Special Issue "Soft Materials and Systems"

A special issue of Polymers (ISSN 2073-4360).

Deadline for manuscript submissions: closed (31 May 2018).

Special Issue Editors

Prof. Dr. Jianfeng Zang
Website
Guest Editor
School of Optical and Electronic Information, Huazhong University of Science and Technology, 1037 Luoyu Rd, Wuhan, China 430074
Interests: smart soft materials; flexible electronics; soft robots; soft metamaterials; 2D materials
Prof. Dr. Zhigang Wu
Website
Guest Editor
School of Mechanical Science and Engineering, Huazhong University of Science and Technology, 1037 Luoyu Rd, Wuhan, China 430074
Interests: soft robotics; bio-inspired soft microsystems; stretchable electronics; microfluidics; biomedical devices; non-conventional manufacturing technologies
Special Issues and Collections in MDPI journals
Prof. Dr. Jeong-Yun Sun
Website
Guest Editor
Department of Materials Science and Engineering, Seoul National University, South Korea
Interests: soft materials; soft machines; bio-inspired materials; stretchable ionics

Special Issue Information

Dear Colleagues,

This Special Issue is jointly supported by “The 3rd International Symposium of Flexible and Stretchable Electronics 2017” held in Wuhan, China, on June 29–30, 2017. Soft materials like gels, elastomers, biological tissues, colloids, and liquid metals give large deformation in response to external stimuli such as force, heat, light, sound, electric field, magnetic field, ions, pH, and solutions. The deformation of soft materials or systems may generate novel properties and functions. The integration of soft materials with functional elements such as quantum dots, nanowires, graphene, and other two-dimensional (2D) materials offers unprecedented opportunities to existing technologies and industries, addressing the challenges from flexible and stretchable electronics, photonics, phononics, energy, and health care.

This Special Issue invites original papers and reviews reporting on recent progress in but not limited to the flowing areas:

  • Soft and nature-inspired materials with striking new properties and functions enabled by deformation
  • Soft and smart devices: robots, actuators, sensors, flexible and stretchable electronics
  • Mechanics of interface between soft materials and hard materials
  • 2D materials and devices: synthesis and electronic and optoelectronic applications
  • Processing and manufacturing
  • Applications: wearable, body-attachable, or implantable electronics, health care, compliant energy sources, flexible display, smart skin
Prof. Dr. Jianfeng Zang
Prof. Dr. Zhigang Wu
Prof. Dr. Jeong-Yun Sun
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Polymers is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1800 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • soft and nature-inspired materials
  • flexible and stretchable electronics
  • soft robots and actuators
  • flexible sensors
  • large deformation mechanics
  • 2D materials and devices
  • processing and manufacturing
  • wearable electronics
  • compliant energy sources
  • smart skin

Published Papers (11 papers)

