IL308535A - Implementing functional electronic devices in fiber-reinforced polymers through simultaneous sintering and hardening - Google Patents

Description

m1ML/23 Embedding Functional Electronic Devices in Fiber Reinforced Polymers Through Simultaneous Sintering and Curing Field of the Invention id="p-1" id="p-1" id="p-1" id="p-1" id="p-1"

[0001] The present Application relates to the field of materials engineering and more specifically, but not exclusively, to a method of embedding functional electronic devices in composite structures, through simultaneous sintering of conductive ink and curing of layers of the composite structure.

Background of the Invention id="p-2" id="p-2" id="p-2" id="p-2" id="p-2"

[0002] Composite structures are multilayered structures made of different materials that are bonded together. One common composite structure is a fiber- reinforced polymer, which is made of a polymer matrix reinforced by a fiber.

Commonly, the polymer is an epoxy or phenolic resin, and the fiber is made of carbon fiber or glass fiber. One method of manufacturing fiber-reinforced polymers includes: forming one or more prepreg layers containing the fiber and a partially cured polymer, shaping the prepreg layers, and curing the polymer through the application of energy, such as UV radiation or heat. When the curing is performed with heat, typical cure temperatures range from approximately 60 °C to 180 °C.

Fiber-reinforced polymers are attractive for a variety of industrial functions due to their attributes of strength, durability, and moldability. These include: airplane wings, automobile bodies, and bicycle frames. id="p-3" id="p-3" id="p-3" id="p-3" id="p-3"

[0003] Composite structures may be enhanced to include abilities beyond the carriage of mechanical loads. In particular, it is advantageous to embed materials ISRAEL PATENT OFFICE' 14-11.23 RECEIVED 47274/IL/23 for sensing, communication and other functions between layers of the composite structure. For example, in the context of airplane parts, rendering the skin of the airplane conductive helps protect the internal components of the airplane from lightning. A strain gauge or a capacitive sensor may be used to evaluate whether the composite structure is beginning to deteriorate. Likewise, embedding sensors within the composite structure enables the collection of relevant information regarding the functioning of the device of which the composite structure forms a part. Other possible functions include, but are not limited to: deicing, heating, energy harvesting, or actuating of mechanical structures within an airplane wing. id="p-4" id="p-4" id="p-4" id="p-4" id="p-4"

[0004] Despite these potential benefits, manufacturing electronic devices within composite structures poses significant practical challenges. In particular, it is challenging to introduce the wiring and sensor between layers of the composite structure while preserving the structural integrity of the composite structure. The thickness of the wiring potentially causes a separation between the layers of the composite structure during the curing process. In addition, and more significantly, soldering of the sensor to an external power source causes the introduction of a foreign body (i.e., the solder material) within the polymer matrix. As a result, the strength of the composite structure is sacrificed. id="p-5" id="p-5" id="p-5" id="p-5" id="p-5"

[0005] One approach for effectively introducing conductive wiring in fiber- reinforced polymers is to introduce the conductive wiring as a conductive ink. The conductive ink is spread onto the surface of the polymeric resin. The curing process also sinters the ink. To date, this solution has been applied to create a circuit trace pattern for imparting lightning protection on airplane bodies. 47274/IL/23 Summary of the Invention id="p-6" id="p-6" id="p-6" id="p-6" id="p-6"

[0006] The present invention addresses a method of production of functional electronic devices within a composite structure. The method includes patterning of conductive ink onto prepreg laminates, placement of electronic devices on the prepregs adjacent to the conductive ink, and simultaneously sintering the ink and curing the polymer to form a composite structure. Particularly advantageously, an electrical connection is formed between the ink and the sensors without sacrificing the structural integrity of the composite structure. id="p-7" id="p-7" id="p-7" id="p-7" id="p-7"

[0007] According to a first aspect, a method of embedding an electronic device in a composite structure is disclosed. The method includes: placing the electronic device on one or more prepreg layer(s) of the composite structure, printing a trace of conductive ink on the prepreg layer, wherein, following the placing and printing steps, the electronic device and the trace are in contact with each other; simultaneously curing the composite structure, sintering the ink, and forming an electrical connection between the sintered ink and the electronic device. id="p-8" id="p-8" id="p-8" id="p-8" id="p-8"

