Skin-like electronics that can adhere seamlessly to human skin or within the body are highly desirable for applications such as health monitoring, medical treatment, medical implants and biological studies, and for technologies that include human-machine interfaces, soft robotics and augmented reality. Rendering such electronics soft and stretchable-like human skin-would make them more comfortable to wear, and, through increased contact area, would greatly enhance the fidelity of signals acquired from the skin. Structural engineering of rigid inorganic and organic devices has enabled circuit-level stretchability, but this requires sophisticated fabrication techniques and usually suffers from reduced densities of devices within an array. We reasoned that the desired parameters, such as higher mechanical deformability and robustness, improved skin compatibility and higher device density, could be provided by using intrinsically stretchable polymer materials instead. However, the production of intrinsically stretchable materials and devices is still largely in its infancy: such materials have been reported, but functional, intrinsically stretchable electronics have yet to be demonstrated owing to the lack of a scalable fabrication technology. Here we describe a fabrication process that enables high yield and uniformity from a variety of intrinsically stretchable electronic polymers. We demonstrate an intrinsically stretchable polymer transistor array with an unprecedented device density of 347 transistors per square centimetre. The transistors have an average charge-carrier mobility comparable to that of amorphous silicon, varying only slightly (within one order of magnitude) when subjected to 100 per cent strain for 1,000 cycles, without current-voltage hysteresis. Our transistor arrays thus constitute intrinsically stretchable skin electronics, and include an active matrix for sensory arrays, as well as analogue and digital circuit elements. Our process offers a general platform for incorporating other intrinsically stretchable polymer materials, enabling the fabrication of next-generation stretchable skin electronic devices.

More recently, there is a shift in research focus to exploiting the mechanical deformability of conjugated polymers due to the rapidly growing demand for wearable and implantable devices. [35][36][37][38][39] Since human bodies and organs are soft, curved, and constantly moving, flexible and stretchable devices are essential for comfort conformability, precise measurement, and longevity of bioelectronics such as medical implants, wearable biosensors, and prosthesis. [40,41] Currently, this has been achieved utilizing geometric approaches such as metallic serpentine or Kirigami interconnects [41][42][43] and induced buckling [44][45][46] to impart stretchability on rigid silicon-based devices. However, next-generation wearable and implantable electronics will benefit from intrinsic stretchability and unique biological properties such as self-healing and biodegradability for high-density and biocompatible devices. Conjugated polymers are attractive candidates to this end for several reasons: They have relatively low tensile modulus (≈1 GPa or lower)compared to that of silicon and inorganic semiconductors (≈100 GPa), which provides a softer interface suitable for bioelectronics. 2. Polymers possess great potential in incorporation of tough, elastic, and self-healing properties via molecular design, polymer chain entanglement, cross-links, and noncovalent interactions. Most biological tissues are also polymeric in nature. 3. With the advancements in organic chemistry, polymer chemical structures are highly tunable, hence biocompatible and biodegradable if desired. [47] 4. Polymers can be designed to be solution processable which allows them to be printed and patterned over large areas. [48][49][50][51][52] However, the major challenge faced in developing such polymers lies in maintaining good electrical and mechanical properties simultaneously. Due to the extended π-conjugation of the polymer backbone that is vital for good electronic properties, semiconducting polymers are often rigid and semicrystalline. For TFT application, semiconductor design usually aims to achieve highly crystalline morphology to obtain high charge carrier mobility.Conjugated polymers have evolved significantly in the past decade and have proven to be more than poorly conducting plastics. Instead, improved understanding has resulted in respectable charge-carrier mobilities and power-conversion efficiencies achieved by various donor-acceptor-type semiconducting polymers. However, their advantages in mechanical flexibility and deformability seem to have conflicting molecular design requirements from those for high charge-carrier transporting properties. It is therefore a challenge to enhance the mechanical compliance of semiconducting polymers suitable for stretchable device applications. This progress report starts with a brief introduction to fracture mechanics and mechanical characterization techniques for thin polymer films, in order to consider the limitations and rationalization of current definition and parameters for stretchability. It t...

