Flexible and Stretchable Medical Devices E-Book

• Herausgeber: Wiley-VCH Verlag GmbH & Co. KGaA
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 778

Veröffentlichungsjahr: 2018

Beschreibung

The book introduces flexible and stretchable wearable electronic systems and covers in detail the technologies and materials required for healthcare and medical applications. A team of excellent authors gives an overview of currently available flexible devices and thoroughly describes their physical mechanisms that enable sensing human conditions.
In dedicated chapters, crucial components needed to realize flexible and wearable devices are discussed which include transistors and sensors and deal with memory, data handling and display. Additionally, suitable power sources based on photovoltaics, thermoelectric energy and supercapacitors are reviewed. A special chapter treats implantable flexible sensors for neural recording.
The book editor concludes with a perspective on this rapidly developing field which is expected to have a great impact on healthcare in the 21st century.

Leseprobe

Table of Contents

Title Page

Copyright

Preface

Acknowledgments

Chapter 1: History of Flexible and Stretchable Devices

References

Chapter 2: Carbon Nanotube Based Flexible and Stretchable Electronics

2.1 Introduction

2.2 Carbon Nanotube Networks for Applications in Flexible Electronics

2.3 Carbon Nanotube Networks for Applications in Stretchable Electronics

2.4 Scalable Fabrication Process—Printing

2.5 Conclusions and Outlook

References

Chapter 3: Organic-Based Transistors and Sensors

3.1 Introduction

3.2 Materials Consideration for Flexible Organic-Based Transistors

3.3 State-of-the-Art Designs and Fabrication of Organic-Based Transistors

3.4 Fabrication Techniques for Organic-Based Transistors and Sensors

3.5 Flexible Organic Transistor-Based Sensors

3.6 Summary and Outlook

References

Chapter 4: Printed Transistors and Sensors

4.1 Introduction

4.2 Printing Technologies for Electronics

4.3 Printed Transistors

4.4 Printed Biosensors

References

Chapter 5: Flexible Photovoltaic Systems

5.1 Introduction

5.2 Flexible Inorganic Photovoltaic Systems

5.3 Flexible Organic Photovoltaic Systems

5.4 Flexible Organic–Inorganic Hybrid Photovoltaic Systems

5.5 Summary and Conclusion

References

Chapter 6: Materials Design for Flexible Thermoelectric Power Generators

6.1 Introduction

6.2 General Principles

6.3 Thermoelectric Materials Design

6.4 Outlook for Flexible Thermoelectric Generators

References

Chapter 7: Flexible Supercapacitors Based on Two-Dimensional Materials

7.1 Introduction

7.2 Flexible Supercapacitors Based on 2D Materials

7.3 Conclusions

References

Chapter 8: Organometal Halide Perovskites for Next Generation Fully Printed and Flexible LEDs and Displays

8.1 Introduction

8.2 Single Layer Perovskite LEDs

8.3 Current Challenges

8.4 Conclusions and Outlook

Acknowledgments

References

Chapter 9: Flexible Floating Gate Memory

9.1 Introduction

9.2 Device Operation of Floating Gate Memory

9.3 Charge Injection Mechanism in Floating Gate Memory

9.4 Flexible Nanofloating Gate Memory

9.5 Characterization of Floating Gate Memory

9.6 Flexibility of Floating Gate Memory

9.7 Conclusion

References

Chapter 10: Flexible and Stretchable Wireless Systems

10.1 Introduction

10.2 The Basics of Wireless Systems

10.3 Flexible, Stretchable Circuits

10.4 Flexible Antennas

10.5 Stretchable Antennas

10.6 Future Outlook

References

Chapter 11: Conductive Nanosheets for Ultra-Conformable Smart Electronics

11.1 Introduction

11.2 Fabrication of Conductive Nanosheets

11.3 Characterization of Conductive Nanosheets

11.4 Applications of Conductive Nanosheets

11.5 Concluding Remarks

Acknowledgments

References

Chapter 12: Flexible Health-Monitoring Devices/Sensors

12.1 Introduction

12.2 Flexible Sensors for Health Monitoring

12.3 Multifunctional Flexible Sensors for Multiple Bio-Signals

12.4 Practical Applications of Flexible Health-Monitoring Devices

12.5 Conclusions and Future Perspective

References

Chapter 13: Stretchable Health Monitoring Devices/Sensors

13.1 Introduction

13.2 Materials for Stretchable Health Monitoring Devices

13.3 Health Monitoring Applications of Stretchable Devices

13.4 Future of Stretchable Electronic Devices

References

Chapter 14: Flexible/Stretchable Devices for Medical Applications

14.1 Introduction

14.2 Materials, Synthesis and Composites for Flexible/Stretchable Systems

14.3 Electronic/Optoelectronic Devices, Sensors and Systems

14.4 Multifunctional Electronic Sensors and Power Scavenging Circuit for the Heart

14.5 Electrophysiology and Optogenetics for the Brain

14.6 Communication and Regulation for the Nervous System

14.7 Skin-Like Electronics/Optoelectronics

14.8 Transient, Bioresorbable Systems

14.9 Conclusion and Outlook

References

Chapter 15: Implantable Flexible Sensors for Neural Recordings

15.1 Introduction

15.2 Flexible Needle Electrodes

15.3 Flexible ECoG Electrodes

15.4 Functionalities of Flexible Substrates

15.5 Flexible Devices for Chronic Applications

15.6 Summary

References

Chapter 16: Perspective in Flexible and Stretchable Electronics

Index

End User License Agreement

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Guide

Cover

Table of Contents

preface

Begin Reading

List of Illustrations

Chapter 1: History of Flexible and Stretchable Devices

Figure 1.1 History of flexible electronics using organic and inorganic materials.

Chapter 2: Carbon Nanotube Based Flexible and Stretchable Electronics

Figure 2.1 Various methods for preparing carbon nanotube networks and representative electronic applications. Dry process (a–d): (a) photograph of directly CVD grown freestanding SWCNT thin film. (Ma

et al

. 2007 [19].

Figure 2.2 High performance TFTs using semiconducting carbon nanotubes (sSWCNTs). (a) Schematics of TFTs on rigid (i) and flexible (ii) substrates. The SEM (iii) and AFM (iv) images of the channel region are also shown. (b) Electrical characteristics of the flexible TFTs. (i)

I

d

-V

g

curves of transistors with different channel lengths; (ii) TFT on/off current ratio as a function of channel length; (iii) normalized on-current and transconductance as functions of reciprocal channel length; (iv) field-effect mobility, calculated based on

C-V

measurement (green), as a function of channel length. Also shown are the mobility obtained using parallel plate model (blue) and cylindrical model (red). (Wang

et al

. 2013 [7].

Figure 2.3 Flexible logic gates using CNT-TFTs. (a) Optical image demonstrating the flexibility of the circuits, where the sample is wrapped onto a glass tube with a diameter of 10 mm. (b) Inverter voltage transfer characteristics (VTC) measured with a

V

DD

of 3 or 5 V. Inset shows the schematic and the optical picture of the diode-loaded inverter. (c) Inverter VTC measured while the circuit is bent to various curvature radii. Inset shows the inverter threshold voltage and gain as a function of curvature radius. (d) Inverter durability under cyclic bending test, showing stable performance for up to 2000 cycles. (e, f) Output characteristics of diode-loaded 2-input NOR (e) and NAND (f) gates.

V

DD

is 5 V for both circuits. Insets are the corresponding schematic and optical micrographs. (Wang

et al

. 2012 [14d].

Figure 2.4 More sophisticated flexible integrated circuits using CNT-TFTs. (a) Optical image of a 4-to-16 decoder. (b) Transient response of the decoder. (Cao

et al

. 2008 [14b].

Figure 2.5 Flexible AMOLED display using CNT TFT array as active matrix. (a) Left: photograph of a completed flexible AMOLED display comprising 16 × 16 pixels driven by a CNT TFT active matrix with a size of ∼3 × 3.5 cm

2

; middle: optical micrograph of one pixel before evaporating the emissive layer; right: SEM image showing the channel region of the CNT TFT. (b) Photograph of a single-color (green) AMOLED display being fully turned on and at bent state, demonstrating a yield, defined as the fraction of lighting pixels, of >97%. The voltages applied to the scan and data lines are −5 and 10 V, respectively. (c, d) Photograph of a full-color (red, green, blue) AMOLED display with all pixels being turned on and at flat (c) and bent (d) states. (Wang

et al

. 2013 [35].

