METHOD FOR PRODUCING AN ELASTIC AND FLEXIBLE FIBER WITH OPTICAL, ELECTRICAL OR MICROFLUIDIC FUNCTIONALITY

Abstract

The invention relates to a method for manufacturing an elastic and flexible fiber with a pre-de-termined non-circular cross-sectional geometry, the method comprising extrusion of an elasto-mer from a nozzle onto a substrate, wherein the pre-determined non-circular cross-sectional geometry of the fiber is determined by the height and velocity of the nozzle relative to the sub-strate. The invention relates to an elastic and flexible fiber produced using the method, wherein the fiber comprises an elongated indentation along a length of the fiber (groove). The invention relates to methods for producing preferably biocompatible microfluidic, electrically conducting or light-guiding fibers using the methods of the invention. The invention further re-lates to elastic and flexible fibers produced by the method.

Claims

1. Method for manufacturing an elastic and flexible fiber with a pre-determined non-circular cross-sectional geometry, the method comprising extrusion of an elastomer from a nozzle onto a substrate, wherein the pre-determined non-circular cross-sectional geometry of the fiber is determined by the height and velocity of the nozzle relative to the substrate, and wherein the fiber comprises an elongated indentation along a length of the fiber.

2. Method according to preceding claim, wherein the elastomer is extruded onto the substrate as a continuous filament by a (preferably automated) translational relative motion of the nozzle relative to the substrate, followed by a hardening of the elastomer after extrusion to produce an elastic fiber.

3. Method according to any one of the preceding claims, wherein the fiber has a maximum cross-sectional width of 10-2000 μm, preferably wherein the fiber has a maximum cross-sectional width of 50-1500 μm, more preferably about 300-1000 μm.

4. Method according to any one of the preceding claims, wherein the nozzle is essentially circular in cross-section, and preferably has a smallest internal diameter of 10-500 μm, preferably 100-300 μm, more preferably about 150-250 μm and preferably has a smallest outer diameter, greater than the internal diameter, of 20-1000 μm, preferably 100-700 μm, more preferably about 300-500 μm.

5. Method according to any one of the preceding claims, wherein the elastomer is an extrudable elastomer, allowing the elastomer to flow through the nozzle when pressure is applied and to harden into an elastic and flexible form after the elastomer is deposited on the substrate.

6. Method according to any one of the preceding claims, wherein the elastomer has a shear rate dependent viscosity and/or is a thermoplastic elastomer.

7. Method according to any one of the preceding claims, wherein the elastomer is selected from the group consisting of silicone rubber (such as polydimethylsiloxane), a (preferably biocompatible) viscoelastic polymer, polyurethane rubber, a hydrogel or microgel (such as based on polyacrylic acid), colloidal suspension (such as containing silicate particles), a polymer precursor and/or a melt (such as wax).

8. Method according to any one of the preceding claims, wherein: a. the translational speed of the nozzle relative to the substrate is slower than the extrusion speed, with which the elastomer leaves the nozzle, and/or b. the distance between the nozzle and the substrate is less than the inner diameter of the nozzle.

9. Method according to any one of the preceding claims, wherein the fiber has an elliptical cross-sectional geometry or is a flattened cylinder (ribbon), preferably wherein the ratio of width to height of the cross-sectional geometry of the fiber is 1.5 or more, preferably 2 or more.

10. Method according to any one of the preceding claims, wherein: determining the velocity of the nozzle relative to the substrate comprises setting the velocity V of the nozzle according to Equation 1: $V=\frac{v}{c}$ wherein v is the translational speed of the nozzle relative to the substrate and c is the extrusion speed, with which the elastomer leaves the nozzle, and/or wherein determining the height of the nozzle relative to the substrate comprises setting the height H of the nozzle according to Equation 2: $H=\frac{h}{\alpha \mathrm{din}}$ wherein h is a distance between the nozzle and the substrate, din is an inner diameter of the nozzle and α is 1 or a die-swelling factor that determines a post-extrusion expansion of the ink, wherein: when both V and H are more than about 1, the fiber has an essentially circular cross-sectional geometry, and when V and/or H are about 1 or less, the fiber has a non-circular cross-sectional geometry, wherein the elliptical or flattened cylinder (ribbon) form is obtained by setting V and H according to Equation 3: $V<\frac{1}{H^2}$ or wherein the elongated indentation along a length of the fiber (groove) is obtained by setting V and H according to Equation 4: $V<\frac{\pi }{4}\frac{din}{\mathrm{dout}}\frac{1}{H}$ wherein din is an inner diameter of the nozzle, dout is an outer diameter of the nozzle.

