Silk fibroin-based flexible pressure sensors: processing and application

doi: 10.1088/2752-5724/ad5f48

College of Polymer Science and Engineering, Sichuan University, State Key Laboratory of Polymer Materials Engineering, Chengdu 610065, Sichuan, People’s Republic of China

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Corresponding author: E-mail: kaike@scu.edu.cn

摘要

Abstract: AbstractWith the advent of the internet of things and artificial intelligence, flexible and portable pressure sensors have shown great application potential in human-computer interaction, personalized medicine and other fields. By comparison with traditional inorganic materials, flexible polymeric materials conformable to the human body are more suitable for the fabrication of wearable pressure sensors. Given the consumption of a huge amount of flexible wearable electronics in near future, it is necessary to turn their attention to biodegradable polymers for the fabrication of flexible pressure sensors toward the development requirement of green and sustainable electronics. In this paper, the structure and properties of silk fibroin (SF) are introduced, and the source and research progress of the piezoelectric properties of SF are systematically discussed. In addition, this paper summarizes the advance in the studies on SF-based capacitive, resistive, triboelectric, and piezoelectric sensors reported in recent years, and focuses on their fabrication methods and applications. Finally, this paper also puts forward the future development trend of high-efficiency fabrication and corresponding application of SF-based piezoelectric sensors. It offers new insights into the design and fabrication of green and biodegradable bioelectronics for in vitro and in vivo sensing applications.
- silk fibroin /
- flexible pressure sensor /
- piezoelectric /
- biodegradable /
- healthcare
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Figure 1. An overview of silk fibroin-based pressure sensors, including the performance of silk fibroin, fabrication and applications of silk fibroin-based pressure sensors. Demos of the application of silk fibroin-based pressure sensors in daily life. An array sensor attached to the back of the hand for remote control of the drone. Reprinted from [11], © 2020 Elsevier B.V. All rights reserved. Smart mask detecting breathing conditions and a smart glove detecting the degree of finger bending. Reproduced from [12], with permission from Springer Nature. Versatile E-skins for simultaneous temperature and stress monitoring. Reprinted with permission from [13]. Copyright (2017) American Chemical Society. Smart gloves for identifying the type of materials in contact with the object. Reprinted with permission from [14]. Copyright (2024) American Chemical Society. Devices for recognition of Morse codes and gestures. Reproduced from [15], with permission from Springer Nature. Reproduced from [16]. CC BY 4.0.

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Figure 2. Structure and properties of silk fibroins. (a) Hierarchical structure of silk fibers. Schematic illustration of the chemical structure heavy chain (b) and β-crystallite (c) of silk fibroin. Reprinted with permission from [30]. Copyright (2019) American Chemical Society. (d) Bond length and bond angle in parallel and antiparallel fibroin β-sheets. Reprinted from [31], Copyright © 2015 Elsevier Ltd. All rights reserved. (e) Typical repeating protein amino acid sequence of the cocoon silk of the domestic silkworm Bombyx mori (Amino acids in red indicate sequence motifs that are recognized in the literature as being involved in β-sheets.). Reprinted from [32], Copyright © 2007 The Biophysical Society. Published by Elsevier Inc. All rights reserved. (f) Schematic diagram of the piezoelectric effect. Reprinted from [33], © 2022 Elsevier Ltd. All rights reserved.

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Figure 3. Timeline of studies on the piezoelectric properties of silk fibroin.

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Figure 4. Preparation of RSF pressure sensors by casting and spin coating. (a) The relationship between processing parameters, structure (degree of orientation, content of β-sheet folding) and piezoelectric properties of RSF membranes. The relationship between processability and piezoelectric coefficient (middle). Exponential dependence of the tensile ratio with the shear piezoelectric coefficient and content of β-sheet folding (right) [81] John Wiley & Sons. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (b) Preparation process, stress-strain curve and cross-sectional structure of RSF-GO composite film. Reproduced from [113] with permission from the Royal Society of Chemistry. (c) Schematic diagram of the preparation of RSF films by centrifugal casting. Mechanical property and transparency curves of centrifugal cast SF film and ordinary cast SF film. [114] John Wiley & Sons. © 2015 Wiley Periodicals, Inc. (d) Schematic diagram of enhancing the piezoelectric intensity of RSF films by using ferroelectric nanoparticles. Piezoelectric output of different films with NPs, AgNW and PVP. The piezoelectric response of SF film with 30 wt.% KNN:Mn to the human foot pedaling. Reprinted from [115], Copyright © 2015 Elsevier Ltd. All rights reserved.