Figure 2.6 User-interactive electronic skin (E-skin) using CNT-TFTs. (a) Left: schematic diagram showing the exploded view of one pixel, consisting of a CNT-TFT, an OLED and a layer of PSR vertically integrated on a polyimide substrate; middle: circuit diagram of the active matrix user-interactive E-skin; right: photograph of a working E-skin. The OLED pixels are turned on locally where the surface is touched and the light intensity reflects the magnitude of the applied pressure. (b) Individual pixel comprising an OLED connected in series to a piece of pressure sensitive rubber (PSR): schematics of the circuit and device structure (left), current–voltage curves under various magnitudes of applied pressure (middle), current and luminance as functions of applied pressure under a bias of 10 V. (d) Optical (middle) and electrical (right) readout from the E-skin system when an L-shaped PDMS slab is used to apply pressure onto the sample. (Wang

et al

. 2013 [35].

Figure 2.7 Carbon nanotube TFTs enabled flexible active matrix optical and X-ray imagers. (a) Photograph of a flexible imager driven by CNT TFTs being bent. (b, c) Schematics of the device structure and operating mechanism of a green light (b) and X-ray (c) imager. (d) Transfer characteristics of one pixel under different illumination intensities (green light). Inset shows the circuit diagram of a pixel. (e) Measured current of one pixel under a reverse bias of 2 V as a function of X-ray dosage. The dashed line marks the dark current level of the organic photodetector. (f) Two dimensional current profile which successfully reflects the “T”-shaped shadow mask placed between the light source and imager. (Takahashi

et al

. 2013 [34b].

Figure 2.8 Carbon nanotube network as stretchable transparent electrodes for skin-like pressure sensor applications. (a) Schematics (left) and corresponding AFM images (right) showing the microscopic morphology evolution of CNT network on PDMS substrate when subjected to uniaxial and biaxial stretching and releasing tests. (b) Optical images and two-dimensional pressure profile obtained from a transparent pressure sensor array. (Lipomi

et al

. 2011 [13b].

Figure 2.9 Stretchable conductors based on CNT-polymer composites. (a) Elastic conductors comprising super-growth SWCNTs uniformly dispersed in a fluorinated rubber: left, optical images of screen printed patterns with a feature width of 100 µm; middle, conductivity as a function of tensile strain for three samples with different SWCNT content; right, stretchability and conductivity as a function of SWCNT content. (Sekitani

et al

. 2009 [40b]. Reproduced with permission of Nature Publishing Group.) (b–d) Highly conductive and stretchable hybrid composites of silver flake and MWCNTs decorated with self-assembled silver nanoparticles. (b) Diagram of the synthesis process. (c) Conductivity as a function of tensile strain. (d) SEM images of the nanocomposites at a strain of 50%. (Chun

et al

. 2010 [40c].

Figure 2.10 Stretchable resistive (a–f) and capacitive (g–k) strain gauges using CNT networks. (a) Schematics of the fabrication process and working mechanism of the resistive strain gauge. (b)

In situ

SEM image of the SWCNT array under a 100% strain, showing the transverse fracture of the film. (c) Relative change in resistance for up to 10 000 stretching cycles with strains of 100%, 150%, and 200%. (d–f) A strain gauge attached to a bandage and adhered to the throat (d) used to detect human breathing (e) and phonation (f). (Yamada

et al

. 2011 [15a].

Figure 2.11 Stretchable CNT TFTs enabled by wrinkled Al

2

O

3

gate dielectrics. (a) Deposition and transfer of Al

2

O

3

, which is randomly wrinkled during the transfer process. The SEM image shows the surface morphology of the wrinkled Al

2

O

3

. (b, c) Schematics, optical micrographs, and transfer characteristics of the fully-fabricated CNT TFTs when stretched along the channel length direction up to a strain of 16% (b) or along the channel width direction up to a strain of 20% (c). (Chae

et al

. 2013 [14e].

Figure 2.12 Intrinsically stretchable CNT-TFTs. (a) Schematics of the fabrication process where CNT film is transferred to prestrained (50%) PDMS substrate, forming buckled structure after the prestrain in PDMS is released. (b)

In situ

SEM images of the buckled CNT film obtained after prestretched to 25%, 50%, and 65%. (c, d) Photographs of the TFTs before (c) and after (d) stretched to 30%. (e, f) Stretching test showing the transfer characteristics (e), mobility, on-current, and off-current (f) of the CNT-TFTs at various tensile strains up to 57.2%. (Xu

et al

. 2014 [14f].

Figure 2.13 Intrinsically stretchable thin-film transistors based on super-growth carbon nanotube electrodes and sSWCNT channel. (a) Schematics of the fabrication process, involving dispersing the super-growth CNTs, spray coating the dispersion, patterning the conductive CNT film and ink-jet printing the semiconducting SWCNTs. (b–d) Photographs of the conductive CNT film just coated on Si (b), after patterning (c) and being peeled off from Si (d). (e) Transfer characteristics of a stretchable TFT while the device is stretched to >110% strain. (Sekiguchi

et al

. 2015 [43].

Figure 2.14 Intrinsically stretchable CNT TFTs fabricated by solution process with silver nanowires (AgNWs) as electrodes. (a) Schematics of the fabrication process involving spraying coating AgNWs, drop casting sSWCNTs, spin coating gate dielectric and lamination. (b, c) Electrical characteristics of a device being stretched to 50% strain along the channel length (b) and channel width (c) direction. (d) Optical micrograph of the devices while being stretched to 40% strain along the channel length direction. (Liang

et al

. 2015 [44b], https://www.nature.com/articles/ncomms8647. Used under CC BY 4.0 https://creativecommons.org/licenses/by/4.0/.)

Figure 2.15 Solution processed intrinsically stretchable CNT TFTs with unsorted carbon nanotubes as electrodes. (a) Schematics of the fabrication process involving spray coating unsorted CNTs, spin coating semiconducting CNTs, spin coating gate dielectric and device assembling by transferring. (b) Optical images of the fully fabricated devices. Scale bar, 4 mm. (c, d) Transfer characteristics of a device being stretched to 100% strain along the channel length (c) and channel width (d) direction. (Chortos

et al

. 2015 [44a].

Figure 2.16 Ink-jet printed SWCNT TFTs. (a, b) Snapshots of DMF based SWCNT ink droplet for channel semiconductor (a) and ionic liquid droplet for gate dielectric (b) during the ink-jet printing process. (c) Optical micrograph of ink-jet printed TFT array with gold contact electrodes. (d) AFM image of the printed SWCNT network in the channel of the TFT. (e) Off/off ratio (top), hole mobility (middle), and channel resistance (bottom) as functions of channel length. (Okimoto

et al

. 2010 [23a].

Figure 2.17 Printed CNT-TFT and integrated circuits using ion-gel dielectrics. (a) Schematic diagram and optical micrograph of the printed CNT TFT on flexible substrate with ion-gel dielectric and PEDOT:PSS gate electrode. The top right AFM image shows the printed CNT network. (b) Representative transfer (left) and output (right) characteristics of the printed TFT. (c) Photograph and circuit schematic of a ring oscillator with printed TFTs. (Ha

et al

. 2010 [23b].

Figure 2.18 Fully-printed highly flexible and foldable CNT TFTs and integrated logic gates. (a) Schematics of the fabrication process. (b) Transfer characteristics of a TFT being bent to different curvature radii. Inset, photograph of the circuits being wrapped around a cylinder with a curvature radius of 13 mm. (c) Voltage transfer characteristics (VTCs) of a resistive loaded p-type only inverter measured at various curvature radii. Inset, inverter gain as a function of curvature radius. (d) Transfer characteristics of a TFT after being folded for multiple times. Inset, optical image of a circuit being folded over the edge of a piece of printing paper. (Cai

et al

. 2015 [47].

Figure 2.19 CNT-TFTs fabricated using a roll-to-plate printing process for applications in E-skin. (a) Schematics of the simplified gravure printing process (roll-to-plate). (b) Photograph of the roll-to-plate printer. (c) SEM image of the printed CNT network in the channel region of the TFT. (Lau

et al

. 2013 [31e].