11. Method according to any one of the preceding claims, wherein a nozzle or object of essentially the same dimensions is passed over the fiber after the elastomer is deposited on the substrate, and preferably before elastomer hardening, thereby removing elastomer (e.g. by engraving and/or suctioning) and producing an elongated indentation along a length of the fiber (groove).

12. Method according to any one of the preceding claims, wherein said elongated indentation subsequently closes at the outer edge of the cross-sectional geometry of the fiber to form an elongated (microfluidic) lumen inside the fiber.

13. Method according to any one of the preceding claims, wherein the fiber comprises an elongated indentation along a length of the fiber (groove), and an elastic, and preferably flexible, electrically conductive material is positioned in the elongated indentation, followed by sealing said elongated indentation by depositing additional elastomer onto the fiber, preferably using the method of any one of the preceding claims, thereby sealing said electrically conductive material inside the fiber.

14. Method according to any one of the preceding claims, wherein the fiber comprises an elongated indentation along a length of the fiber (groove) and an elastic, and preferably flexible, light guiding material is positioned in the elongated indentation, followed by sealing said elongated indentation by depositing additional elastomer onto the fiber, preferably using the method of any one of the preceding claims, thereby sealing said light guiding material inside the fiber.

15. Elastic fiber with a non-circular cross-sectional geometry, produced using the method of any one of the preceding claims.

16. Elastic and flexible extruded fiber with a non-circular cross-sectional geometry and a maximum cross-sectional width of 10-2000 μm, preferably produced using the method of any one of the preceding claims, wherein the fiber comprises: a. a base element comprising an elongated indentation along a length of the base element (groove), wherein the base element is obtained by extruding an elastomer from a nozzle onto a substrate thereby forming an elongated indentation, and/or optional subsequent engraving of the extruded base element to form an elongated indentation, and b. at least one sealing element bound to the base element, wherein the sealing element is obtained by extruding an elastomer from a nozzle onto the base element, wherein the sealing element is positioned to form a sealed elongated lumen along a length of the fiber between the elongated indentation of the base element and the sealing element, wherein the lumen optionally comprises an elastic and flexible electrically conductive material or an elastic and flexible light guiding material.

17. Use of the elastic fiber according to any one of the preceding claims as: an optical fiber, when a light guiding material is sealed inside the fiber, produced according to claim 14, a microfluidic channel, when an elongated (microfluidic) lumen is present inside the fiber, preferably produced according to claim 12, a pneumatic actuator, when an elongated lumen is present inside the fiber, preferably produced according to claim 12, and pressure can be applied in the lumen, or a thermal exchange device, when an elongated lumen present inside the fiber, preferably produced according to claim 12, is used to circulate a fluid with temperature different from that of the surroundings, an electrical interconnect, when an electrically conductive material is present inside the fiber, produced according to claim 13, A resistive strain sensor, when an electrically conductive material is present inside the fiber, produced according to claim 13.

Description

FIGURES

[0136] The invention is further described by the following figures. There are intended to represent a more detailed illustration of a number of preferred non-limiting embodiments or aspects of the invention without limiting the scope of the invention described herein.

[0137] FIG. 1: Nozzle speed and height control filament geometry. (a) Schematic illustration of a typical DIW set-up. Key parameters that influence the extruded filament are the nozzle's inner diameter d.sub.in, its height above the substrate h, and its translational speed. (b) As determined by oscillating plate rheometry, the two model inks SE1700 and Carbopol investigated here exhibit shear rate dependent viscosity. (c) Catalogue of filament cross sections produced by varying the speed and height of the print nozzle. Bottom row: optical micrographs of filaments printed with SE 1700 and with Carbopol. Filaments can be classified as ellipse (i), ribbon (ii), groove (iii) and discontinuous (iv). (d) Phase diagram of transitions between the cross-section geometries obtained with the SE1700 material. Data points in square boxes indicate the parameters used to produce the examples in (c). Filaments are identified as ribbon when the ratio between the width and height of cross sections is larger than 2. The shaded area indicates the parameter space for which Equation (1) predicts groove filaments.