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Figure 5. RSF pressure sensors made by electrospinning. (a) Schematic diagram of the fabrication and sensing mechanism, and pressure sensing performance of the AFMS. Reproduced from [12], with permission from Springer Nature. (b) Schematic diagram of the preparation, mechanical properties and piezoelectric output performance of SF-NFSs. Reproduced from [131] with permission from the Royal Society of Chemistry (c) Schematic diagram of the preparation of OSFM-SE. Effect of the orientation of RSF fibers and electrode surface microstructure on piezoelectric properties of OSFM-SE. Reprinted from [97], © 2022 Elsevier Ltd. All rights reserved.

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Figure 6. Fabrication of RSF pressure sensors by other methods. (a) Schematic diagram of the fabrication of RSF nanogenerators by using electrodeposition. Atomic force microscopy data and output open circuit voltage of the as-electrodeposited RSF nanogenerators. Reproduced from [141] with permission from the Royal Society of Chemistry. [126] John Wiley & Sons. © 2021 John Wiley & Sons Ltd. (b) Schematic diagram of the preparation of SF-based multi-channel cryogel scaffolds. Stereo micrographs and cyclic compressive stress-strain curves of SF/PEDOT scaffolds. Reprinted from [88], © 2022 Elsevier B.V. All rights reserved. (c) Schematic illustration of the silk-fabric piezoelectric nanogenerator (PENG) fabrication. FE-SEM image of and piezoelectric output performance of degummed silk fabrics. Reproduced from [142]. CC BY 4.0. (d) Schematic diagram of the manufacturing of PZ-EG-skin. The output voltage response to force for the PZ-EG-skin with 4.8% ZnONRs. Output voltages of PZ-EG- skin plotted with time at 15 N for the sensors with different ZnONR contents. Reprinted from [143], © 2020 Elsevier Ltd. All rights reserved. (e) Schematic diagram of the in-situ molding and piezoelectric output performance of SFBT hydrogel. Reprinted from [144], © 2024 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

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Figure 7. Applications for SF-based resistive and capacitive pressure sensors. (a) Schematic diagram of the structure of PSGP flexible conductive hydrogel. PSGP flexible conductive hydrogel is used to detect human facial expressions. Reprinted with permission from [151]. Copyright (2020) American Chemical Society (b) Schematic diagram of the preparation of the combo E-skin sensor. Combo E-skin sensor is used for detecting air blowing and finger compression. As well as ice water. Reprinted with permission from [13]. Copyright (2017) American Chemical Society. (c) Schematic diagram of the preparation of RSF/ITO composite membranes. The detection results of a touch sensor array on a finger touch. Schematic diagram of the structure of the touch sensor array and the touch array’s detection signal for touch and swipe events. Reprinted with permission from [156]. Copyright (2022) American Chemical Society. (d) Schematic diagram of the fabrication of an interdigital capacitive pressure sensor and the sensor circuit, as well as the final assembled pressure sensor. Controlling the switch of the LED light by touching the sensor with fingers. Reprinted from [153], © 2020 Elsevier B.V. All rights reserved. (e) AFMS is used to monitor respiration as a smart mask and to identify the bending angle of flexion of finger joints as smart gloves. Reproduced from [12], with permission from Springer Nature.