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Research

Open AccessArticle
A Hyaluronic Acid Based Injectable Hydrogel Formed via Photo-Crosslinking Reaction and Thermal-Induced Diels-Alder Reaction for Cartilage Tissue Engineering
Polymers 2018, 10(9), 949; https://doi.org/10.3390/polym10090949 - 27 Aug 2018
Cited by 3
Abstract
A hyaluronic acid (HA) based injectable hydrogel with gradually increasing mechanical properties was synthesized via photo-crosslinking reaction and thermal-induced Diels-Alder (DA) reaction. The injectable hydrogel can quickly gelate within 30 s by photo-crosslinking of HA-furan under the catalysis of lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP). This [...] Read more.
A hyaluronic acid (HA) based injectable hydrogel with gradually increasing mechanical properties was synthesized via photo-crosslinking reaction and thermal-induced Diels-Alder (DA) reaction. The injectable hydrogel can quickly gelate within 30 s by photo-crosslinking of HA-furan under the catalysis of lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP). This injectable property is beneficial to keep the encapsulated cell activity and convenient for clinical operation. And the mechanical properties can be control from 4.86 to 10.66 kPa by exposure time. Then, the thermal-induced DA click chemistry further occurs between furan groups and maleimide groups which gradually promoted the crosslinking density of the injectable hydrogel. The mechanical properties of the injectable hydrogel can be promoted to 21 kPa. ATDC-5 cells were successfully encapsulated in the injectable hydrogel and showed good activity. All the results suggested that the injectable hydrogel with gradually increasing mechanical properties formed by photo-crosslinking reaction and thermal-induced DA reaction has a good prospect of application in cartilage tissue engineering. Full article
(This article belongs to the Special Issue Soft Materials and Systems)
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Open AccessArticle
Synthesis and Characterization of the Conducting Polymer Micro-Helix Based on the Spirulina Template
Polymers 2018, 10(8), 882; https://doi.org/10.3390/polym10080882 - 07 Aug 2018
Cited by 3
Abstract
As one of the most interesting naturally-occurring geometries, micro-helical structures have attracted attention due to their potential applications in fabricating biomedical and microelectronic devices. Conventional processing techniques for manufacturing micro-helices are likely to be limited in cost and mass-productivity, while Spirulina, which [...] Read more.
As one of the most interesting naturally-occurring geometries, micro-helical structures have attracted attention due to their potential applications in fabricating biomedical and microelectronic devices. Conventional processing techniques for manufacturing micro-helices are likely to be limited in cost and mass-productivity, while Spirulina, which shows natural fine micro-helical forms, can be easily mass-reproduced at an extremely low cost. Furthermore, considering the extensive utility of conducting polymers, it is intriguing to synthesize conducting polymer micro-helices. In this study, PPy (polypyrrole), PANI (polyaniline), and PEDOT (poly(3,4-ethylenedioxythiophene)) micro-helices were fabricated using Spirulinaplatensis as a bio-template. The successful formations of the conducting polymer micro-helix were confirmed using scanning electron microscopy (SEM). Fourier transform infrared spectroscopy (FTIR) and Raman and X-ray diffraction (XRD) were employed to characterize the molecular structures of the conducting polymer in micro-helical forms. In the electrochemical characterization, the optimized specific capacitances for the PPy micro-helix, the PANI micro-helix, and the PEDOT micro-helix were found to be 234 F/g, 238 F/g at the scan rate of 5 mV/s, and 106.4 F/g at the scan rate of 10 mV/s, respectively. Therefore, it could be expected that other conducting polymer micro-helices with Spirulina as a bio-template could be also easily synthesized for various applications. Full article
(This article belongs to the Special Issue Soft Materials and Systems)
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Open AccessArticle
Self-Erasable Nanocone Antireflection Films Based on the Shape Memory Effect of Polyvinyl Alcohol (PVA) Polymers
Polymers 2018, 10(7), 756; https://doi.org/10.3390/polym10070756 - 09 Jul 2018
Cited by 2
Abstract
Arrays of nanostructure that are capable of broadband antireflection and light trapping properties are implemented in photovoltaic and photonic devices. However, most of the existing antireflection films have been hindered by a complicated fabricated method and structurally rigid. Here, we report a simple [...] Read more.
Arrays of nanostructure that are capable of broadband antireflection and light trapping properties are implemented in photovoltaic and photonic devices. However, most of the existing antireflection films have been hindered by a complicated fabricated method and structurally rigid. Here, we report a simple preparation method for self-erasable nanocone antireflection films using the surface replication method. Arrays of nanocone with sub-100 nm surface features could be replicated easily on the shape memory polyvinyl alcohol (PVA) film, and are erased by thermal stimulation. The reflectivity of self-erasable antireflection film can be switched from the 4.5% to 0.6% averaged over the visible spectral range by controlling the temperature below or above 80 °C. Theoretical simulations have been demonstrated. The unique smart film is expected to be used to further extend the application of smart optical windows and digital screens. Full article