[0138] FIG. 2: Post-extrusion modification of filament cross sections. Optical micrographs depicting the use of the printing nozzle as a stylus. Depending on the height of the stylus above the substrate, cross-sectional geometries including grooves and channels can be produced with the SE1700 silicone.

[0139] FIG. 3: Direct writing of functional elastic fibers. (a) Micrographs of an optical fiber produced by combining filaments with groove and ribbon cross sections. The optical core is formed by a high refractive index silicone. The optical fiber is coupled to a white light source. (b) Representative optical fiber subjected to tensile strain. The attenuation ratio P/P.sub.0, is the ratio between transmitted power during stretch and at rest as measured at the fiber's free end. (c) Micrographs of an electrical interconnect fiber. Here the groove is filled with platinum powder with average particle size of approximately 1 μm. (d) Representative interconnect under tensile strain. Here R/Ro is the ratio between the interconnect resistance at stretch and at rest. (e) Micrographs of freeform microfluidics. As demonstrated by the infusion of a blue food dye, the channel lumen remains patent even at sharp turns of the fiber.

[0140] FIG. 4: Integration of multi-modal fibers by DIW. (a) Cross section of a pneumatically actuated optical fiber illustrating the off-axis position of the microfluidic channel. (b) Front (upper panels) and side (bottom panels) views of the actuator at rest and during the application of compressed air. (c) Quantification of tip deflection as a function of applied air pressure.

[0141] FIG. 5: Demonstration of thermal actuation and sensing using printed fibers and a packaged sensor. (a) A flower-shaped microchannel loop circulates thermal exchange liquid (ethanol). In the middle of the loop is a digital temperature sensor which is linked to printed electrical interconnects by conductive epoxy (Epo-Tek H27D). The sensor-actuator system conforms to the surface of a dome-shaped gelatin model of brain tissue. (b) Thermal response of the model brain tissue to the circulation of chilled ethanol.

[0142] FIG. 6: Schematic overview of various fields of application of the fibers described herein.

[0143] FIG. 7: Determining die-swelling factor and extrusion speed for SE1700 silicone. (a) Use of the dimensionless height and speed requires direct measurement of two parameters, C (ink extrusion speed) and a (die-swelling factor), which are specific to the ink, printing nozzle and applied pneumatic pressure. First, we print a section of filament in the stable coiling regime (upper panel) which ensures the filament is not stretched by the translational motion of the nozzle. The extrusion speed C is calculated by dividing the total length of the printed filament l, by the amount of time that the nozzle was extruding material Δt. The air pressure in the nozzle is set at 5 bar, which is maintained in subsequent experiments. The die-swelling factor α, is calculated by dividing the cross-section diameter of the filament by the inner diameter of the nozzle d.sub.in. Using a nozzle with inner diameter of 210 μm, we obtain a value for the die-swelling factor of 1.19. (b) For inks with a “pot-life” the extrusion speed C can change as a function of polymerization time. We quantify this for the printable silicone elastomer SE1700 (Dow Corning) over an 8-hour period. For extended printing runs, changes in extrusion speed should be taken into account when calculating the dimensionless parameter V.

[0144] FIG. 8: Phase diagram obtained with Carbopol hydrogel. The transitions between the cross-section geometries for the Carbopol (at 1.2 bar) ink closely follow those observed for SE1700. Filaments are identified as ribbon when the ratio between the width and height of cross sections is larger than 2. The shaded area indicates the parameter space for which Equation (1) predicts groove filaments.

[0145] FIG. 9: Transitions between ribbon, ellipse and circular cross sections. At a constant nozzle height H*, the cross-sectional geometry of the printed filament can be tuned by varying the nozzle speed V*. The transition from ribbon to ellipse to circular cross-sections is gradual. We assign the ribbon geometry for cross sections preferably where width/height >2. Ellipses have 1<width/height <2 and circular cross sections are characterized by width/height=1.

[0146] FIG. 10: White light attenuation in printed optical fibers. To calculate the attenuation in optical fibers, we use the cut-back method. The length of the fiber is reduced in steps. The optical power detected at the free end of the fiber is measured at each step. Attenuation in the fiber is thus calculated according to 10 log.sub.10(P/P.sub.0)/Δ/in units of dB/cm.