[0008] In another implementation according to the first aspect, the placing step comprises forming the electronic device in an additive manufacturing process. id="p-9" id="p-9" id="p-9" id="p-9" id="p-9"

[0009] In another implementation according to the first aspect, the method further includes placing one or more additional prepreg layers above the conductive ink and electronic device. The curing step comprises curing the additional layers together with the first prepreg layer. id="p-10" id="p-10" id="p-10" id="p-10" id="p-10"

[0010] In another implementation according to the first aspect, the curing step comprises applying heat at a temperature between 130°C and 180 °C. 47274/IL/23 id="p-11" id="p-11" id="p-11" id="p-11" id="p-11"

[0011] In another implementation according to the first aspect, the conductive ink is a copper-based ink. id="p-12" id="p-12" id="p-12" id="p-12" id="p-12"

[0012] In another implementation according to the first aspect, the functional electronic device is a strain gauge, a capacitative sensor, or a piezoelectric sensor. id="p-13" id="p-13" id="p-13" id="p-13" id="p-13"

[0013] In another implementation according to the first aspect, the step of simultaneously curing, sintering, and forming further comprises: setting values of temperature, pressure, vacuum, and dwell time, so as to control the completeness of the sintering of the ink and a degree of release of volatiles from the ink and prepreg into the composite structure.

Brief Description of the Drawings id="p-14" id="p-14" id="p-14" id="p-14" id="p-14"

[0014] FIG. 1 illustrates steps in a method for generating composite structures with functional electronic devices embedded therein, according to embodiments of the present disclosure; id="p-15" id="p-15" id="p-15" id="p-15" id="p-15"

[0015] FIG. 2 schematically steps of forming a composite structure with an electronic device embedded therein, according to embodiments of the present disclosure; id="p-16" id="p-16" id="p-16" id="p-16" id="p-16"

[0016] FIGS. 3A-3D illustrate traces of electromagnetic ink at various stages of sintering, according to embodiments of the present disclosure; id="p-17" id="p-17" id="p-17" id="p-17" id="p-17"

[0017] FIG. 3E illustrates the different absorbance, across the electromagnetic spectrum, of the traces of FIGS. 3A-3D, according to embodiments of the present disclosure; 47274/IL/23 id="p-18" id="p-18" id="p-18" id="p-18" id="p-18"

[0018] FIG. 4 illustrates an air bubble formed between layers of a composite structure; id="p-19" id="p-19" id="p-19" id="p-19" id="p-19"

[0019] FIGS. 5A-5D illustrate stages of production of a fiber-reinforced polymer connected to a flexible printed circuit board; id="p-20" id="p-20" id="p-20" id="p-20" id="p-20"

[0020] FIGS. 6A-6B illustrate stages of production of a fiber-reinforced polymer with an electronic device therein; id="p-21" id="p-21" id="p-21" id="p-21" id="p-21"

[0021] FIG. 7 illustrates an example of an additively manufactured sensor; according to embodiments of the present disclosure; id="p-22" id="p-22" id="p-22" id="p-22" id="p-22"

[0022] FIG. 8 illustrates a frequency-selective surface that may be embedded within a composite structure, according to embodiments of the present disclosure; and id="p-23" id="p-23" id="p-23" id="p-23" id="p-23"

[0023] FIGS. 9A-9B illustrate a prior art sintered connection between an electronic device embedded within a composite structure and external wiring.

Detailed Description of the Invention id="p-24" id="p-24" id="p-24" id="p-24" id="p-24"

[0024] The present Application relates to the field of materials engineering, and more specifically, but not exclusively, to a method of embedding functional electronic devices in a composite structure, through simultaneous sintering of conductive ink and curing of the polymer of the composite structure. id="p-25" id="p-25" id="p-25" id="p-25" id="p-25"

[0025] Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not necessarily limited in its application to the details of construction and the arrangement of the components and/or methods set forth in the following description and/or illustrated in the drawings and/or the 47274/11/23 Examples. The invention is capable of other embodiments or of being practiced or carried out in various ways . id="p-26" id="p-26" id="p-26" id="p-26" id="p-26"