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Figure 8. Applications of SF-based piezoelectric and triboelectric pressure sensors. (a) Encapsulated EG skin in detecting different human activities, e.g. muscle movement, elbow bending, respiration and the response of chicken breast tissue during respiration. Reprinted from [143], © 2020 Elsevier Ltd. All rights reserved. (b) The output signal of the smart glove made by SF-based flexible piezoelectric sensors under different gestures. Reproduced from [16]. CC BY 4.0. (c) OSFE-SE Detection of different pressure in different locations in the oral cavity by the OSFE-SE pressure sensor. Schematic diagram of remote signal acquisition by the OSFE-SE and transmitting via a Bluetooth device. Reprinted from [97], © 2022 Elsevier Ltd. All rights reserved. (d) Schematic diagram of piezoelectric smart patch based on RSF/CNTs and the detection of diaphragmatic contraction in rats once inserted into the subcutaneous pocket of the rat’s abdomen. Reproduced from [117]. CC BY 4.0. (e) Schematic diagram of a remote emergency call microsystem for TPNG for real-time detection of falls and the corresponding demos in the fall alarm. Reprinted from [154], © 2018 Elsevier Ltd. All rights reserved. (f) Schematic illustration of the working principle of the SF-TENG when manually tapping on the SF-TENG for Morse code compilation and joint curvature detection of human finger and leg. Reproduced from [15], with permission from Springer Nature. (g) A photo of a STENG-based tactile sensor on the skin and the use STENG to control the drone movements. Reprinted from [11], © 2020 Elsevier B.V. All rights reserved. (h) Output of current signals for sound vibrations of different animals (giraffes, cows, and horses) and friction surfaces with different roughness and STFT analysis of texture perception signals with different roughness by SFGH. Schematic diagram of SFBS gloves and photos of rehabilitation training while grabbing bottles by a smart glove made by SFGH. Reprinted with permission from [14]. Copyright (2024) American Chemical Society.

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Table 1. Solvent systems for the dissolving of silk fibroin.

System	Composition	Examples	Advantages	Disadvantages
Inorganic salt system	Inorganic salt/H₂O	LiBr/H₂O, ZnCl₂/H₂O	Low price, excellent solubility	Cumbersome post-processing, less molecular chain damage
	Inorganic salt/organic solvent	CaCl₂/MeOH
	Inorganic salt/H₂O/organic solvent	CaCl₂/H₂O/ethanol
	Inorganic salt/inorganic acids	CaCl₂/FA, LiBr/FA
Inorganic acids system		FA	No post-processing	Poor solubility, partial molecular chain damage
Alkali system		NaOH/H₂O	Less molecular chain damage	Poor solubility
Organic solvent system		HFIP, HFA-3H₂O	Less molecular chain damage and good stability	Toxic, volatile, high-cost, weak solubility
Ionic liquids (ILs) system		BmimCl, EmimCl, AmimCl	Excellent thermal stability, weak volatility and easy regeneration	Harsh processing conditions, degradation of SF at high temperature, hard to process at room temperature

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Table 2. A summary of methods for the fabrication of RSF and its composites.

Processing	Advantages	Disadvantages
Solution casting	Simple operation, good plasticity	Very limited structural regulation
Spin coating	Certain structural regulation ability, uniform thickness and a smooth surface	Limited structural regulation (better than solution casting)
Electrospinning	Effective structural control under multiple processing external field	Low β-sheet folding (fast solvent evaporation rate), more defects
3D printing	High degree of freedom, good repeatability and accuracy of 3D structure	Weak microstructural regulation ability
Electrodeposition/ electrogelation	Effective structural regulation, higher β-sheet folding	Low-degree macromolecular chain orientation
Freeze drying	Enhancement of piezoelectric properties (stress concentration in porous structure), enhancement of tensile properties	Low β-sheet folding, low freedom degree of 3D structure control
Formation of hydrogel via crosslinking	Excellent mechanical properties	Totally different performance with and without water

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Table 3. Applications of silk fibroin-based pressure sensors in daily life.