[0147] FIG. 11: Laminar flow inside printed channels. To investigate how fluid flows inside printed channels, we inject two solutions of food dye of different colors at one end of a channel at a flow rate of 0.1 ml/min for each solution. In optical micrographs, we observe that the two colors do not mix as they flow. This demonstration of laminar flow is taken as an indication of good flow homogeneity inside SE1700 silicone printed channels.

[0148] FIG. 12: Mechanical properties of functional fibers. (a) Up to at least 30% tensile strain, printed fibers exhibit close to linear elastic response. Stress-strain curves are obtained at elongation rate of 0.1 mms.sup.−1. (b) Elastic moduli of the three types of fiber (extracted from (a)) compared to the values for the silicones they are made from.

[0149] FIG. 13: Schematic overview of a selection of preferred cross-sectional geometries of the fibers of the invention. Schematic 1 shows an elliptical shape. In some embodiments, a “lower” or bottom surface of the fiber is flattened due to the flat surface of the substrate employed. Depending on substrate shape, the surface may also form other shapes. The elliptical shape is therefore primarily elliptical in the upper region opposed from the substrate. Schematic 2 shows a ribbon or flattened cylinder shape. Schematic 3 shows an elongated indentation (groove) 10 in the fibers produced in a single pass of the nozzle by reducing nozzle height and/or reducing nozzle velocity. Schematic 4 shows an elongated indentation (groove) in the fibers produced by a second pass of the nozzle, thereby engraving an elongated indentation (groove) or engraved channel 12. Of note, due to “engraving”, the second pass of the nozzle typically results in an indentation with a sharper corner to the upper edge of the groove (also referred to as an “apex”) 11. This sharp or sharper corner or apex is typically sharper than the rounded edge 14 of the groove produced by a single pass of the nozzle, as shown in in 3. Schematic 5 shows an elongated indentation (groove) in the fibers produced by a second pass of the nozzle, whereby the groove has closed to form a lumen 13 or microfluidic channel, whereby the apexes formed by the engravement have “fallen” together to form a sealed lumen. Schematic 6 shows a fiber with an elongated indentation (groove) 10 produced in a single pass of the nozzle, which is considered here a base element 16 and additionally a sealing element 15 extruded over the groove, thereby sealing the groove to form a lumen 13. Schematic 7 shows a fiber with an elongated indentation (groove) produced in a second pass of the nozzle by engraving, which is considered here a base element 16 and additionally a smaller sealing element 17 extruded over the groove, thereby sealing the groove to form a lumen 13. By producing a deeper groove using engraving a smaller sealing element, fitting essentially within an upper region of the groove, can be applied leading to a generally thinner fiber with respect to total height. Schematic 8 shows a fiber with an elongated indentation (groove) produced in a second pass of the nozzle by engraving, which is considered here a base element 16 and additionally a sealing element 15 extruded over the groove, thereby sealing the groove to form a lumen 13. Schematic 9 shows a fiber with an elongated indentation (groove) produced in a second pass of the nozzle by engraving, which is considered here a base element 16 and additionally a sealing element 15 extruded over the groove, thereby sealing the groove to form a lumen 13. In this case, the sealing element 15 also comprises a lumen. Fibers of this structure can be produced by a first pass of the nozzle to produce an initial fiber or base element 16, a second pass of the nozzle to produce a groove in the base element 16, a third pass to extrude a sealing element 15 over the groove of the base element 16, and a fourth pass of the nozzle in order to produce a groove in the sealing element 15, thereby forming two lumens 13.

EXAMPLES

[0150] The invention is further described by the following examples. These are intended to present support for the workability of a number of preferred non-limiting embodiments or aspects of the invention without limiting the scope of the invention described herein.

Example 1: Height and Velocity Determination

[0151] In standard DIW operations, filaments have close to circular cross sections with diameter αd.sub.in. Here d.sub.in is the inner diameter of the nozzle and α is the die-swelling factor, which describes the post-extrusion expansion of the ink [16]. Stable printing is achieved by setting the translational speed of the print head v to be close to the extrusion speed C, with which ink leaves the nozzle. At the same time, the height of the nozzle above the substrate h is kept similar to αd.sub.in (FIG. 1a). Following Yuk and Zhao, we introduce the dimensionless nozzle speed V*≡v/c and height H*≡h/αd.sub.in and point out that for conventional DIW with circular cross-section filaments, both parameters are adjusted to be close to unity [15]. The extrusion speed C may depend on the ink, nozzle and applied pressure. For inks that age, C can change over time. Values for C and α can be determined experimentally by printing simple test structures (FIG. 7). In some embodiments, V* and H* as described herein correspond to V and H, respectively, as used throughout the description.