[0026] In particular, in the examples described herein, a fiber-reinforced polymer is selected as an example of a composite structure. The fiber-reinforced polymer may be made of carbon fiber or glass fiber and may incorporate any suitable polymer or resin. However, the electronic devices may be implemented on any suitable composite structure, using the methods described herein, so long as that composite structure meets the requirements of structural stability under the temperature and pressure conditions required to sinter the ink. id="p-27" id="p-27" id="p-27" id="p-27" id="p-27"

[0027] FIG. 1 illustrates steps in a method 100 for embedding an electronic device in a composite structure. FIGS. 2A-2F schematically illustrate steps in performance of this method, with reference to the composite materials. For the sake of simplicity, the figure refers to a single prepreg layer, but the skilled person will easily appreciate that the same process can be carried out on a plurality of such layers. id="p-28" id="p-28" id="p-28" id="p-28" id="p-28"

[0028] Referring to FIG. 2A, the process begins with the preparation of a prepreg layer 12a. The prepreg layer is made of a reinforcing material that is pre- impregnated with an epoxy or other resin. In the examples that follow, the reinforcing material is carbon fiber. Other reinforcing materials, such as glass fibers, glass cloth, aromatic polyamide fibers, or basalt fibers, may alternatively be used. In addition, although a single prepreg layer is shown in the illustration of FIG. 2A and the subsequent illustrations, this is merely exemplary and it is understood that many 47274/IL/23 prepreg layers may be layered on each other, as needed, prior to placement of any electronic components. id="p-29" id="p-29" id="p-29" id="p-29" id="p-29"

[0029] Referring to FIG. 2B and to step 101 of method 100 in FIG. 1, one or more traces of conductive nanoparticle ink 14 (also referred to herein as a "nanoink") are printed onto the surface of the prepreg layer 12a. The conductive ink may be deposited, for example, through inkjet printing. Any other suitable method, such as screen printing, plating, spin coating, or sputtering, may also be used. id="p-30" id="p-30" id="p-30" id="p-30" id="p-30"

[0030] The conductive ink includes nanoparticles of a conductive metal.

Preferably, the metal is copper or silver. These nanoparticles are suspended in a carrier solution, which may include, for example, grease, epoxy, resin, and/or a solvent. The ink is not yet conductive when applied. Following a sintering process, the nanoparticles fuse together and become conductive. id="p-31" id="p-31" id="p-31" id="p-31" id="p-31"

[0031] In particularly advantageous embodiments, the conductive ink has a sintering temperature of below approximately 180 °C. This sintering temperature is significantly lower than the sintering temperature of most conductive inks, which is approximately 300 °C. In experiments conducted by the inventors of the present disclosure, a proprietary conductive ink with copper nanoparticles was used. id="p-32" id="p-32" id="p-32" id="p-32" id="p-32"

[0032] Referring now to FIG. 2C and to step 102, an electronic device 16 is placed on the prepreg layer 12a. The electronic device 16 may be any suitable sensor configured to deliver an electronic output, including, for example, a strain gauge, a piezoresistive sensor, a capacitative sensor, or a temperature sensor. The electronic device 16 may also be, for example, an actuator, a LED, a semiconductor device, or an external power line. It is possible for more than one electronic device to be laid 47274/1L/23 adjacent to a trace - e.g., multiple actuators, multiple sensors, or one sensor and one power line. id="p-33" id="p-33" id="p-33" id="p-33" id="p-33"

[0033] Optionally, the electronic device 16 may be additively manufactured directly on top of the prepreg layer 12a. Alternatively, the electronic device 16 may be manufactured separately and placed subsequently on the prepreg layer. id="p-34" id="p-34" id="p-34" id="p-34" id="p-34"

[0034] The electronic device 16 and conductive traces are placed such that the electronic device 16 and at least one trace 14 are in contact with each other. For the purposes of forming this contact, typically, trace 14 is layered prior to placement of the electronic device 16, so as to ensure even printing of trace 14. However, it is equally possible to place the electronic device 16 first, and to layer the trace above it. id="p-35" id="p-35" id="p-35" id="p-35" id="p-35"