(This article belongs to the Special Issue Soft Materials and Systems)
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Open AccessArticle
High-Performance Pressure Sensor for Monitoring Mechanical Vibration and Air Pressure
Polymers 2018, 10(6), 587; https://doi.org/10.3390/polym10060587 - 27 May 2018
Cited by 2
Abstract
To realize the practical applications of flexible pressure sensors, the high performance (sensitivity and response time) as well as more functionalities are highly desired. In this work, we fabricated a piezoresistive pressure sensor based on the micro-structured composites films of multi-walled carbon nanotubes [...] Read more.
To realize the practical applications of flexible pressure sensors, the high performance (sensitivity and response time) as well as more functionalities are highly desired. In this work, we fabricated a piezoresistive pressure sensor based on the micro-structured composites films of multi-walled carbon nanotubes (MWCNTs) and poly (dimethylsiloxane) (PDMS). In addition, we establish efficient strategies to improve key performance of our pressure sensor. Its sensitivity is improved up to 474.13 kPa−1 by minimizing pressure independent resistance of sensor, and response time is shorten as small as 2 μs by enhancing the elastic modulus of polymer elastomer. Benefiting from the high performance, the functionalities of sensors are successfully extended to the accurate detection of high frequency mechanical vibration (~300 Hz) and large range of air pressure (6–101 kPa), both of which are not achieved before. Full article
(This article belongs to the Special Issue Soft Materials and Systems)
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Open AccessArticle
A Sandwich-Structured Piezoresistive Sensor with Electrospun Nanofiber Mats as Supporting, Sensing, and Packaging Layers
Polymers 2018, 10(6), 575; https://doi.org/10.3390/polym10060575 - 23 May 2018
Cited by 7
Abstract
Electrospun nanofiber mats have been used as sensing elements to construct piezoresistive devices due to their large surface area and high porosity. However, they have not been utilized as skin-contact supporting layers to package conductive nanofiber networks for the fabrication of piezoresistive sensors. [...] Read more.
Electrospun nanofiber mats have been used as sensing elements to construct piezoresistive devices due to their large surface area and high porosity. However, they have not been utilized as skin-contact supporting layers to package conductive nanofiber networks for the fabrication of piezoresistive sensors. In this work, we developed a sandwich-structured pressure sensor, which can sensitively monitor human motions and vital signs, with electrospun nanofiber mats as supporting, sensing, and packaging layers. The nanofiber mats were prepared by electrospinning with biocompatible poly (l-lactide) (PLA), silk fibroin (SF), and collagen (COL) as raw materials. The synthesized PLA–SF–COL mat possesses a non-woven structure with a fiber diameter of 122 ± 28 nm and a film thickness of 37 ± 5.3 μm. Polypyrrole (PPy) nanoparticles were grown in-situ on the mat to form a conductive layer. After stacking the pristine and conductive mats to form a PLA–SF–COL mat/(PPy-coated mat)2 structure, another layer was electrospun to pack the multilayers for the construction of a sandwich-structured piezoresistive sensor. The as-prepared device can sensitively detect external pressures caused by coin loading and finger tapping/pressing. It can also tolerate more than 600 times of pressing without affecting its sensing capability. The human body-attached experiments further demonstrate that the sensor could real-time monitor finger/arm bending, arterial pulse, respiration rate, and speaking-caused throat vibration. The electrospinning-based fabrication may be used as a facile and low-cost strategy to produce flexible piezoresistive sensors with excellent skin-compatibility and great pressure sensing capability. Full article
(This article belongs to the Special Issue Soft Materials and Systems)
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Open AccessArticle
Modeling the Viscoelastic Hysteresis of Dielectric Elastomer Actuators with a Modified Rate-Dependent Prandtl–Ishlinskii Model
Polymers 2018, 10(5), 525; https://doi.org/10.3390/polym10050525 - 14 May 2018
Cited by 12
Abstract
Dielectric elastomer actuators (DEAs) are known as a type of electric-driven artificial muscle that have shown promising potential in the field of soft robotics. However, the inherent viscoelastic nonlinearity makes the modeling and control of DEAs challenging. In this paper, we propose a [...] Read more.