Application types	Concrete applications	Materials	Sensing types	Structure	Sensor performance	Refs.
Human health monitoring (in Vitro)	Joint movements, emotions, pulse, and respiration	RSF/PAM/GO/PEDOT:PSS	Res.	Hydrogel	2%-600% (strain), 0.5-119.4 kPa (pressure)	[151]
	Finger movement, human-computer interaction	Flat silk cocoon/PDMS	Res.	Carbonized fiber network	0.01 kPa^-1, 0-680.57 kPa	[152]
	Physiology and exercise detection	RSF/CNTs	Cap.	Casted film	20% (capacitance change)	[15]
	Smart mask for respiration monitoring, smart gloves for finger bending detection	RSF/PEO/AgNWs	Cap.	Electrospun film	2.27 pF/kPa (0-10 kPa), 0.255 pF/kPa (10-53 kPa)	[12]
	Human gait, fall monitoring for the elderly	RSF/CNFs	Pie.	Sponges	2.95 ± 0.03 V	[118]
	Insufflation and cheek movements	RSF	Pie.	Electrospun film	4.5 V, 0.078 ± 0.01 V kPa^-1	[95]
	Body movements etc.	RSF	Pie.	Dip-coated film	8-10 pC N^-1, 35.31 μW cm^-2	[121]
	Body joint movements, energy supply	RSF	Pie.	Electrospun film	7 V, 150 nA	[84]
	Human motion detection, lighting LEDs, charging capacitors	Degummed SF	Tri.	Woven silk fabric	1.52 μW cm^-2	[142]
	Human joint motion detection	RSF/PVDF	Tri.	electrospun film	16.5 V, 290 nA	[132]
Human health monitoring (in Vivo)	Muscles, elbow movements, and respiration	RSF/FA/glycerol	Pie.	Hydrogel	1 mW cm^-2	[143]
	Disposable oral medical equipment	RSF	Pie.	Electrospun film	30.6 mV N^-1, 5.9 mW m^-2, 3.4 ms	[97]
	Intestinal suture, surgical adhesive, monitoring of tissue activity, electrical stimulation	RSF/GNR/Ca²⁺	Pie.	Casted film	20 mV	[116]
	Surgical sutures and monitoring of respiration	RSF/CNTs	Pie.	Casted film	\	[117]
	heart tissue engineering	RSF/PEO/BaTiO₃	Pie.	Casted film	1100 mV	[120]
Remote control	Switching on /off LED by finger touching	RSF/AgNWs	Cap.	Casted film	\	[153]
E-skin	Temperature and pressure detection	RSF/PET/PDMS	Res.	Carbonized silk fiber membrane	\	[13]
E-skin	Electronic skin (pressure sensing)	RSF/PU/glycerol/AgNWs /PDMS	Tri.	Casted film	13 V, 0.4 μA, 1.7 nC	[11]
Object identification	Recognize animal sounds, object identification, smart gloves for guiding stroke rehabilitation	SF/glycerol	Tri.	Casted film	1.083 kPa^-1, 50-400 Hz	[14]
Information conversion	Gesture monitoring gloves	RSF/PVDF	Tri.	Electrospun film	500 V, 12 μA, 0.31 mW cm^-2	[154]
	Morse code compilation, finger and knee flexion detection	RSF/CNTs	Tri.	Casted film	13.5 V, 26.7 nA, 4.5 nC	[15]
	Wearable keyboardless input system (WKIS)	RSF/PU/glycerol/AgNWs	Tri.	Casted film	4 V, 1.8 nA	[155]
Other	4 × 4 array touch sensor	RSF/ITO	Cap.	Casted film	\	[156]
	Micromachined ultrasonic transducer	RSF	Pie.	Spin coated film	56.2 pm V^-1	[128]
	Pressure sensing, recognition of the preying of spiders	RSF/ITO	Tri.	Spin coated film	5 V, ∼520 μA	[157]
Note: Res. Resistive. Cap. Capacitive. Pie. Piezoelectricity. Tri. Triboelectricity. PU, Polyurethane. Voltage and current for piezoelectric and triboelectric pressure sensors are open circuit voltage and short circuit current, respectively. The symbol of represents no reporting of the sensing performance.

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