[0035] Referring now to FIG. 2D and step 103, optionally, additional prepreg layers 12b and 12c are laminated on top of the first prepreg layer 12a, the conductive ink 14, and the electronic device 16. When additional prepreg layers are added, the traces and electronic device become embedded within a final composite structure. This embedding may be preferred in order to protect the electronic components from the environment, as well as to the ability to obtain sensorial data from the depth of the structure, such as the temperature, mechanical motion, pressure, etc., which is not readily available according to the prior art, which is mostly limited to data obtainable at or close to the surface. However, the processes described herein work equally well even if no additional layers of prepreg material are applied, such as for electronic devices that are designed to be attached to an exterior surface of the composite structure. 47274/1L/23 id="p-36" id="p-36" id="p-36" id="p-36" id="p-36"

[0036] Referring to FIG. 2E and step 104, the entire layered apparatus is cured under suitable temperature and pressure conditions. For example, the curing may take place in an autoclave. In preferred embodiments, the heating takes place at a temperature of between 130 °C and 180 °C. This temperature is sufficient to cure the polymer of the composite structure, without causing damage thereto, and is also sufficient to sinter the conductive nanoparticles. The curing may thus be characterized as a "sintering-curing" process, or, for short, "sin-cur." The result of this process is the cured fiber-reinforced polymer, with conductive wiring and an electronic device formed within, as shown in FIG. 2F. id="p-37" id="p-37" id="p-37" id="p-37" id="p-37"

[0037] Advantageously, the sin-cur process not only suffices to cure the composite structure and sinter the conductive ink, but it also forms an electronic connection between this wire and the electronic device 16. This electronic connection is formed so long as there is physical contact between the ink and the electronic device 16. This contact may be minimal. Experimental results demonstrated the formation of a working electronic connection with a circular contact pad having a diameter of 2 mm. However, it is assumed that even smaller contact areas are sufficient. In addition, the electronic connection is formed regardless of the type of conductive metals that are present in the wire and the electronic device 16. For example, working connections were formed with copper as well as with silver. id="p-38" id="p-38" id="p-38" id="p-38" id="p-38"

[0038] Notably, no additional soldering process is required in order to form this electronic connection. This is particularly advantageous because prior art processes for supplying electronic connections to sensors embedded in composite 47274/IL/23 structures required soldering. The soldering process destabilizes the composite structure. An illustration of the destabilizing nature of soldering is illustrated in FIGS. 9A-9B. In FIG. 9A, a composite structure 900 is shown, with external wires 9 soldered to internal wires 901, at soldering points 903. As can be seen, the composite structure is structurally weakened at the soldering points 903. In FIG. 9B, a soldered connection is shown between a sensor 904 and wires 901. The solder constitutes foreign object debris (FOD) within the structure of the composite structure, which causes de-laminations between the layers of the composite structure. By contrast, the methods of the present disclosure enable obtaining of electrical connectivity without soldering at all during the standard curing process of the composite structure matrix. By contrast, following the sin-cur process described herein, the cured fiber-reinforced polymer remains as strong and resistant to strain as one without the embedded wiring and sensors. id="p-39" id="p-39" id="p-39" id="p-39" id="p-39"

[0039] Simultaneously achieving all of these results - sintering, curing, forming the electrical connection, and maintaining structural integrity - requires precise control of process parameters. These process parameters may necessarily vary depending on the materials used, for both the composite structure and the conductive ink. The process parameters include: temperature, vacuum, pressure on the prepreg layers, dwell time, and use of specific assisting materials during the prepreg lamination process. id="p-40" id="p-40" id="p-40" id="p-40" id="p-40"

[0040] With respect to temperature, temperature conditions are necessarily set according to required values for the composite matrix curing on one hand, and the simultaneous sintering of the ink on the other. The ink releases volatiles when 47274/IL/23 exposed to elevated temperatures. These volatile compounds may introduce destabilizing air bubbles into the composite material. Accordingly, an ideal temperature is to be selected, balancing the conditions for a successful Sin-Cur process with the avoidance of volatile release. id="p-41" id="p-41" id="p-41" id="p-41" id="p-41"

[0041] Regarding vacuum, the sin-cur process is preferably conducted under vacuum conditions. These vacuum conditions enable the efficient evacuation of the sintered ink volatiles, as well as the "standard" volatiles that are released from the prepreg layers in the matrix curing reaction. These vacuum conditions are synchronized with the curing regime, whereby the viscosity of the matrix is maintained at minimal values so that the time window for evacuation of volatiles is sufficient to enable the escape of all the volatiles. Otherwise, volatiles could become trapped inside the solid cured matrix. Control of viscosity is, in turn, enabled by controlling the process temperature profile, as will be discussed further below. id="p-42" id="p-42" id="p-42" id="p-42" id="p-42"