Dielectric elastomer actuators (DEAs) are known as a type of electric-driven artificial muscle that have shown promising potential in the field of soft robotics. However, the inherent viscoelastic nonlinearity makes the modeling and control of DEAs challenging. In this paper, we propose a new phenomenological modeling approach with the Prandtl–Ishlinskii (P–I) model to characterize the viscoelastic hysteresis nonlinearity of DEAs. Differently from the commonly used physics-based models, the developed phenomenological model, called the modified rate-dependent P–I model (MRPIM), produces behavior similar to that of physics-based models but without necessarily considering physical insight into the modeling problem. In this way, the developed MRPIM can characterize the asymmetric and rate-dependent viscoelastic hysteresis with a relative simple mathematical format using only the experimental data. To validate the development, experimental tests were conducted with seven different frequencies; four were selected to identify the model parameters and the rest of the data were used to further verify the model. Comparisons between the model prediction and experimental data demonstrate that the MRPIM can precisely describe the viscoelastic hysteresis effect of DEAs with a maximum prediction error of 9.03% and root-mean-square prediction error of 4.50%. Full article
(This article belongs to the Special Issue Soft Materials and Systems)
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Open AccessArticle
Fabrication of Stretchable Copper Coated Carbon Nanotube Conductor for Non-Enzymatic Glucose Detection Electrode with Low Detection Limit and Selectivity
Polymers 2018, 10(4), 375; https://doi.org/10.3390/polym10040375 - 28 Mar 2018
Cited by 4
Abstract
The increasing demand for wearable glucose sensing has stimulated growing interest in stretchable electrodes. The development of the electrode materials having large stretchability, low detection limit, and good selectivity is the key component for constructing high performance wearable glucose sensors. In this work, [...] Read more.
The increasing demand for wearable glucose sensing has stimulated growing interest in stretchable electrodes. The development of the electrode materials having large stretchability, low detection limit, and good selectivity is the key component for constructing high performance wearable glucose sensors. In this work, we presented fabrication of stretchable conductor based on the copper coated carbon nanotube sheath-core fiber, and its application as non-enzymatic electrode for glucose detection with high stretchability, low detection limit, and selectivity. The sheath-core fiber was fabricated by coating copper coated carbon nanotube on a pre-stretched rubber fiber core followed by release of pre-stretch, which had a hierarchically buckled structure. It showed a small resistance change as low as 27% as strain increasing from 0% to 500% strain, and a low resistance of 0.4 Ω·cm−1 at strain of 500%. This electrode showed linear glucose concentration detection in the range between 0.05 mM and 5 mM and good selectivity against sucrose, lactic acid, uric acid, acrylic acid in phosphate buffer saline solution, and showed stable signal in high salt concentration. The limit of detection (LOD) was 0.05 mM, for the range of 0.05–5 mM, the sensitivity is 46 mA·M−1. This electrode can withstand large strain of up to 60% with negligible influence on its performance. Full article
(This article belongs to the Special Issue Soft Materials and Systems)
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Open AccessArticle
On-Demand Multi-Resolution Liquid Alloy Printing Based on Viscoelastic Flow Squeezing
Polymers 2018, 10(3), 330; https://doi.org/10.3390/polym10030330 - 16 Mar 2018
Cited by 3
Abstract
Recently, high-resolution patterning techniques of stretchable electronics advanced extensively. An important trend is to fabricate complex circuits with varied sizes in a small area, which is a technical challenge to current conductive ink printing technologies. Here, we introduce a new strategy for multi-resolution [...] Read more.
Recently, high-resolution patterning techniques of stretchable electronics advanced extensively. An important trend is to fabricate complex circuits with varied sizes in a small area, which is a technical challenge to current conductive ink printing technologies. Here, we introduce a new strategy for multi-resolution liquid alloy printing, which can tune the resolution of printed liquid alloy trace in real time with the squeezing effect of compound viscoelastic flow. A newly developed coaxial nozzle with the inner nozzle extension (CNINE) is used to wrap and squeeze liquid alloy steadily and effectively. By controlling the working parameters and compound flow properties, liquid alloy patterns with different widths are obtained continuously. This work offers a new way to rapidly manufacture complex stretchable electronics patterning in multi-resolution. Full article
(This article belongs to the Special Issue Soft Materials and Systems)
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Open AccessArticle
Paper Electrodes Coated with Partially-Exfoliated Graphite and Polypyrrole for High-Performance Flexible Supercapacitors
Polymers 2018, 10(2), 135; https://doi.org/10.3390/polym10020135 - 31 Jan 2018
Cited by 11
Abstract
Flexible paper electrodes for supercapacitors were prepared with partially-exfoliated graphite and polypyrrole as the active materials. Graphite was coated on paper with pencil drawing and then electrochemically exfoliated using the cyclic voltammetry (CV) technique to obtain the exfoliated graphite (EG)-coated paper (EG-paper). Polypyrrole [...] Read more.