Figure 2.20 Heterogeneous integration of CNT (p-type) and ZTO (n-type) semiconductors for printed complementary integrated circuits. (a) Schematic diagram of the CMOS inverter. (b) Output characteristics of the printed n-type and p-type TFT. (c) VTC (left) and voltage gain (right) of the printed inverter under various supply voltages. (d) Printed 5-stage ring oscillator using the complementary inverters. Top: optical micrograph; bottom: output signal of the oscillator driven by a

V

DD

of 8 V. (Kim

et al

. 2014 [50a].

Chapter 3: Organic-Based Transistors and Sensors

Figure 3.1 Gate electrode engineering achieving highly flexible magnetic sensors using the gate suspension technique. Silver nanoparticles are immersed in a solution of PMDS yielding a magnetically active composite.

Figure 3.2 Transistor structures preferred for sensitive materials (a) bottom-contact top gate, (b) top-contact top gate. Contact is in reference to the position of source and drain with the semiconductor.

Figure 3.3 Transfer characteristics curves for mobility extraction from high-performance conjugated polymer OFETs showing occasional non-ideal transfer characteristics: (a) CDT-BTZ bottom-gate, top-contact FET. (b) DPP-T-TT bottom-gate, top-contact FET. (c) IDTBT top-gate, bottom-contact FET.

Figure 3.4 Representative modifications of OFET design for sensing applications. The red circle indicates the interface of the functional layer (OSC: organic semiconductor, ECP: electrically conductive polymer) and the electrolyte-sample solution.

Figure 3.5 Printing technologies for roll-to-roll printing of flexible electronics under investigation.

Figure 3.6 Illustration of a piezoelectric-based strain sensor (a) strain is applied from both ends of the sensor, and (b) is the performance of the devices under different strain.

Figure 3.7 Demonstration of an elastic strain sensor based on heptazole: (a) device structure and junctions layout, (b) strain gauge mounted on a fore arm, (c) muscle movement detected by the sensor.

Figure 3.8 An ultralightweight device for pressure sensing (a) device weight compared to a feather in air, (b) structure of a single pressure sensor in the array on a flexible foil.

Figure 3.9 (a) A pressure sensor equipped with a flexible and microstructured dielectric layer. (b) Structure of an OLED-based pressure sensor. (c) A kind of pressure sensor mimicking the chameleon's skin. The organic layer is oxidized upon applied pressure resulting into a color change.

Figure 3.10 Bending insensitive pressure sensor. (a) Array of nanofibers insensitive to bending strain. (b) Structure of the sensor showing different layers encapsulating a transistor. The fiber layer underlies the source electrode making the device insensitive to bending strain.

Figure 3.11 (a) Strucrture of a stretchable temperature sensor. (b) Strain distribution of the sensor during the stretching process.

Figure 3.12 (a) Device layout of an organic electrochemical transistor-based biosensor for enzymatic sensing. The presence of analytes can be detected from the change in at the gate–electrolyte interface. The doping and dedoping of the ionic transporting channel layer is used to detect enzymatic-specific analyte. (b) Glucose monitoring using the detection of proton via fluorescence quenching.

(

c) Fabricated biocompatible electrodes for brain activity detection. PEDOT:PSS is used to coat the contacts for ionic detection and soft interfacing.

Figure 3.13 (a) Device layer of an μ-ILEDs-based wireless optogenetics system. The joules heaters are separated from medicine reservoirs by an expandable layer. The reservoirs and the encapsulating Cu-membrane can be wirelessly triggered to rupture by the heating and expansion of the expandable membranes. (b) Demonstration of the implatation of the optogenetic device in the spinal cord of a mouse.

Figure 3.14 Demonstration of an optical sensor based on OLED coupled with a PD; (a) the bandage consists of one light source and four photodiodes to detect the movement of muscle fibres. (b) A robotic arm mimicking the arm movement of a volunteer.

Chapter 4: Printed Transistors and Sensors

Figure 4.1 (a) Schematic illustration of inkjet process. (b) Ink spreading after application to the hydrophilic substrate. (c) Clumping together of the applied inks onto the hydrophobic surface.

Figure 4.2 Examples of patterns obtained from inkjet-printed silver nanoparticle inks by changing drop spacing and substrate temperature. The surface of the substrate was coated by PVP. A relatively high substrate temperature (60 °C) and appropriate drop spacing (60 µm) enables narrow and uniform lines.

Figure 4.3 Schematic illustration of wettability contrast inkjet printing technique. The inks only remain on the hydrophilic surfaces, and so fine patterning can be obtained when wettability contrast surfaces are used.

Figure 4.4 Schematic illustration of gravure offset printing.

Figure 4.5 (a) Schematic illustration of reverse-offset printing: ink coating (left), patterning (middle), and transfer (right). (b) Top-view photograph of reverse-offset printed electrodes formed on 120 mm × 120 mm glass substrates. (c) Photograph of printed electrodes with a width/spacing of 15/0.6 µm. (d) Cross-sectional SEM image of the printed electrodes. (Fukuda

et al

. 2015 [28].

Figure 4.6 Fabrication process of fully printed organic TFTs.

Figure 4.7 (a) Schematic illustration explaining the relation between solvent evaporation speed and final shape of drying films. (b) Profiles of inkjet-printed silver electrodes dried at relative humidity levels 30%, 80%, 85%, and 90 %RH. The drying temperature and drying time were 30 °C and 30 min, respectively. (Fukuda

et al

. 2013 [34].

Figure 4.8 Location of the neutral strain position of multilayer stack film.

Figure 4.9 (a) Adhesion strength of printed silver electrodes on PVP interlayer as a function of sintering temperature. (b) Relative adhesion strength versus surface energy of the underlayers. (c) Cross-sectional TEM images of the interface between the silver electrode layers and PVP.

Figure 4.10 Photographs of printed silver source electrodes before (left) and after (right) the application of 1.6% tensile strain (

R

= 4.0 mm). (a) Silver electrodes with high adhesion and (b) with low adhesion. (Fukuda

et al

. 2013 [43].

Figure 4.11 (a) Schematic illustration of flow of semiconducting solution. The dispensing point of the semiconducting ink solution was moved away from the center of the channel. Both bank layers and source/drain electrodes define the solution flow as shown by the red arrows in the Figure (b) Polarization microscope image of printed DTBDT-C

6

layer on parylene-C surface with source/drain electrodes. (c) Circuit diagram of the TFT array. (d) Transfer characteristics of 100 TFT devices in the array. (e) Distribution of threshold voltage as-measured results for 100 TFT devices in the array. (Fukuda

et al

. 2015 [44].

Figure 4.12 (a) Photograph of organic fully printed circuits fabricated on ultrathin films. (b) Photograph (left) and transfer characteristics (right) of TFTs wrapped around a copper wire with a radius of 140 µm. The TFT was measured in the bent and unbent states, with no discernible changes in the characteristics due to bending. (c) Schematic illustration of compression test. The compression forms out-of-plane wrinkles in the device, indicating the multiple bends with small bending radii of less than 10 µm. (d) Transfer characteristics of a TFT device operated under no strain (blue) and 54% compressive strain (red), demonstrating the mechanical stability of the fully printed TFT. (e) Mechanical durability during repeated compression and relaxation. The normalized mobility was plotted as a function of the number of cycles. (Fukuda

et al

. 2014 [47].

Figure 4.13 (a) Photograph of ultraflexible

D

flip–flop circuits. (b) Circuit diagram of a

D

flip–flop. (c) Dynamic characteristics with a clock frequency of 1 Hz and operating voltage of 10 V. (Takeda

et al

. 2016 [49].

Figure 4.14 Schematic illustration of the organic TFTs with extended-gate electrodes.

Figure 4.15 (a) Circuit diagram of fabricated differential amplifier. (b) Schematic illustration of the biosensor with differential amplifier. Two extended-gate electrodes (Au) were connected to each input of the differential amplifier. (c) Input–output characteristics of the differential amplifier upon titration with target IgG in a Dulbecco's phosphate-buffered saline (D-PBS) solution with 0.1 wt% bovine serum albumin (BSA). The blue line represents [target IgG] = 0 µg mL

−1

, and the red line represents [target IgG] = 50 µg mL

−1

. (d) Differential output voltage for the target IgG at various concentrations in a D-PBS solution with 0.1 wt% BSA. (Fukuda

et al

. 2015 [44].