[0042] "Pressure" refers to the direct application of positive pressure on the prepreg layers during the sin-cur process. Pressure values are set to allow the compaction of the prepreg layers on one hand and assist the sintering of the ink on the other hand. Generally, pressure encourages the ink particles to crowd and bond to one another. Also, applying pressure conditions during the sin-cur process permits to use lower sintering temperature values. However, pressure values that are too high cause dissipation of ink particles in the epoxy matrix, and prevent a continuum of ink particles and a successful sin-cur process. Therefore, similar to temperature, the pressure parameter also presents complex behavior of dynamics of local optimum values for a desired sin-cur process to occur. 47274/1L/23 id="p-43" id="p-43" id="p-43" id="p-43" id="p-43"

[0043] Regarding duration or dwell time, it is necessary for the temperature, pressure, and vacuum conditions to be maintained for enough time for the desired effect or chemical reaction to take place. It is therefore desirable to plan time durations (dwell times) in specific temperatures during the sin-cur process to allow all three of the following processes to occur: 1) matrix curing, 2) ink sintering and 3) escaping of volatiles during the curing and sintering (which is made possible by matrix viscosity dynamics, as discussed above). A successful sin-cur process with correct temperature values and dwell times, and of course, appropriate pressure and vacuum values, enables the abovementioned phenomena to occur during the process and to the desired extent. id="p-44" id="p-44" id="p-44" id="p-44" id="p-44"

[0044] Assisting materials include ,peel-ply* fabrics, teflon layers, and release agents that are used in composite parts manufacturing. In relation to the sin-cur process, the assisting materials enable the final composite part to have a desired fiber-to-matrix ratio, and prevent any leakage of the ink or adhesion of the ink to unwanted surfaces (for example the tooling material of the part mold or any mechanical connector or joint that is present in the part near the composite layers), which could cause peeling of the ink from the composite surface. id="p-45" id="p-45" id="p-45" id="p-45" id="p-45"

[0045] In general, the curing process may take any amount of time from several minutes to several hours, and may be performed at temperatures ranging from 130 °C to 180 °C. Also, typical (but not limitative) ranges for pressure and vacuum are 1 - 7 bar and down to -1 bar, respectively. In addition, different prepreg materials differ in the recommended matrix cure cycle regarding temperature values, temperature dwell times, vacuum values and pressure values. Hence, every 47274/IL/23 different prepreg matrix requires a "custom made" sin-cur process that provides the desired composite material structural properties and the desired electrical properties of the embedded conductors and devices. id="p-46" id="p-46" id="p-46" id="p-46" id="p-46"

[0046] To exemplify one challenge that may result from insufficient control of the sintering process, reference is made to FIGS. 3A-3E. FIGS. 3A-3D illustrate traces of conductive ink following sintering that was performed to various degrees of success. In FIG. 3D, trace 301 is shown prior to sintering. In FIG. 3C, trace 302 is minimally sintered, in FIG. 3B, trace 303 which is sintered to an intermediate degree, and in FIG. 3A, trace 304 is completely sintered. The traces with incomplete sintering underwent the sintering process under insufficient conditions of temperature, pressure, or dwell time, as discussed above. The differences in sintering correlate to differences in conductivity. FIG. 3E illustrates the differences in conductivity at different wavenumbers for the differently sintered conductive inks of FIGS. 3A-3D.

Although the scale of the wavenumbers is the same for all four graphs, the scale of the absorbance is different for each graph. As can be seen, the ink prior to sintering, shown in graph 311, barely shows any absorbance. Graphs 312 and 313, corresponding to traces 302 and 303, show additional absorbance, although still insufficient for the functioning of an electronic device. Graph 314 corresponds to the fully-sintered trace 304, and demonstrates sufficient conductivity to enable proper functioning of a device within the composite structure. id="p-47" id="p-47" id="p-47" id="p-47" id="p-47"