Flexible paper electrodes for supercapacitors were prepared with partially-exfoliated graphite and polypyrrole as the active materials. Graphite was coated on paper with pencil drawing and then electrochemically exfoliated using the cyclic voltammetry (CV) technique to obtain the exfoliated graphite (EG)-coated paper (EG-paper). Polypyrrole (PPy) doped with β-naphthalene sulfonate anions was deposited on EG-paper through in-situ polymerization, leading to the formation of PPy-EG-paper. The as-prepared PPy-EG-paper showed a high electrical conductivity of 10.0 S·cm−1 and could be directly used as supercapacitor electrodes. The PPy-EG-paper electrodes gave a remarkably larger specific capacitance of 2148 F∙g−1 at a current density of 0.8 mA∙cm−2, compared to PPy-graphite-paper (848 F∙g−1). The capacitance value of PPy-EG-paper could be preserved by 80.4% after 1000 charge/discharge cycles. In addition, the PPy-EG-paper electrodes demonstrated a good rate capability and a high energy density of 110.3 Wh∙kg−1 at a power density of 121.9 W∙kg−1. This work will pave the way for the discovery of efficient paper-based electrode materials. Full article
(This article belongs to the Special Issue Soft Materials and Systems)
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Open AccessArticle
Ultra-Stretchable Piezoelectric Nanogenerators via Large-Scale Aligned Fractal Inspired Micro/Nanofibers
Polymers 2017, 9(12), 714; https://doi.org/10.3390/polym9120714 - 15 Dec 2017
Cited by 11
Abstract
Stretchable nanogenerators that directly generate electricity are promising for a wide range of applications in wearable electronics. However, the stretchability of the devices has been a long-standing challenge. Here we present a newly-designed ultra-stretchable nanogenerator based on fractal-inspired piezoelectric nanofibers and liquid metal [...] Read more.
Stretchable nanogenerators that directly generate electricity are promising for a wide range of applications in wearable electronics. However, the stretchability of the devices has been a long-standing challenge. Here we present a newly-designed ultra-stretchable nanogenerator based on fractal-inspired piezoelectric nanofibers and liquid metal electrodes that can withstand strain as large as 200%. The large-scale fractal poly(vinylidene fluoride) (PVDF) micro/nanofibers are fabricated by combination of helix electrohydrodynamic printing (HE-Printing) and buckling-driven self-assembly. HE-Printing exploits “whipping/buckling” instability of electrospinning to deposit serpentine fibers with diverse geometries in a programmable, accurately positioned, and individually-controlled manner. Self-organized buckling utilizes the driven force from the prestrained elastomer to assemble serpentine fibers into ultra-stretchable fractal inspired architecture. The nanogenerator with embedded fractal PVDF fibers and liquid-metal microelectrodes demonstrates high stretchability (>200%) and electricity (currents >200 nA), it can harvest energy from all directions by arbitrary mechanical motion, and the rectified output has been applied to charge the commercial capacitor and drive LEDs, which enables wearable electronics applications in sensing and energy harvesting. Full article
(This article belongs to the Special Issue Soft Materials and Systems)
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Open AccessArticle
Thermal Transport in Soft PAAm Hydrogels
Polymers 2017, 9(12), 688; https://doi.org/10.3390/polym9120688 - 08 Dec 2017
Cited by 18
Abstract
As the interface between human and machine becomes blurred, hydrogel incorporated electronics and devices have emerged to be a new class of flexible/stretchable electronic and ionic devices due to their extraordinary properties, such as softness, mechanically robustness, and biocompatibility. However, heat dissipation in [...] Read more.
As the interface between human and machine becomes blurred, hydrogel incorporated electronics and devices have emerged to be a new class of flexible/stretchable electronic and ionic devices due to their extraordinary properties, such as softness, mechanically robustness, and biocompatibility. However, heat dissipation in these devices could be a critical issue and remains unexplored. Here, we report the experimental measurements and equilibrium molecular dynamics simulations of thermal conduction in polyacrylamide (PAAm) hydrogels. The thermal conductivity of PAAm hydrogels can be modulated by both the effective crosslinking density and water content in hydrogels. The effective crosslinking density dependent thermal conductivity in hydrogels varies from 0.33 to 0.51 Wm−1K−1, giving a 54% enhancement. We attribute the crosslinking effect to the competition between the increased conduction pathways and the enhanced phonon scattering effect. Moreover, water content can act as filler in polymers which leads to nearly 40% enhancement in thermal conductivity in PAAm hydrogels with water content vary from 23 to 88 wt %. Furthermore, we find the thermal conductivity of PAAm hydrogel is insensitive to temperature in the range of 25–40 °C. Our study offers fundamental understanding of thermal transport in soft materials and provides design guidance for hydrogel-based devices. Full article
(This article belongs to the Special Issue Soft Materials and Systems)
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