Chapter 5: Flexible Photovoltaic Systems

Figure 5.1 Schematic illustration of a typical solar cell.

Figure 5.2 The photovoltaic effect of the p–n junction.

Figure 5.3 (a) Current–voltage curves of a solar cell under different levels of illumination. (b) The current density-voltage (black) and power density-voltage (gray) characteristics of an ideal solar cell.

Figure 5.4 Schematic illustration of a flexible triple-junction silicon solar cell, where µ-Si represents microcrystalline silicon.

Figure 5.5 (a) Optical image, (b) schematic illustration, and (c) photovoltaic performance of mechanically flexible modules that incorporate arrays of interconnected microcells. (Yoon

et al

. 2008 [10].

Figure 5.6 Schematic illustration of a typical flexible CIGS thin-film solar cell.

Figure 5.7 The CIGS nanoink of

Nanosolar.

Figure 5.8 Benzene: (a) Kekulé structure, consisting of two resonance (mesomerism) structures. (b) Visualization of the location of the lowest bonding π-orbital. (c) The position of the six energy levels resulting from the overlap of the six p

z

orbitals. The more nodes found in the wave function, the higher is the energy.

Figure 5.9 (a) Illustration of bulk heterojunction with bi-continuous interpenetration network of polymer and acceptor. (b) Schematic energy level diagram of a bulk heterojunction organic photovoltaic under a bias voltage. Red arrows indicate the transfer direction of the electrons and blue arrows point in the transfer direction of the holes.

Figure 5.10 Examples for introducing functional substituents to increase

V

oc

of the OPV based on conventional P3HT donors [30–32].

Figure 5.11 (a) Scheme of the ultralight and flexible organic solar cell. (b) Extreme bending flexibility demonstrated by wrapping a solar cell around a 35-µm-radius human hair. Scale bar is (also in c,d) 2 mm. (c) Stretchable solar cells made by attaching the ultrathin solar cell to a prestretched elastomer. (d) The device attached to the elastomeric support, under three-dimensional deformation by pressure from a 1.5 mm-diameter plastic tube. (e) SEM image of the PET surface of the solar cell in compressed state. (Kaltenbrunner

et al

. 2012 [37].

Figure 5.12 Tensile test of PBDTTTPD:PC

61

BM and PBDTTTPD:P(NDI2HD-T) blend films. (a) Strain–stress curves and (b) toughness of PBDTTTPD:PCBM and PBDTTTPD:P(NDI2HD-T) blend films. (Kim

et al

. 2015 [38], https://www.nature.com/articles/ncomms9547. Used under CC BY 4.0 https://creativecommons.org/licenses/by/4.0/.)

Figure 5.13 Structural representation of an organic–inorganic perovskite. (Green

et al

. 2014 [47].

Figure 5.14 (a) Schematic diagram of regular planar structure; (b) schematic diagram of inverted planar structure. (Liu

et al

. 2016 [51].

Figure 5.15 The schematic diagram of the working mechanism for perovskite solar cells with n–i–p structure. (Green

et al

. 2014 [47].

Figure 5.16 Energy level diagram of HTLs (left) and ETLs (right) used in PerSCs and perovskite light absorbers (middle). The unit for each value is (eV) but omitted. (Kim

et al

. 2016 [52].

Figure 5.17 Schematic procedure for different preparation methods for perovskite films. (Zheng

et al

. 2015 [54].

Figure 5.18 (a) Conceptual schematic diagram of a smart watch integrated with a flexible perovskite solar cell. (b) Cross-sectional SEM image of the inorganic–organic halide perovskite planar heterojunction flexible solar cell and schematic of the flexible device structure. Scale bar: 200 nm. (c) An actual bent image of a flexible perovskite solar cell. (d) Normalized

PCE

measured after bending the substrate within a specified radius of 400–1 mm. The inset shows the real images attached on the human neck, wrist, and finger corresponding to 400, 10, and 4 nm bending radii, respectively. The error bar represents standard deviation from 4 devices. (e) Normalized

PCE

of flexible perovskite devices as a function of bending cycles with different radii of 400, 10, and 4 mm. The inset shows the real images taken during the bending tests. The error bar represents the standard deviation from four devices. (Kim

et al

. 2015 [62].

Figure 5.19 Ultrathin and light perovskite solar cells. (a) Schematic of the solar cell stack. (b) Free-standing 3-µm-thick solar cells with gold top metal. Scale bar, 1 cm. (c) Perovskite solar foil with low-cost copper back contacts. Scale bar, 1 cm. (d) Snapshot of the model plane during solar-powered outdoor flight. Scale bar, 10 cm. (e) Close-up photograph of the horizontal stabilizer with integrated solar panel. Scale bar, 2 cm. (Kaltenbrunner

et al

. 2015 [63].

Figure 5.20 (a) Schematic, (b) energy level diagram (c,d) cross-sectional SEM images of the fiber-shaped perovskite solar cell. (e) Photograph of a textile. (Qiu

et al

. 2014 [64].

Figure 5.21 (a) Image and structure of a flexible PET substrate with embedded Ag-mesh with detail parameters.

Chapter 6: Materials Design for Flexible Thermoelectric Power Generators

Figure 6.1 Basic thermoelectric circuits. (a) A closed circuit with two similar conductors. (b) A closed circuit with two different conductors. (c) An open circuit with two different conductors.

Figure 6.2 A practical view of thermoelectricity. (a) A schematic of, and (b) a band-based scheme for, carrier diffusion in a given n-type thermoelectric material.

Figure 6.3 Typical density of state for (a) metals, (b) one-dimensional semiconductors, and (c) molecules and insulators. Dashed lines show the gradient at the Fermi level.

Figure 6.4 Heat-to-electricity conversion efficiency as a function of

ZT

between 80 and 20 °C.

Figure 6.5 (a) Unipolar and (b) bipolar (π-type) thermoelectric modules.

Figure 6.6 A typical molecular structure of PEDOT:PSS.

Figure 6.7 Typical n-type organic compounds (P(BDIOD-T2, PDI, PyDI-5FPE, BDPPV, and FBDPPV) and a dopant (N-DMBI).

Figure 6.8 Representative schematic illustration of carbon nanotube formation from a 2D graphene sheet with two arm-chair edges.

Figure 6.9 Representative schematic illustration of the DOS of semiconducting carbon nanotubes with (a) a smaller diameter and (b) a larger diameter, and (c) metallic carbon nanotubes.

Figure 6.10 (a) Semiconducting fraction

α

dependence of Seebeck coefficient (

S

) at 300 K. (b) Schematic serial model for junction networks consisting of semiconducting (s) and metallic (m) SWCNTs, where temperature (

T

) varies along the series of SWCNT bundles. (Nakai

et al

. 2014 [34].

Figure 6.11 Typical organic n-type dopants.

Figure 6.12 A schematic illustration of charge balanced n-type SWNT anion–metal cation complexes.

Figure 6.13 A thermal stability. (a) Temporal changes of the electrical conductivity, the Seebeck coefficient and power factor of n-type films derived from eDIPS-CNTs treated with KOH/benzo-18-crown ether at 100 °C. (b) Additives used in this study. Temporal changes of n-type films from (c) KH-CNTs 1.3 ± 0.2 nm in diameter; (d) Tuball-CNTs 1.8 ± 0.3 nm in diameter; (e) eDIPS-CNTs 2.0 ± 0.7 nm in diameter at 150 °C. Importantly, filled circles in (c–e) indicate the samples doped with KOH/benzo-18-crown ether. (Nonoguchi

et al

. 2016 [40].

Figure 6.14 (a) A flexible thermoelectric generator made of p-type and n-type carbon nanotubes as thermoelectric layers. (b) Power generation and LED lightening under a hot air blow with the thermoelectric generator. (The original pictures were supplied from Sekisui Chemical CO., LTD and Nara Institute of Science and Technology.)

Figure 6.15 Multifunctional application of MFSOTE-based-sensing array. Graphs (a,b) show a photograph and circuit diagram, respectively, of an MFSOTE array with 4 × 4 pixels. Graphs (c,d) show schematic illustrations of the integrated array: (c) all the devices connected in series can be used as an electricity generator for harvesting energy. (d) The device switches to a self-powered dual-parameter sensor model when subjected to an external pressure. The insets in graph (c,d) show magnified views of the circuit connection between two pixels under unloading and loading conditions, respectively. (e) The thermoelectric voltage (top), temperature (middle), and pressure (bottom) responses of a MFSOTE array to five finger-touch cycles. The bottom surface of the MFSOTE array is heated with a Peltier element. (Zhang

et al

. 2015 [46], https://www.nature.com/articles/ncomms9356. Used under CC BY 4.0 https://creativecommons.org/licenses/by/4.0/.)