The cross-linking of conjugated polymers has been demonstrated to be an effective strategy to improve its elastic properties to give deformable semiconductors for plastic electronics. While there have been extensive studies of the structural requirements of the polymer host for good film ductility, no work to date has focused on the relevance of the structural design or chemistry of these cross-linker additives. In this study, urethane groups and tertiary carbon atoms are inserted into the alkyl backbone of perfluorophenyl azide-based cross-linkers to investigate the importance of cross-linker crystallinity with respect to polymer morphology and hence mechanical and electrical properties. Linear cross-linkers with hydrogen bonding from urethane groups readily phase separate and recrystallize in the polymer network to form cross-linked domains that obstruct the strain distribution of the polymer film. Branch cross-linkers with tertiary carbon on the other hand form an evenly cross-linked network in the polymer blend stemming from excellent miscibility and show a 4-fold increase in fracture strain. Furthermore, a stable hole mobility of 0.2 cm2 V–1 s–1 is achieved up to ε = 100%, and a stable hole mobility of 0.1 cm2 V–1 s–1 after 2000 cycles of ε = 25% on fully stretchable organic field-effect transistors.

The ultimate control over chain self-assembly is key to unravel and optimize the relationship between film microstructure and charge carrier mobility in solution processable conjugated polymer semiconductors. Here we employ preparatory size exclusion chromatography to produce fractions of a poly(thieno)thiophene polymer, coded PBTTT-C12, with varying number-average molecular weight, M n, from 5.8 to 151 kDa and low polydispersity index of 1.1–1.4. Solution processing of these samples into bottom-contact, bottom gate, field effect transistors reveals a strong dependence of transistor performance on the molecular weight. Further analysis of the films’ microstructure and crystallinity show three distinct regions: fiber formation (ca. 5–20 kDa), terrace formation (20–50 kDa), and a rough morphology (50–150 kDa). The performance of low-M n films was found to increase rapidly with increasing chain length, and while the best transistor performance was found with the terrace morphology, films not exhibiting the terraced morphology (using 80 kDa polymer) were capable of similar performance. In addition, by blending only 5 wt % of a high molecular weight fraction into a low-M n film, we demonstrate the ability to drastically increase the measured charge carrier mobility of the low-M n material without attaining a terraced morphology. This illustration suggests a viable route to easily increase the processability and transistor performance of low molecular weight conjugated polymeric or oligomeric semiconductors. In addition, GIXRD and thermal analysis of select fractions further indicate that the films of higher molecular weight exhibit a reduced side-chain crystallinity due to chain entanglement; the degree of backbone crystallinity remains more constant.

Diketopyrrolopyrrole (DPP) based donor-acceptor conjugated polymers, with increasing amount of weak H-bonding units, namely 2,6-pyridine dicarboxamide (PDCA), inserted as end groups in alkyl sidechains were prepared and investigated. In contrast to previously reported DPP polymers containing PDCA units as conjugation breakers along the polymer backbone, PDCA in the alkyl sidechains readily produced almost quantitative formation of intermolecular H-bonding even at low PDCA unit content (< 10 mol%) as shown by Fourier-transform infrared spectroscopy (FTIR). The efficient intermolecular H-bonding was further supported by the appearance of a pronounced vibronic shoulder in the UV-Vis spectra and a reduction of interlamellar spacing (from 24.02 Å to 22.87 Å) compared to the neat DPP polymer. Increasing mol% of PDCA units in sidechains of DPP conjugated polymers also has a clear effect on the thermal and mechanical properties of the films as investigated by Dynamic Mechanical Analysis (DMA). Polymers with a high loading of PDCA showed a linear increase in both tan delta intensity and temperature at which softening of film crosslinking occurs. In particular, at a comparable mol%, polymers with PDCA units along the conjugated backbone showed a lower transition intensity and on average a 10 °C to 20 °C higher temperature required for H-bonding breaking. FTIR coupled with crack onset measurements showed that H-bonding breaking during tensile deformation happens only at strains close to crack onset. All these observations suggest that molecular engineering of conjugated polymers bearing H-bonding units has a strong influence on microstructure, thermal and mechanical properties of solution processed films and final energy dissipation mechanisms in stretchable electronics applications as well.