[0152] As model inks, we use two different materials: SE1700 (Dow Corning), a printable silicone elastomer in the polydimethylsiloxane family and Carbopol (Lubrizol), a microgel based on cross linked polyacrylic acid [17]. Both inks exhibit shear rate dependent viscosity (FIG. 1b). This property allows for the ink to flow through the nozzle when pressure is applied and to “set” in the shape of a filament after the ink has exited the nozzle.

[0153] We start by noting that when H* or V* are close to or smaller than unity, non-circular cross section filaments are produced. They can be described as elliptical cylinder, ribbon, groove and discontinuous. The same combination of H* or V* produces a similar effect in both of our model inks (FIG. 1c). To investigate how the interplay between, H* or V* influences which of the cross-sectional geometries is produced, we construct a phase diagram concentrating on the parameter space close to unity (FIG. 1d). For these experiments, we use a nozzle with an outer diameter of 210 μm and 430 μm respectively and SE1700. The phase diagram obtained with Carbopol appears nearly identical and is presented in FIG. 8.

[0154] When H* or V* are less than unity, the ink is squeezed and forced to deform between the nozzle and the substrate, which results in a filament flattened in the shape of a ribbon (FIG. 1d, blue squares). Filaments are identified as ribbon when the ratio between the width and height of cross sections is larger than 1.5, preferably larger than 2. By increasing V* at a constant H* we observe a gradual transition to ellipse and circular cross sections (e.g. FIG. 1d, boxed symbols ii and i and FIG. 9). The transition between the ribbon and groove geometries is sharp and resembles a phase transition. Ribbon filaments become grooves when excess ink fully fills the space under the nozzle and starts to accumulate at the sides. Using an ink conservation argument, the condition for creating the groove filament can be expressed as follows (Equation 5):

[00005]$V^{*}=\frac{\pi }{4}\frac{din}{\alpha \mathrm{dout}}\frac{1}{H^{*}}$

[0155] The phase boundary predicted by Equation (5) is presented as a solid red line in FIG. 1(d) and FIG. 8 and is in good agreement with experimental observations of ribbon versus groove filaments.

Example 2: Post-Extrusion Modification of the Filament Cross Section

[0156] Post-extrusion modification of the filament cross section is demonstrated below. To do this, we use the nozzle as a stylus by setting to zero (no ink leaves the nozzle). Passing the stylus over a freshly printed (not yet polymerized) ribbon/ellipse filament of SE1700 silicone can create a groove in it (FIG. 2). Interestingly, we observed that engraving deep grooves causes collapse of their walls leaving behind a lumen. The lumen persists even after thermal polymerization of the ink.

Example 3: Functional Core-Shell Fibers

[0157] Next, we combine ribbon and groove filaments to form functional core-shell fibers (FIG. 3). As illustrated in FIG. 3(a), grove filaments can aid patterning a functional core material by restricting its spreading to the confines of the grove. Combined with a ribbon filament printed on top, the grove filaments serve as cladding protecting the core. In the case of conductive fibers, the cladding functions as electrical insulation, in the case of optical fibers, it provides a step in refractive index.

Example 3: Optical Fibers

[0158] For optical fibers, we create a core by filling groove filaments with a high refractive index silicone (OE 6520, Dow Corning, RI=1.54 compared to RI=1.44 for SE 1700). The optical core cannot be printed as a continuous filament due to the low viscosity of the optical silicone. Instead, it is dosed (using a printing nozzle) inside the groove where it spreads. A ribbon filament is printed on top, acting as the optical seal. The entire fiber is then polymerized by heating. Optical fibers produced in this way display average attenuation of 0.72±0.06 dBcm.sup.−1 (FIG. 10) for white light, which is similar to other polymer or hydrogel-based waveguides fabricated by molding or soft lithography [18][19][20].

[0159] Printed optical fibers remain functional when tied in a knot, or when stretched to at least 30% tensile strain (FIG. 3b). Strain of 30% results in 0.90±0.03-fold change (decrease) of the transmitted optical power during the first stretch cycle. Within experimental error, strain induced attenuation remains unchanged during the 1000th stretch cycle.

