Glass has proven to be one of the most inevitable materials for the fast-emerging field that involves fabrication of micromanipulation smart devices.

Why glass and what are the possibilities?

It has a desirable optical and good isolation property [1], [2], which in addition to their biocompatibility, chemical stability, hydrophilicity, and optical transparency, makes them suited for various applications ranging from Optical/Bio-MEMS, micro-actuators, micro-sensors, and the Lab-on-chip device. The development of microfabrication in glass techniques over the years enabled different microcomponents and applications ranging from microfluidic (microchannels) [3], [4], [5], [6], optical (optical alignment, waveguide, and positioners) [7], [8], [9], [10], mechanical parts (nozzles, gears) [11], [12], [13], [14], micro-actuators [15], [16], [17], [18], [19], [20], [17], [21] and sensors [22], [23], [24], [25], [26], [27], [28], [29], [30], [31].

Figure 1. Different 3D miniature devices and monolithic microstructure fabricated in glass A) Volumetric 3D printing of silica glass with microscale computed axial lithography to fabricate a 3D transparent and complex microfluidic structure, having trusses and lattice with minimum feature sizes of 50µm, [32], B) Microfluidic device using laser micro-welding process in which the two glass plates are permanently bonded together without using any adhesives nor intermediate layer [33], C) SEM image of a 500nm diameter silica micro/nanofibre (MNF) tied into a ring and placed on a 60µm diameter human hair, with the MNF often used for optical sensors [34], D) Intertwined microfluidic spiral channels in fused silica glass with a channel width of 74µm, which is filled with dyes (scale: 140µm) [34], E) Quartz glass chip for cell sorting application with a dimension of 34mm × 12 mm × 2mm, fabricated by selective laser-induced etching [35], F) Top and side view of a nested nozzle in quartz glass for biological application (diameter: 10mm, height: 7mm), fabricated by selective laser-induced etching [35], G) Quartz glass connector for capillary electrophoresis, diameter 15mm, thickness 2mm, fabricated by selective laser-induced etching [35] H) Transparent suspended microchannel resonator (SMR) in fused silica with fluidic channels with a cross-section around 10µm x 5µm flowing underneath [36], I) Monolithic 3D micromixer with an impeller for glass microfluidic systems using selective laser-induced etching [37], J) Microfluidic mixer with 5 inlets and 1 outlet (channels diameter of 100µm) [38], K) Passive compliant tool for retinal vein cannulation (RVC) that relies on a buckling mechanical principle [39], L) 3D complex lab-on-a-chip (smallest channel diameter of 3µm) [38], M) Optically transparent glass micro-actuator fabricated by femtosecond laser exposure and chemical etching (missing reference), N) 3D microfluidic channel fabricated by using selective laser-assisted etching [40].

Figure 1 presents some of the 3D miniaturized and monolithic devices in the literature that are fabricated with glass using different approaches. Figure 1A show the possibility of 3D volumetric additive manufacturing of silica glass with microscale computed axial lithography for microstructures with minimum feature sizes of 50µm. It is also possible to use a maskless approach to fabricate in glass by laser micro-welding which usually involves non-adhesive bonding of two pre-printed surfaces (see Fig. 1B) The case of 500 nm diameter silica micro/nanofibre for optical sensors, which is fabricated by taper-drawing glass fiber at high temperature is presented in Fig. 1C. It is also possible to fabricate arbitrary 3D suspended hollow microstructures in transparent fused silica glass using stereolithography as shown in Fig. 1D. Using selective laser-induced etching, this presents the most predominant approach which is used from Fig. 1E to Fig. 1N, for designing different complex 3D monolithic micro-structures in a glass.

Figure 2. {Illustration of glass beam bending experiments undergoing several stress deformation for loading and unloading without breaking~ (see video). The beam is characterized by a 40µm thick flexure in its thinnest part, Image~\copyright~2011 Optical Society of America [41].

While conventional glass at the macro scale is rigid and brittle, conversely, at the micro-scale level, thin glass is flexible with high tensile strength [42], [43]. These inherent properties of glass on a micro-scale are beneficial in the design of miniaturized 3D structures. Therefore, one can take advantage of glass at the micro-scale to design and fabricate miniaturized flexible microstructure [34]. Consequently, this solidifies the possibility of designing and fabricating a continuum robot using thin glass. Moreover, for CTRs, higher dexterity in a tightly confined region requires a small radius of curvature [44]. For this reason, we ask the question of whether tubes made of glass, with a sub-millimeter diameter can permit a small radius of curvature without breaking. The proposed glass for the robot design is fused silica. Its surface stress can significantly be reduced by using a protective polymer coating, which is common in optics fiber to increase its mechanical rigidity. In addition, its mechanical strength can be further improved by minimizing the flaws in glass and enabling a low surface area because a small size limits the risk of the presence of flaws [45], [46], [41]. All these factors guarantee the ability of capillary glass to withstand high bending stress (see Fig. 2}). It also allows the capillary glass to have flexibility similar to the spring steel. Fused silica has a non-linear elastic property and the applied strain determines the elastic modulus [47],[48], with the maximum bend radius of curvature, deduced by considering tensile strength in the equation below.

Figure 3. The relationship between the glass capillary bending stress and the obtainable bend radius of curvature for the glass diameters.

\(\sigma\) is the surface stress, \(E_o\) is Young’s modulus at zero strain (70GPa), \(r_c\) is the capillary radius, and \(r\) is the bending radius of curvature. \(\alpha=2.30\) and \(\beta=8.48\) are the second-order and third-order nonlinear material coefficients, respectively.

For the proposed miniaturized CTR, when considering the glass tube diameters used, the relationship between their bending stress and the obtainable radius of curvature, as derived using the equation above is presented in Fig. 3. % Although theoretically, the ultimate tensile strength of fused silica can reach 4.83GPa (green dash line), we considered nominal bending stress of 0.69GPa (black dash line); this consideration is in line with the Polymicro proof test Polymicro Technologies. The figure explains why it was possible to obtain a small bend radius of curvature in glass down to 5mm with tube diameter below 440µm (full detail discussed next), which is very flexible to sustain bending stress below nominal value without fracture. This is highlighted and considered as our target area in Fig. 3. Considering its ultimate tensile strength, the figure indicates that it can withstand more stress in cases of path contact during deployment and manipulation. % For both glass and Nitinol, the precurvature limits, or the obtainable minimum radius of curvature, depend on the available tube diameter; a smaller tube diameter guarantees a smaller radius of curvature without plastic deformation. Fig. 3 also demonstrated that it is theoretically possible for a 1mm glass tube to sustain bending stress over a 10mm radius of curvature like that of Nitinol, which has a minimum radius of curvature of 15 mm in literature [49]. The various approach explored to obtain a pre-curved tube using thin glass for the proposed miniaturized concentric tube robot (CTR) is discussed in detail in the subsequent blog post. Whereas for the parallel continuum robot (PCR), a standard optical fiber was used directly for the robot modeling, design, and experimental prototype, which is detailed in another blog post below. Overall, we demonstrated the possibility to actualize the first of its type, a miniaturized continuum robot using glass material. This landmark achievement and scientific breakthrough has huge prospects in the robotics and material domain as regards microsurgery or micromanipulations.

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