Chapter 7: Flexible Supercapacitors Based on Two-Dimensional Materials

Figure 7.1 Ragone plot showing the specific power against specific energy for various electrical energy storage systems. (Simon and Gogotsi 2008 [18].

Figure 7.2 Models of the electrical double layer at a positively charged surface: (a) the Helmholtz model, (b) the Gouy–Chapman model, and (c) the Stern model, showing the inner Helmholtz lane (IHP) and outer Helmholtz plane (OHP). The IHP refers to the distance of closest approach of specifically adsorbed ions (generally anions) and OHP refers to that of the nonspecifically adsorbed ions. The OHP is also the plane where the diffuse layer begins. (d) the double layer distance described by the Helmholtz model.

ψ

0

and

ψ

are the potentials at the electrode surface and the electrode/electrolyte interface, respectively. (Zhang and Zhao 2009 [19].

Figure 7.3 (a) Schematic drawing of stacking graphene film and cross-sectional SEM image of the reduced graphene oxide films (inset: SEM image of its surface).

Figure 7.4 Schematic depiction of the operating principle of: (a) the stacked-geometry conventional supercapacitors, and (b) in-plane micro-supercapacitors. (Qi

et al

. 2016 [23].

Figure 7.5 (a) Schematic showing the fabrication process for nanoporous gold/MnO

2

hybrid materials by directly growing MnO

2

(orange) onto nanoporous gold. (b) Bright-field TEM image of the nanoporous gold/MnO

2

hybrid nanostructure, which can be identified by the contrast between the bright MnO

2

filler and the dark gold skeleton. (c) Photograph of a nanoporous gold/MnO

2

-based supercapacitor. (d) Crystal structure of NiMoO

4

(e) crystallized NiMoO

4

nanosheets arrays. (f) Cycling stability of NiMoO

4

at a current density of 5 A g

−1

(the inset shows the photograph of the device). (a–c: Lang

et al

. 2011 [28].

Figure 7.6 (a) Cross-sectional SEM image of the MnO

x

/graphene oxide nanohybrid paper. (b) Schematic diagram showing how MnO

x

nanofibers are embedded among reduced graphene oxide sheets. (c) Schematic illustration of the flexible asymmetric supercapacitor based on MnO

x

/graphene oxide nanohybrid paper as the positive electrode and reduced graphene paper as the negative electrode in a diluted ionic liquid (1 M EMIMBF

4

in acetone nitrile) electrolyte, wrapped in a flexible packaging. (d) Photo of the freely bended asymmetric supercapacitor device. (Hu

et al

. 2015 [229].

Figure 7.7 Schematic drawing of the different electrode arrays and their relevant strain distribution by the finite element modeling analysis for (a) in-plane structure, (b) wrinkled structure, and (c) suspended wavy structure. (d) The peak strain comparison of in-plane structure, wrinkled structure, and suspended wavy structure, respectively. Liquid-crystal display (LCD) lit by the stretchable micro-supercapacitors under (e) 0 strain and (f) 100% strains (insets show the distance). (Qi

et al

. 2015 [87].

Chapter 8: Organometal Halide Perovskites for Next Generation Fully Printed and Flexible LEDs and Displays

Figure 8.1 One potential application for stretchable OLED displays, which are light weight and can be stretchable from a small stowed volume into large-size displays for various portable electronics and next generation wearable medical devices.

Figure 8.2 (a) The atomic structure of perovskite, A is a +1 cation, B is a +2 cation, and X is a −1 halide ion. (b) The emission wavelength as a function of halide composition. (c) Perovskite containing solution, and bulk crystals grown using solution processing.

Figure 8.3 (a) The multilayer architecture commonly found in perovskite solar cells. Different layers have been labeled with common materials used in that layer. When a forward bias is applied, electrons are injected from the cathode side and holes are injected from the anode side into the perovskite emissive layer. Light emission is generated when the electrons and holes meet and combine radiatively. The use of the electron transportation layer and the hole transportation layer is to lower the energy barriers between the perovskite and the electrodes, assuring that both can be injected at a sufficiently low voltage. (b) Different multilayer devices illuminated; the far right shows perovskite LEDs in flexible applications.

Figure 8.4 (a) The p–i–n junction formation formed in single layer perovskite LEDs. (b) Multiple color devices fabricated using the single layer approach, on the right is a flexible device (c) the band structure of the p–i–n junction. The high levels of ion accumulation near the electrodes allow for a tunnel junction to form.

Figure 8.5 (a) The printing process outlined as a five-step process (b) large area covered via the printing process, under UV illumination it is apparent that it is extremely uniform. (c) Letter fabrication via mask patterning in the top electrode printing step.

Figure 8.6 The device structures of various LED technologies. Perovskites have proven to have the simplest device structure to date containing only one layer between the electrodes.

Chapter 9: Flexible Floating Gate Memory

Figure 9.1 (a) Schematic diagram of the FET structure. (b) Typical transfer curve of FET. (c) Schematic diagram of the floating gate memory structure. (d) Typical transfer curve of the floating gate memory.

Figure 9.2 (a) Schematic diagram of F–N tunneling in the floating gate memory; (b) Schematic diagram of direct tunneling in the floating gate memory.

Figure 9.3 (a) Schematic illustration of the nanoparticle printing process. (b) Optical image of the flexible µCP floating gate memory. (c) Threshold voltage of the µCP floating gate memory with respect to the elapsed time. Inset: the pulse sequence for retention test. (d) Threshold voltage of the µCP floating gate memory as a function of the number of P/E cycles. Inset: the test pulse sequence for endurance test. (e) Comparison of the retention properties of different floating gate memories. (f) Flexibility test of the µCP floating gate memory. (Wei

et al

. 2012 [3].

Figure 9.4 Schematic diagram of the flexible substrate at tensile state and compressive state.

Figure 9.5 (a) Schematic diagram of the flat floating gate memory; (b) schematic diagram of the floating gate memory at negative strain; (c) schematic diagram of the floating gate memory at positive strain; (d) electrical performances of the flat floating gate memory; (e) electrical performances of the floating gate memory at negative strain; (f) electrical performances of the floating gate memory at positive strain. (Zhou

et al

. 2013 [44].

Chapter 10: Flexible and Stretchable Wireless Systems

Figure 10.1 (a) Generation of electromagnetic waves in free space from an antenna. (b) The Thevenin-equivalent circuit of an antenna as seen from the side of the transmission signal.

Figure 10.2 (a) The relationship between flexural rigidity of a material with decreasing thickness and energy release rate. (Rogers

et al

. 2011 [2].

Figure 10.3 (a) Design, optical images, and scanning electron micrograph (SEM) images of stretchable sensors fabricated using lateral spring structures of silicon nanomembranes. (Kim

et al

. 2014 [21].

Figure 10.4 (a) Schematic illustration of the inkjet printing process. (b) Optical images of flexible antennas fabricated using inkjet printing on Kapton. (Ahmed

et al

. 2015 [68].

Figure 10.5 (a) Stretchable antenna fabricated using Ag nanowire and PDMS composite. (Song

et al

. 2014 [88].

Figure 10.6 (a–c)

S

11

for stretchable antenna measured under various conditions. (d) Variation of gain of the antenna with stretching cycles. (e) Radiation pattern of the stretchable antenna for stretched and unstretched conditions. (Hussain

et al

. 2015 [90].

Chapter 11: Conductive Nanosheets for Ultra-Conformable Smart Electronics

Figure 11.1 General features of a “conductive nanosheet”; a free-standing conductive polymer ultrathin film based on poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS).

Figure 11.2 (a) Schematic illustration of the spin-coating process for the preparation of a free-standing single-layered conductive nanosheet. (b) A free-standing PEDOT:PSS nanosheet in water. PEDOT:PSS nanosheets collected onto (c) paper, (d) human skin, and (e) steel mesh (scale bar 100 µm). (Greco

et al

. 2011 [49].

