Flexible electronics is an emerging technology with an exciting array of applications, ranging from paper-like displays, skin-like smart prosthesis, organic light emitting diodes (OLEDs), to printable solar cells. These potential applications will profoundly impact various facets of our daily life, and excite our curiosity on: what's the future of newspapers and books? Will OLEDs replace light bulbs and fluorescent lamps, and emerge as future lighting source? Can we power electronic devices everywhere cordlessly? Significant progress has been made in the past several years, especially as sizable investments flux in. For example, Polymer Vision just released the first commercial product of rollable display (as shown in the figure) after secured $26M investment in January 2007. The future success of this emerging technology largely relies on:

• New architecture design and new materials choice, to enable the functionality and improve the reliability of flexible devices.
• Revolutionary fabrication process, to reduce the manufacturing cost for massive production of flexible devices.
  

New architecture design and new materials choice
The electronic materials used in the current microelectronics technology (e.g., Si, SiO2, Cu) are inorganic. These inorganic materials are excellent in electrical performance, but are poor in mechanical deformability. The discovery of conductive and semiconductive polymers provokes the enthusiasm to build flexible devices entirely out of organic materials. So far, however, the performance of such organic electronics are still unsatisfactory. For example, the best available conductive polymer is still two orders of magnitude less conductive than typical metals. Therefore, a suitable solution for flexible electronics will be the organic/inorganic hybrid architecture. For example, thin films of indium tin oxide (ITO ) deposited on polyethyleneterephthalate (PET) are commonly used as transparent conductors in flexible display design.
Flexible electronic devices will be subject to large, repeated deformation during manufacturing and use (for example, a cell phone with rollable screen). While organic materials are compliant, and can recover from large strains, most inorganic electronic materials are stiff, and fracture at small strains (typically < 1%). How to use these materials to make electronic devices with reliable deformability under cyclic loadings remains uncertain. Furthermore, the organic/inorganic hybrids exhibit rich mechanical responses under loading, many of which have not been well understood.

Revolutionary fabrication process
Current IC manufacturing is a batch process: one component at a time. Many of the fabrication steps involve the use of chambers in the billion dollar fab facility. Such a manufacture process is not suitable for making flexible electronics. For example, the processing temperature in current fabrication steps is often too high for organic materials; the size of the resulting device is limited by the chamber size, while flexible electronics, such as thin film solar cells, require the distribution of electrical components over large area. Therefore, novel fabrication processes are desirable to manufacture rugged, large area, and flexible electronic devices in a cost-effective and time-efficient way. There has been a surge of interest in developing a roll-to-roll process in which multiple functional layers of inorganic electronic materials are patterned and printed on a plastic substrate, resulting in lightweight, rugged and low-cost devices. Still, many issues need to be addressed, such as layer-to-layer registration, damage due to handling, and adhesion quality. Scientists are also exploring other innovative processes to fabricate flexible electronics through direct growth, or self assembly.

Promising opportunities and great challenges co-exist for the flexible electronics technology. Many of such challenges find their origins in the mechanical response of new architecture made of hybrid materials. More opportunities will emerges as the understanding of such mechanical response advances. The March issue of jClub includes three papers to reflect various aspects of the challenges and opportunities in the emerging field of the mechanics of flexible electronics. The three papers under discussion are:

1. Electromechanical properties of transparent conducting substrates for flexible electronic displays, Cairns, D.R. and Crawford, G.P. Proc. IEEE, 93, 8, 1451- 1458 (2005)

The paper starts with a nice brief introduction to the flexible display technology and then focuses on the electromechanical properties of the flexible anodes (e.g., ITO-coated PET) in flexible displays. The thin coatings of ITO (~100 nm thick) are brittle and crack at a tensile strain of ~2.3%. Of particular interest is Section V of the paper. Under cyclic loading, even when the strain is much lower than the ITO virgin cracking threshold , the brittle ITO films on PET substrates show fatigue fracture behavior. For example, SEM images after 100K cycles clearly show the fatigue cracks in the ITO films (Fig. 8). An open question worth of discussion is that, what is the deformation mechanism of the fatigue of a brittle ITO film on a compliant PET substrate?

2. Calculation of adhesive and cohesive fracture toughness of a thin brittle coating on a polymer substrate, N.E. Jansson, Y. Leterrier, L. Medico and J.-A.E. Manson, Thin Solid Films, 515, 4, 2097-2105 (2006)

A typical inorganic/organic hybrid structure in flexible electronics often consists of a thin film of inorganic materials (e.g. SiNx) on a relatively thick organic substrate (e.g., polymers). The thin film fracture toughness as well as the film/substrate interfacial adhesion are important properties that govern the durability of the film (often the functional layer in a device) under mechanical loads. Determination of these fracture parameters for thin films with a sub-micron thickness is rather challenging for both experiments and modeling. Often the fracture of the film and the interfacial debonding co-evolve during the deformation. The polymer substrate deforms plastically under large deformation, an important effect on the fracture process that is not well studied. In this paper, the fragmentation test and finite element method are combined to simultaneously derive both the adhesive fracture toughness of the interface between a thin brittle coating and a polymer substrate and the cohesive fracture toughness of the thin film coating. A cohesive zone model is used to simulated the debonding process. The approach adopted in this paper may be of interest of many fellow iMechanicians for further discussion.

3. Self-assembled single-crystal silicon circuits on plastic, Stauth, S.A., Parviz, B.A., Proc. Nat. Acad. Sci., 103, 38, 13922 -13927 (2006)

The authors demonstrate a new way to fabricate circuits on a plastic substrate by self-assembly. Thousands of single-crystal silicon transistors and resistors are integrated onto flexible plastic substrates. The micron-scale components self-assemble onto etched channels in the plastic substrates to form circuits. The assembly process is driven by the capillary forces and controlled by using differently shaped components, including circles, triangles, squares, and rectangles, that selectively assemble in matching substrate channels. A mechanics model is set up to examine the role of both capillary and fluidic forces during the self-assembly. The possible interests of discussion can be either the optimization of materials properties and experimental designs to improve the efficiency of this specific assembly process, or other potential fabrication processes enabled by self-assembly.

While some discussion topics are proposed above, we welcome discussions on any aspects of these papers, or the general field of the mechanics of flexible electronics.

(Image credit: Polymer Vision)