Example 3: Electrical Interconnects

[0160] Similarly, electrical interconnects can be printed by filling groove filaments with a conductive material. Here we use compacted platinum microparticles as the conductive core while the groove and ribbon filaments act as electrical passivation (FIG. 3c). Elastic interconnects produced in this way display average conductivity of 22±3.5 Scm-1 which is similar to the conductivity observed in other metal microparticle based conductive composites [21].

[0161] Electrical interconnects remain conductive when stretched to at least 30% tensile strain (FIG. 3d). Strain of 30% results in 6.80±1.86-fold increase in resistance, which changes to 14.17±2.02 after 1 000 strain cycles. Finally, we demonstrate that using the printing nozzle as a stylus, freeform microfluidic channels can be produced. Printed channels are observed to be free of obstructions (FIG. 3e) and are able to support laminar flow (FIG. 11). The three types of fiber presented here exhibit nearly linear elasticity (up to at least 30% tensile strain) with elastic moduli in the range of 2.28 to 3.00 MPa, which is consistent with the behavior of the constituent silicone elastomers (FIG. 12).

Example 4: Optical Fiber with Steerable Tip

[0162] Fibers themselves may perform multi-modal sensing and actuating tasks. Here we present two examples where fibers of different modalities are integrated. In the first demonstration, we fabricate an optical fiber with steerable tip that may find applications in endoscope systems with adaptive illumination [22]. We print a composite fiber consisting of a microfluidic channel on top of an optical core (FIG. 4a). By blocking one end of the microfluidic channel and connecting the other to a syringe, we create a pneumatic actuator. Because of the off-axis position of the microfluidic channel, inflation with air results in bending of the entire structure and deflection of the tip of the optical fiber (FIG. 4b). The applied pressure can control the amount of tip deflection (FIG. 4c).

Example 5: Thermal Modulation

[0163] In our second demonstration, we fabricate a system for delivering and monitoring thermal modulation on soft curvilinear surfaces (FIG. 5a). A microfluidic channel is printed in the shape of a flower. It circulates a liquid that facilitates thermal exchange. In the center we position a packaged digital temperature sensor interfaced with printed elastic interconnects.

[0164] We apply our thermal actuator to the surface of a gelatin brain model. By flowing chilled ethanol at a rate of 1-2 mLmin-1 through the thermal exchange loop, we achieved a temperature drop of 3.06° C. in the brain model as reported by the integrated temperature sensor (FIG. 5b). Focal cooling by only several degrees executed form the surface of the cortex has been shown to be effective for seizure suppression in several species including humans [23][24][25]. Thermal neuromodulation is a promising strategy for treating intractable focal epileptic seizures that remains less investigated due to lack of suitable implantable technology [26].

SUMMARY OF THE EXAMPLES

[0165] In summary, we demonstrate a strategy for rational control of the cross sectional geometry of filaments printed with SE1700 silicone and Carbopol hydrogel. We integrate filaments in core-shell functional fibers and freeform microfluidics. As an alternative method to co-extrusion with specialized co-axial nozzles, our approach relies on simple circular nozzles. Using groove and ribbon filaments we demonstrate production of fibers with optical, electrical and microfluidic functionality. Freeform fibers may be integrated in webs of multi-modal sensors and actuators. We envisage applications in soft robots as well as in implantable systems to deliver multimodal therapeutic programs to soft organs in the body.

Experimental Section:

Ink Preparation:

[0166] Silicone elastomers, SE 1700 and OE 6520 (Dow Corning), are prepared by mixing catalyst and base at a ratio of 1:10 and 1:1 respectively, followed by degassing. Carbopol (EDT 2020, Lubrizol Corporation) is prepared by vigorous mixing in water at a concentration of 2% w/w, followed by the addition of NaOH until pH 7.0 is achieved.

Electrical Interconnects:

[0167] For electrical interconnects, conductive ink is fabricated by mixing Platinum powder (particle diameter 0.2-1.8I.Im, ChemPur, Germany) with tri(ethylene glycol) monoethyl ether (TGME, Merck KGaA) followed by sonication. The Platinum content in the suspension is around 15% by weight. The platinum suspension is deposited in groove filaments by ink-jet or by pipetting. The printed lines are then heated to 120° C. for 5 min to evaporate TGME leaving behind compacted dry Platinum powder inside groove filaments.