Figure 11.3 (a) Scheme of bilayer PEDOT:PSS/PLA nanosheets subtractive patterning through inkjet printing of a deactivating (overoxidizing) solution of NaClO. (b) Free-standing patterned nanosheet (IIT logo) floating in water. Inkjet patterned nanosheets (interdigitated electrodes pattern with 200 µm gap size) collected onto (c) paper and (d) a steel mesh. (e) Demonstration of functioning of nanosheet as a suspended and bendable conductor membrane: patterned nanosheet (two conducting lines) suspended over a plastic frame and connected to an LED. (Greco

et al

. 2013 [50].

Figure 11.4 (a) Tabletop Mini-Labo™ Test Coater, Yasui Seiki Co., Japan (upper) and schematic illustration of a Roll-to-Roll (R2R) gravure-printing system (bottom). R2R process uses a gravure roll with an engraved texture that allows to print the ink (polymer solution) onto the flexible substrates. (b) R2R processed conductive nanosheets supported on a roll substrate. (c) A free-standing R2R-processed single-layered conductive nanosheet in acetone. (d) Detachment of a PEDOT:PSS/PDLLA bilayered conductive nanosheet from a poly(propylene) (PP) substrate by a “tape method”. Four pieces of Scotch® adhesive tape were placed on the nanosheet-printed side of the PP substrate to form a supporting frame for detachment (i). The PP substrate was peeled off from the frame (ii) to obtain a free-standing nanosheet supported by an adhesive tape frame (iii). (e) An R2R conducting nanosheet peeled off from the substrate. (f) Example of a large-area conductive nanosheet transferred onto human skin. (a–f: Zucca

et al

. 2015 [54].

Figure 11.5 (a) The relationship between the thickness

t

and the conductivity

σ

of free-standing single-layered conductive nanosheets prepared by a spin-coating process. Nanosheets supported onto PDMS before release, (solid squares); nanosheets after release in water, re-collection on glass (solid circles), and after a subsequent thermal treatment, (solid triangles). (b) Schematic representation of the percolative mechanism in single-layered PEDOT:PSS nanosheet structure composed of PEDOT-rich particles surrounded by the PSS matrix. Length of conducting pathways between neighbor PEDOT particles (dashed line) increases as its thickness comes up to a percolation threshold, thus establishing multiple parallel pathways. (Greco

et al

. 2011 [49].

Figure 11.6 Conductivity

σ

of free-standing single-layered conductive nanosheets prepared by an R2R gravureprinting process as a function of their thickness

t

; comparison among different formulations of a PEDOT:PSS dispersion: pristine (solid circles), 5% BG-doped (solid squares), and 5% DMSO-doped (black circles). On the right side detailed trends for secondary doped (up) and pristine (bottom) formulations of PEDOT:PSS.

Figure 11.7 (a) AFM topographic images (1.5 µm × 1.5 µm) of single-layered PEDOT:PSS nanosheet (spin-coating-processed) collected on Si after its release in water. Surface topography made up of individual grains is evidenced. (Greco

et al

. 2011 [49].

Figure 11.8 Mechanical properties of the single-layered PEDOT:PSS conductive nanosheets with different thicknesses

t

evaluated by the SIEBIMM [76] measurement. The relationship between thickness

t

and (a) the buckling wavelength

λ

, that is, the distance between two consecutive ripple maxima and (b) the Young's modulus En of the nanosheets. Inset shows an example of strain-induced buckling on a 77.1 nm thick conducting nanosheet sample (topographic image obtained by AFM). (a and b: Greco

et al

. 2011 [49].

Figure 11.9 (a) Electrochemical properties of conductive nanosheets. The nanosheet, patterned with an interdigitated electrodes design, was suspended onto a Teflon ring that provided electrical contact: left side set as working electrode, right side as counter electrode. Nanosheet was immersed in a 0.1 M NaCl solution in water and Cyclic voltammograms (CV) were recorded at different scan rates. Pictures depict the electrochromic behavior of PEDOT:PSS/PLA patterned nanosheets at each redox state during CV experiment. Electrochemical switching of wettability on conductive nanosheets surface; (b) variation of contact angle

θ

depending on the redox state of free-standing or supported nanosheets: (c) side-view of a free-standing PEDOT:PSS/PLA patterned nanosheet showing different contact angle for drops placed on the reduced or the oxidized portion of the film. (Greco

et al

. 2013 [50].

Figure 11.10 (a) Surface EMG recording of muscle activity on the arm; comparison between (BG-doped) PEDOT:PSS/PDLLA bilayered conductive nanosheets and standard pregelled Ag/AgCl electrodes. The signal-to-noise ratio (SNR) of a recorded signal demonstrating similar performances. (b) Stability of the conductive nanosheet against mechanical stress: normalized resistance variation (

R

/

R

0

) of the nanosheet during repeated (160 times) exercise for wrist contraction/expansion (the nanosheet placed on wrist, flexion ±40°–50° with respect to the relaxed state). (c) Stability of the conductive nanosheet against sweat: normalized resistance variation (

R

/

R

0

) of the nanosheet collected on a silicon-based artificial skin model and submerged in a bath containing water (circles), artificial sweat—pH 5.5 (squares) and pH 8.0 (triangles). (Zucca

et al

. 2015 [54].

Figure 11.11 Pictures of (a) free-standing PEDOT:PSS/iron oxide NPs nanosheet floating in water after release; (b) STEM images showing NPs dispersion within nanocomposite PEDOT:PSS nanosheet; nanosheet collected onto (c) flexible PDMS and (d) paper. (e) SEM image showing the conformal adhesion of nanosheet re-collected on paper. (f) Resistance response to relative humidity variation in pure PEDOT:PSS and PEDOT:PSS/iron oxide NPs composite nanofilms at different NP concentrations (

x

):

x

= 0 (▴); 1 (▪); 5 (♦) mg mL

−1

. (g) Resistance variation (black curve, right vertical axis) of a PEDOT:PSS/iron oxide NPs nanosheet (5 mg mL

−1

) during humidity step-change experiments (gray curve, left vertical axis) in the range of 30–70% RH. (Taccola

et al

. 2013 [51].

Figure 11.12 (a) Fabrication procedure for PEDOT:PSS/SU8 bilayer microactuators: (i) PEDOT:PSS deposition on PDMS layer; (ii) SU-8 deposition and photolithographic patterning; (iii) wet chemical etching of PEDOT:PSS; (iv) bilayer microactuator peeling off by using temporary PVA support layer; (v) free-standing PEDOT:PSS/SU8 microactuators released in water. (b) Picture of PEDOT:PSS/SU8 microactuator released in water after PVA dissolution (in inset the actuator before water immersion supported by PVA layer). (c) Movements of PEDOT:PSS/SU8 microactuators during cyclic voltammetry at different potentials and oxidation states (from oxidation

V

= +0.4 V to reduction

V

= −0.8 V). (© 2013 IOP Publishing Ltd.)

Figure 11.13 (a) Concept of temporary tattoo conductive nanosheet as skin-contact ultra-conformable electrodes. (b) Comparison of recording of electrical activity of muscles performed with EMG dry electrodes based on tattoo nanosheets and standard pregelled Ag/AgCl electrodes. (c) Example of use of EMG dry tattoo electrodes for the control of a prosthetic limb: multiple movements are correctly reproduced by the robotic hand, by using four pairs of tattoo electrodes placed on the forearm of the test subject to pick up muscle activation signals. (Zucca

et al

. 2015 [52].

Chapter 12: Flexible Health-Monitoring Devices/Sensors

Figure 12.1

Human bio-signals and types of healthcare sensors.

Representative bio-signals from the human body sites (a) and types of healthcare sensors depending on the detection approaches for physical, biochemical, and electrophysiological vital signs (b).

Figure 12.2

Flexible pressure and strain sensors with structural modification for detecting human motion and activities

. 1D structures: (a) 1D fiber structures based on graphene at 50% strain and (b) response curves of a wearable sensor corresponding to motion. (a,b: Cheng

et al

. 2015 [37].

Figure 12.3

Detection of physical vital signs with pressure and strain sensors

. (a) Schematic illustration for detection of blood pressure with fingertip skin-inspired ferroelectric skins and the detection of artery pulse pressure (i) and variation in pulse waveforms before and after exercise (ii).

