Y, Xie R, Du Q, Zhang T, Shen Y, Zheng B, Li S, Wu J, Zhang W, Huang W, Huang X, Huo F. Repurposed leather with sensing capabilities for multifunctional electronic skin. Adv Sci. 2018;1801283.Google Scholar
- 20.Hwang SW, Tao H, Kim DH, Cheng H, Song JK, Rill E, Brenckle MA, Panilaitis B, Won SM, Kim YS, Song YM, Yu KJ, Ameen A, Li R, Su Y, Yang M, Kaplan DL, Zakin MR, Slepian MJ, Huang Y, Omenetto FG, Rogers JA. A physically transient form of silicon electronics. Science. 2012;337:1640–4.CrossRefGoogle Scholar
- 28.Veeger L. Ecological procedure to solve the tannery waste problems. J Am Leather Chem Assoc. 1993;88:326–9.Google Scholar
- 42.Renugopalakrishnan V, Bhatnagar RS. Hydrogen-bonded water in collagen structure - a Ft-Ir spectroscopic corroboration. Biophys J. 1984;45:A163.Google Scholar
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.