# Towards a Glass New World: The Role of Ion-Exchange in Modern Technology
## 4 Perspectives & Future Insights
### All-Glass Flexible Photonics  
As happened for electronics, flexibility is also becoming a fundamental paradigm in photonics, favouring the integration of the respective devices in flexible and stretchable substrates and contributing to expand their application scope, from the field of optical interconnections to that of new systems for the continuous monitoring of patient health, or that of implantable devices for minimally invasive medicine, up to the development of a new class of optical sensors and sources [175; 176; 177]. Figure 8: (**a**) Mechanism for the formation of silver nanoparticles in a double ion-exchange using an aluminium thin film as electrode; (**b**) distribution of silver nanoparticles, under the sample surface, following the process (SEM image of the sample cross section near to the surface). Reprinted with permission from [159] © The Optical Society. Polymers are certainly the main substrates for the development of this novel technology thanks to their intrinsic mechanical flexibility. In order to give some examples, it is worth listing the possibility of making silicon photonic devices on plastic substrates by means of low temperature approaches (e.g., transfer-and-bond technique, direct fabrication process flow) [180; 181]; the deposition of glass-based 1D photonic crystals and erbium activated planar waveguides by radio frequency (RF) sputtering on flexible polymeric substrates [182]; the realisation of stretchable platforms in which single-mode photonic devices, made from high-index chalcogenide glasses and low-index epoxy polymers, are monolithically integrated on polydimethylsiloxane (PDMS) elastomer substrate. In the latter case, local substrate stiffening and spiral based waveguides provide protection to functional photonic components and interconnections with very low radiative optical losses, respectively [183; 184]. Polymeric materials, however, exhibit various drawbacks such as a limited range of refractive indices, not negligible optical absorption in the near-infrared region, low process temperature and poor chemical stability. All these factors generally represent serious side effects against their integration and processing with other materials. Conversely, glass, with a wide range of refractive indices, high transparency from visible to infrared, excellent chemical and thermal stability, as well as low permeation rates for ambient gases and water, may represent a valid alternative as flexible platforms for the realisation of photonic devices provided that their brittleness is somehow mitigated [185]. In this context, [PERSON] et al. demonstrated the possibility to obtain single mode, low-loss (<0.15 dB/cm), laser-written channel waveguides in flexible chalcogenide glass tape [186]. Nevertheless, among all the glass formulations that can be used for the purpose of flexible photonics, borosilicate glass represents the most widely used material due to its intrinsic characteristics of mechanical strength. In particular, alkali-free commercial borosilicate glasses with extremely thin thicknesses (<100 um) are currently made available by major glass producers in the format of sheets or rolls. This is the case of Willow(r) glass from Corning and AF32(r) from Schott just to mention a few [187; 188]. The flexibility of these glasses is conferred through particular manufacturing processes--such as down-draw or roll-to-roll (R2R) techniques--capable of guaranteeing ultra-thin thicknesses to the finished product with an extremely high surface quality, similar to that of a pristine glass [189; 190]. As a white sheet of paper waiting to receive the characters from the pen of an inspired writer, so Willow glass with different thickness can be used as a substrate for the realisation of photonic circuits as demonstrated by [PERSON] and co-workers. In this case, the writing instrument is represented by a femtosecond laser capable of locally modifying, in the irradiated area, the refractive index of the pristine glass [191].