Figure 12.4

Types of flexible temperature sensors for body temperature distribution

. (a) Temperature mapping of rat lung using flexible and printable thermal sensors. (Reproduced from Ref. [56]. with permission; Copyright 2015, National Academy of Sciences.) (b) Monitoring of temperature change of neck skin during drinking of hot water with body-attachable temperature sensor. (Trung

et al

. 2016 [61].

Figure 12.5

Flexible pH sensors and detection of biochemical signals

. (a) Schematic illustration of pH-sensitive IZO-based neuromorphic transistor measured in dual-gate synergic modulation mode and pH-dependent transfer curves of the device measured by sweeping control gate voltage (G2) at

V

DS

= 0.1 V with different fixed sensing gate voltages (G1).

Figure 12.6

Flexible glucose sensors and detection of blood sugar level

. (a) Schematic illustration of the tattoo-based glucose sensors (i) and photograph of the sensor attached to human skin (ii). (iii) Chronoamperometric response of the sensor with increase of glucose levels from 0 to 100 µM.

Figure 12.7

Flexible blood oximeters and detection of oxygenation in human body

. (a) Schematic illustration of a model for penetrated light path from oximeter through skin and blood vessel. (b) The absorptivity of deoxygenated and oxygenated hemoglobin in arterial blood. (c) Photograph of integrated OLED and OPD on subject's finger to monitor the level of blood oxygen concentration. (d) The measured pulsating photoplethysmogram (PPG) signal by all-organic pulse oximetry system. The heart rate and saturation level of blood oxygen were converted from the PPG signal. (Lochner

et al

. 2014 [78].

Figure 12.8

Flexible biochemical sensors and detection of various VOCs

. (a) Scanning electron microscope (SEM) and transmission electron microscope (TEM) images of In

2

O

3

/Au nanorods with large surface area and dynamic responses of In

2

O

3

/Au nanorod-based gas sensors under exposure of acetone and ethanol, respectively.

Figure 12.9

Flexible electrophysiological sensors and detection of electrophysiological signals

. (a) Photograph of patterned PEDOT:PSS electrode on knitted textile for long-term monitoring of ECG and acquired ECG signals over 3 days.

Figure 12.10

Multifunctional sensors for detection of multiple bio-signals

. (a) Schematic illustration of bioinspired dry-adhesive-based multifunctional electronic patch for monitoring vital signs, wireless signal transmission, and drug delivery. (b) Detection of acute medical conditions with multifunctional patch. (a,b: Choi

et al

. 2016 [98].

Figure 12.11

Practical applications of health-monitoring devices

. (a) Fabric-based wearable activity monitors for fitness applications. (Jung

et al

. 2014 [38].

Chapter 13: Stretchable Health Monitoring Devices/Sensors

Figure 13.1 Representative materials and structures used in stretchable electronic devices for health monitoring. (a) A highly stretchable elastomer membrane, (Park 2012 [14].

Figure 13.2 Various formats of stretchable health monitoring devices. (A) An epidermal electronic sensor that monitorsbiopotential of the skin. (Jang 2015 [39].

Figure 13.3 Epidermal sensors that can monitor (a) biopotential of skin using metallic meshes (Yeo 2013 [17].

Figure 13.4 Epidermal sensors that can monitor skin temperature using (a) metallic sensors (Webb 2013 [64].

Figure 13.5 Epidermal sensors that can (a) conduct transdermal oximetry measurement using LEDs and photodetectors (Lochner 2014 [104].

Figure 13.6 Skin sensors that can monitor biomolecules in sweat based on (a) colorimetry approach (Huang 2014 [118].

Figure 13.7 Stretchable electronic devices that are used as (a) a neural probe (Guo 2014 [70].

Figure 13.8 Examples of body wearable devices for rehabilitation and health tracking. (a) A carbon nanotube based strain sensor that can be stretched by over 900%. (Ryu

et al

. 2015 [138].

Chapter 14: Flexible/Stretchable Devices for Medical Applications

Figure 14.1

Materials, synthesis and composites for flexible/stretchable systems

. (a) Top: Tilted scanning electron microscope (SEM) image of an array of ultrathin silicon solar microcells tethered to a mother wafer, right after defining individual cells and undercut etching with potassium hydroxide (KOH). The inset represents a magnified SEM image of a representative microscale solar cell (thickness, ∼20 µm). Bottom: Cross-sectional SEM view of multiple stacks of GaAs/AlAs formed by metal organic chemical vapor deposition (MOCVD) during selective etching of the AlAs via hydrofluoric acid (HF). (b) Top: A colorized image of an array of electronic circuits with a pop-up bridge structure to enhance stretchability, conformal interfaces of the system to unusual shapes such as the surface of a golf ball. Bottom: SEM image of an advanced design layout to achieve extreme stretchable capability of an array of silicon CMOS inverters using noncoplanar, serpentine bridges over a large area. Each cell strongly bonds to the PDMS, while serpentine traces are not restricted to the substrate for being stretchable to external forces. (c) Top: A magnified SEM image showing the structure of a representative conductive elastomer, including uniformly distributed single wall carbon nanotubes (SWCTs) in a rubber through multistage manufacturing procedures. Bottom: An optical image of Ag flakes-based elastic ink with high conductivity and stretchability for printed electronics. The electrical and mechanical properties of this elastic conductive ink is determined by its composite materials. (d) Top: Photographs of pliable, patterned SWCTs-based conductors on a PDMS slab for organic electronic devices, with dispersed SWCTs in paste (left) and patterned SWCTS by screen printing (right) in the inset. Bottom: Image of net-shaped conductors enabling excellent contact ability to three-dimensional random configurations, for potentially electronic artificial skin.

Figure 14.2

Electronic/optoelectronic devices, sensors and systems

. (a) An array of stretchable Si CMOS integrated circuits on a PDMS rubber with pop-up bridges as interconnects in a noncoplanar configuration. Chosen materials and design layouts enable to accommodate external mechanical stimulations such as twisting, bending, diagonal stretching, and so on. (b) Description of an artificial digital camera inspired from compound eyes. Combination of silicon photodiodes and blocking diodes in an active matrix array offers elastic hemispherical microlenses system. The inset represents a magnified view of 2 × 2 array of the device connected with serpentine traces for enhanced extension. (c) Photographs of mechanically deformed GaAs- (left) and GaN-based (right) micro-LEDs array. Stretching in a sharp pencil tip and twisting of micro-LEDs exhibit no functional degradation due to the selection of materials and structure configurations. (d) An image of flexible, wearable conductive nets incorporated with organic transistors in the configuration of active matrix addressing on an egg, potentially for pressure, thermal sensors, and artificial electronic skin. (e) Demonstration of stretching tests in uniaxial/biaxial direction of stretchable organic integrated circuits configured in matrix addressing, consisting of organic transistors and SWCTs contacts and interconnects. (f) Photographs of flexible, stretchable large-area organic LED display that is capable of conforming to curvilinear surfaces and/or structures. Deformed states (e.g., folding) do not limit device functionality as shown in the inset.

Figure 14.3

Multifunctional electronic sensors and power scavenging circuit for the heart

. (a) Optical images of three-dimensional multifunctional integumentary membranes (3D-MIMs) supplying interfaces for physiological mapping and stimulation through wrapping up the entire epicardial surface of the heart, with the view of covering both sides (anterior and posterior) of the heart in the inset. The device includes diverse functional components such as ECG sensor, Si strain gauge, micro-LED (μ-LED), pH sensor, temperature sensor (from left to right). (b) Comparison of measured fractional resistance from a Si strain sensor with recorded signals from ECG sensor during dynamic cardiac movements, for example, sinus rhythm (top), ventricular pacing (middle), and ventricular fibrillation (bottom). (c) Image of

in vivo

attachment of PZT-based mechanical energy harvesters on the right ventricle (RV), left ventricle (LV), and free wall of a bovine heart (left). The whole system consists of a rechargeable microbattery, a rectifier, and PZT MEHs on a flexible substrate (top right), and a magnified view of PZT MEHs connected to anisotropic conductive film (ACF) in a flexible format (bottom right). (d) Measurements of voltage output as a function of time for MEHs by varying the heart rate of bovine (green) and ovine (blue). Voltage characteristics were captured from different locations, RV (top), LV (middle), and free wall (bottom) during expansion and relaxation.