Printing:

[0168] Printing is done using the 3D Discovery bio-printer from RegenHU, Switzerland. Print layouts are developed in the BIOCAD software (RegenHU). Studies of filament cross-sections are conducted with the SE 1700 silicone and the Carbopol microgel. We use a plastic conical nozzle with nominal inner diameter of 200I.Im at the tip (corrected to an actual value of 210 μm following precise optical measurements). The pneumatic pressure is set at 5 and 1.2 bar for SE1700 and Carbopol respectively. Printing of the platinum ink is done with the inkjet printing head of the 3D Discovery instrument. The optical silicone OE 6520 is deposited by the nozzle extrusion method, using 100 μim inner diameter, metal nozzles (Poly Dispersing Systems). In all cases, the substrate used for printing is glass treated with 2% sodium dodecyl sulfate (Merck KGaA) to form a debonding layer.

Optical Fibers:

[0169] To create a grove in a freshly printed filament of SE 1700 the nozzle (d.sub.in=210 μm, d.sub.out=430 μm) is used as a stylus. The groove filament is then heat cured at 120° C. for 45 minutes. One end of a cleaved silica optical fiber (ø1501.im, Thor Labs) is placed inside the groove. Optical silicone (OE 6520) is then dispensed inside the groove to form the core of the optical fiber. This structure is heat cured at 120° C. for 45 minutes. A final ribbon shaped filament of SE1700 silicone forms a seal, and is heat cured at 120° C. for 45 minutes. Prior to each printing step, the structure is exposed to a brief oxygen plasma to improve interlayer adhesion. Finally, the free end of the silica optical fiber is coupled to a white light source (SCHOTT, KL 1500 electronic). Light transmission measurements are conducted by inserting the free end of the printed optical fiber into the opening of the integrating chamber of an optical powermeter (PM100D, S142C, Thor Labs). For stretching experiments, the ends of printed fibers are attached to the prongs of a caliper.

Electrical Interconnects:

[0170] Here a groove filament is filled with platinum ink by ink-jetting and heated to 120° C. for 5 minutes to remove the dispersing solvent. A ribbon shaped filament forms the electrical passivation. The two ends of the electrical interconnects are covered with, conductive epoxy (Epo-Tek H27D, Epoxy Technology) to which conventional electrical wires are attached. Printed electrical interconnects are stretched using a Dynamic Mechanical Analysis tester (SHIMADZU, EZ-SX) with a load cell of 20 N. During stretching, resistance measurements are performed using a potentiostat (AUTOLAB PGSTAT204, Metrohm).

Microfluidic Channel:

[0171] Here, a nozzle (d.sub.in=210 μm, d.sub.out=430 μm) is used as a stylus passed through a freshly printed filament of SE 1700 silicone. In doing so, the walls of the groove shaped filament collapse forming a triangular shaped channel. The microfluidic channels are heat cured at 120° C. for 45 minutes.

Steerable Optical Fibers:

[0172] A microfluidic channel is printed directly on top of an optical fiber. The end of the microfluidic channel is sealed with a small droplet of RTV silicone (734 Clear, Dow Corning). The other end is coupled to a needle mounted on a 6 ml syringe. The junction between the syringe and the microfluidic channel is secured with additional blobs of silicone. Air pressure is applied by depressing the syringe piston using a syringe pump (kdScientific).

Thermal Modulation System:

[0173] A microfluidic channel is printed in a flower shape. In the center of the flower we place a packaged digital temperature sensor (DS18B20, Maxim Integrated) interfaced with printed electrical interconnects. The microfluidic channel is coupled with silicone tubes (Fredenberg Medical, Mono Lumen Tubing). The whole system is placed upon gelatin (Sigma-Aldrich, G1890-100G) cast in the shape of a hemisphere (red food dye is added to the gelatin). A peristaltic pump (Bio-Rad, EP-1 Econo Pump) is used to pump chilled Ethanol (−25° C.) through the microchannels at different flow rates.

Imaging of Printed Structures:

[0174] A ZEISS Discovery V2.0 microscope is used for the imaging of filament and fiber cross sections. Other optical images are captured with a macro lens.

Rheology Measurements:

[0175] The rheology of SE1700 and Carbopol is investigated by oscillating plate rheometry (ARES, TA instruments, with parallel plates diameter of 8 mm). In order to quantify the shear dependent viscosity behavior of the samples, the shear rate is stepped between 1 and 100 rad/s.

Statistics:

[0176] Measurements are quoted as averages from at least three independent samples and errors represent standard deviation.

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