. Inspired by daily hand-writing, innovative and ubiquitously available pens can be employed to write conductive patterns on multiple substrates; such a low-cost, fast, and user-friendly direct writing paradigm has recently aroused remarkable research interest as a promising electronics prototyping strategy. In this review, state-of-art advances in techniques for direct writing of electronics are presented, and pros and cons of each fabrication route are discussed. Emerging applications of pen-based writing electronics are also summarized. Based on these, fi nal conclusions, limitations and challenges, as well as ongoing perspectives are illustrated. a polyester-based tattoo-like integrated epidermal electronic system has been developed that can perform real-time muscle movement, heart activities, and brain-wave detection without the engagement of expensive and bulky medical instrumentations, holding promises to empower doctors to timely monitor vital signs of patients in a simple and effi cient way. [ 3 ] For these applications, the ideal electronics should be low-cost, easily accessible, high-performance, rapid prototyping, and user friendly etc. Consequently, increasing attention has been devoted to discovering several factors closely related to fl exible electronics (e.g., substrates, conductive materials, and manufacturing techniques) for fabricating electronics with increased performance and decreased cost. [ 7 ] Among them, the development of fast and cost-effective fabrication strategies has drawn close attentions for realizing lowcost fl exible electronics. [8][9][10] Recently, various techniques have been developed to fabricate fl exible electronics, such as coating, [ 11,12 ] sputtering, [ 13 ] and printing techniques (e.g., gravure printing, [ 14 ] fl exo printing, [ 15 ] offset printing, [ 16 ] inkjet printing, [ 17 ] and screen printing [ 18 ] ). Specifi cally, printing techniques enable rapid prototyping of components and devices with accurate three-dimensional (3D) structures, thus facilitating achievement of functional 3D electronics. [19][20][21] Based on a newly developed embedded 3D printing system, the fabrication of strain and pressure sensors has been demonstrated to be viable within highly stretchable silicon polymer substrates, exhibiting huge potential for the production of fl exible electronics and wearable devices with arbitrary constructs and unique functions. [ 6 ] However, these sophisticated techniques may not live up to the emerging demand of low cost, easy accessibility and rapid prototyping for fl exible electronics fabrication, since the long processing period of coating, the high energy consumption of sputtering, and the inevitable enrollment of expensive professional equipment of printing are all intractable issues that can hardly be overlooked. [ 22,23 ] As an alternative technique and inspired from day-to-day hand writing, various writing instruments, such as brush pen, [ 22,24,25 ] pencil, [26][27][28][29] fountain pen, [ 30,31 ] and ball pen, [32][33][34][35] have been re...