Figure 14.4

Electrophysiology and optogenetics for the brain

. (a)

In vivo

animal (feline model) experiments of a flexible, high-density active neural circuit implanted on the visual cortex of the brain, and insertion of the electrode into the interhemispheric fissure (inset). Images of active multiplexing array before (bottom left)/after (bottom right) insertion into the interhemispheric fissure. (b) Recordings of both spiral waveforms, counterclockwise (top) to clockwise (bottom), from active neural electrodes during electrographic seizure, that is, induced seizure via local remedy of the drug (picrotoxin). (c) Images of sequence in a procedure of injection and retraction of microneedle that allows multifunctional, flexible cellular-scale micro-LEDs system to be implanted desirable locations. Attached silk fibroin films on the backside of the micro-LEDs system gradually dissolved during this procedure, leaving the optogenetic system in the targeted area. (d) Experimental results of colorized data map in the Y-shaped maze for the preference of animal behaviors in different conditions. Red colors indicate preferred locations for longer time period than other colors do.

Figure 14.5

Communication and regulation for the nervous system

. (a) Illustration of a skin-inspired mechanoreceptor system through pressure-induced somatosensory cortex stimulation circuit for an optoelectronic system, incorporating with a wearable pressure sensor, organic oscillator, light-emitting diode, and optical fiber. (b) Measured correlation data of applied pressure to the digital tactile (DiTact) system, sensor output from the LED, and cell potential from the patch clamp. Sensor output signals are closely related to the recorded action potentials from neurons in the magnified view of data (right). (c) Optical images of soft tissue-like electronic dura mater (e-dura) capable of electrical stimulations as well as drug delivery for injuries related to the nervous system. Materials constituting the system include microcracked gold film for interconnects (top right), composite of platinum nano- and microparticles for electrodes/contacts (bottom right), and a silicone rubber for the substrate. (d) Comparison of response to the electrochemical stimulations from implanted e-dura, with severely injured spinal cord in a rat. Measurement recording signals from walking tests represent obvious recovery capability with (left)/without (right) electrochemical stimulation, from flexor/extensor muscle and hindlimb movements.

Figure 14.6

Skin-like electronics/optoelectronics.

(a) Demonstration of a smart wearable wristband for a real-time, wireless integrated healthcare monitoring system. The wristband consists of an array of multifunctional sensors to measure various analytes (glucose, lactate, sodium, potassium) in perspiration and skin temperature, and commercial electronic components-based flexible printed circuit board. (b) Comparison of real-time plots of sodium concentrations for tracing hydration/dehydration levels from flexible sensor arrays during group outdoor running, in the case of water intake and no water intake. (c) Illustrations of ultrathin, flexible polymer light-emitting diodes (PLEDs) laminated onto human skin with conformal contacts, capable of displaying user's health conditions and other information received from integrated sensors. Representative examples of device functionality exhibit a blue logo and red number (inset) with external power connections. (d) Electrical characterizations of different oxygen concentrations in blood through organic photodetectors (OPDs). Light emissions from green and red PLEDs reflected from blood, which depends on the concentration of oxygen in blood (top, 99%: bottom, 90%). (e) Schematic description of a soft, stretchable electronic robot that is capable of displaying color differences in response to changes of pressure. This soft robot includes an actuator system (pneumatic chambers with strain limited layer) for a locomotion that leads to changes of capacitances to display the robot's status or surrounding conditions. (f) A series of optical images to describe soft robot's locomotion with independent color segments. As underlying pneumatic chambers are in inflation/deflation states, top layers including electroluminescent layer become stretched/expanded leading to increase of illuminance of the system.

Figure 14.7

Transient, bioresorbable systems.

(a) Exploded view illustration of a flexible, bioresorbable electronic system, based on ultrathin silicon nanomembranes (Si NMs), nanoporous silicon, silicon oxide, and poly (lactic-co-glycolic acid) (PLGA). The system utilizes a thin diaphragm structure with a piezoresistive component to maximize chemical, mechanical responses. Inset shows a magnified view of active component, Si NM, in optimized position of the trench. (b) Comparison of measured intracranial pressures as a function of time via a commercial sensor with a transient monitor. The recorded pressure from both sensors indicates that the performance of implanted wireless, transient brain sensors is comparable to that of a commercial sensor (blue, commercial ICP sensor; red, transient ICP sensor). (c) Optical images of an array of actively addressable, multiplexed bioresorbable neural electrodes for recording high spatiotemporal electrocorticography (ECoG) signals. The actively controlled region of 8 × 8 cells includes 128 n-channel Si NMs-based transistors for operating the whole system as well as measuring neural signals, which connects to 64 channels through ACFs to communicate external peripheral circuits. The inset shows a magnified view of Si active device region. (d) Spatial color maps indicating measured locations of relative, evoked potential via the neural recording array after stimulating the barrel cortex. Each map exhibits good agreement in between stimulated coordinates of barrel cortex and locations of evoked potentials by a bioresorbable ECoG array.

Chapter 15: Implantable Flexible Sensors for Neural Recordings

Figure 15.1 The structure of a neuron. A typical neuron has four regions: cell body, dendrites, axons, and presynaptic terminals. (Kandel

et al

. 2012 [1].

Figure 15.2 Spatiotemporal resolutions of noninvasive methods [e.g., EEG, fMRI, and magnetoencephalography (MEG)] and invasive electrical measurements [e.g., optical imaging, ECoG, LFPs, and single-neuron action potential recordings (single units or spikes)].

Figure 15.3 Extracellular electrical recordings of brain activities categorized into four primary modalities: EEG, ECoG, LFPs, and single-neuron action potential recordings (single units or spikes).

Figure 15.4 Equivalent circuit model of the recording system in extracellular neural recordings.

Figure 15.5 A polyimide-based flexible intracortical needle-electrode array. (a) The interconnect system in place on the backside of a 12-pin Microtech connector. The integrated polyimide cable (large arrow) leads from the device, while the single traces (small arrow) break out to individual connector posts. (b) A six-site, three-shaft device with an “

S

”-curve for strain relief engineered directly into the cable (Scale bar, 1.5 mm). (c,d) A 2D structure (c) (scale bar, 1.5 mm) and a similar structure “bent” into a 3D configuration (d). (e) Schematic views of the layered construction used in the manufacture of a polyimide electrode array. (f) View of a prototype single-shaft electrode array without an integrated cable “flexed” against a mirrored surface. (g) A continuous six-second sample showing multiple unit activity recorded from rat whisker barrel cortex from one site of a polyimide electrode array (site size ) [12].

Figure 15.6 A 3D flexible mulichannel polyimide needle electrode array fabricated by magnetic batch assembly process. (a) Schematic illustration of a flexible multichannel 3D needle array. (b) 2D planar needles of polyimide formed to 3D needles by the magnetic batch assembly process. (c) Photos of the bending experiment. Photos of a fabricated needle array: (d) the needle array with 2 cm interconnection cable, (e) front and back side of the array before folding, (f) 3D flexible needle array after folding, and (g) recording pads, (h) photos of an insertion experiment: after the electrode insertion to rat's brain, and (i) the inserted regions after the detachment of the needles by tweezers. (j) Neural signals (spontaneous action potentials) recorded via the flexible needle array. The needle array was inserted to the rat's visual cortex [13].

Figure 15.7 A flexible needle electrode by Cheung

et al

. (a,b) Images of the microfabricated needle structures. The needle thickness is 15 µm. Each electrode is a circle of 25 µm diameter. The polyimide is transparent and the overlapping stacked metal lines are visible. (c) A micro surface-mount connector is soldered to the polyimide needle. (d) During recording, the microelectrodes are connected to a head-mounted preamplifier, which is mounted on the distal end of the recording cable and main amplifier. (Cheung

et al

. 2007 [14].

Figure 15.8 Syringe-injectable flexible mesh electronics. (a–c) Schematics of injectable electronics. (d) Bright-field microscopy image of the electronics immediately before injection into solution. (e–g) Injection of mesh electronics into aqueous solution (e,f), (g) Optical image of a 15 mm total width mesh electronics partially injected through a 20 gauge needle into solution, (h,i) Schematic and optical image of

in vivo

stereotaxic injection of mesh electronics into a mouse brain. (j) Acute

in vivo

16-channel recording using mesh electronics injected into a mouse brain. (Liu

et al

. 2015 [15].