[0047] FIG. 4 illustrates another challenge of the sin-cur process that may result in suboptimal performance of the composite structure. Composite structure 400 includes fiber reinforced composite 401, which encloses conductive traces 402. 47274/11/23 Due to the trapping of volatiles during the sintering-curing process, an air bubble 4 formed between the conductive traces 402 and the upper layers of the composite structure. The trapping of volatiles may result from a lack of vacuum, a short time window of minimal matrix viscosity, the exposure to temperatures higher than necessary, and any combination of such conditions. As a result, layers of the prepreg were delaminated, and the integrity of the entire structure was compromised. As stated above, optimizing the parameters of the sin-cur process for the particular materials used therein helps ensure that these challenges do not take place in practice. id="p-48" id="p-48" id="p-48" id="p-48" id="p-48"

[0048] As stated above, the principles of the present disclosure may be applied in order to make various "smart" composite structures. FIGS. 5A-5D and 6A- 6B illustrate two such examples. Referring to FIG. 5A, a base prepreg layer 501a is laid on a surface, and conductive ink 502 is printed thereon. An additional prepreg layer 501b is laid on top of the conductive ink 502. A flexible printed circuit board 503 is laid on the prepreg layer 501a and at least partially on the conductive ink 502.

In the illustrated embodiment, the flexible printed circuit board includes electronic traces embedded within Kapton® (a polyimide film). Optionally, at FIG. SB, the junction of the flexible PCB and the conductive ink is covered with an additional prepreg layer 501c. Although, in the illustrated embodiment, a portion of the conductive ink 502 remains uncovered, this is only in order to demarcate the different layers of the composite structure. Following layering of the additional prepreg laminates, the entire device is heated in the autoclave, thereby producing composite structure 504, as shown in FIG. SC. As further illustrated in FIG. SC, the 47274/IL/23 composite structure 504 includes the conductive traces of the flexible printed circuit board 503, and the conductive line 505 from the sintered conductive ink. The flexible printed circuit board 503 and the sintered conductive ink are electrically connected at location 506. In FIG. 50, the flexible printed circuit board 503 may also include a control panel 507, to which it was previously attached. Thus, the composite structure includes a connection to the outside world, for purposes of delivering power to the sensor and receiving data from the sensor. id="p-49" id="p-49" id="p-49" id="p-49" id="p-49"

[0049] FIGS. 6A-6B illustrate another example of a smart composite structure made according to the principles of the present disclosure. In FIG. 6A, lines of conductive ink 602 are printed on prepreg layer 601. A strain gauge 603 is placed on the prepreg layer 601, and lead wires 604 extend from the strain gauge 603 to above the conductive ink 602. Following the sin-cur process, a composite structure is generated. This composite structure includes the strain gauge 603 embedded therein. An electrical connection is formed between wires 604 and the sintered traces 602, without welding. id="p-50" id="p-50" id="p-50" id="p-50" id="p-50"

[0050] As is readily understood by those of skill in the art, it is possible to combine the embodiments of FIGS. 5A-D and 6A-B. The result of this combination is a composite structure with an internal conductive trace that includes an internal connection both to an internal sensor and to an external control panel. All of the electrical connections in the composite structure are achieved without soldering. id="p-51" id="p-51" id="p-51" id="p-51" id="p-51"

[0051] FIG. 7 illustrates a strain gauge that may be incorporated into a composite structure, according to embodiments of the present disclosure. In the illustrated embodiment, the strain gauge is formed by an additive manufacturing 47274/IL/23 process. More specifically, the strain gauge is additively manufactured onto the prepreg layer prior to curing. id="p-52" id="p-52" id="p-52" id="p-52" id="p-52"

[0052] Various modifications may be implemented to the methods described herein without departing from the principles described herein. For example, in the embodiments described above, the conductive ink and electronic device are deposited onto the base layer while it is still in prepreg form, and additional prepreg layers are then laminated above the conductive ink and electronic device. However, it is also possible for the ink and device to be deposited on a fully cured composite material, followed by layering of prepreg laminates on top of the ink and device, and sintering and curing all of the components in the manner described. id="p-53" id="p-53" id="p-53" id="p-53" id="p-53"

[0053] In another example, in addition to, or instead of, placing electrical conductors and electronic devices between layers of the composite structure, it is possible to place a Frequency Selective Surface (FSS) between layers of the composite structure. A Frequency Selective Surface is a repetitive printed pattern of conductive materials, which serves a filter for certain electromagnetic wavelengths.

The conductive ink may be printed on a prepreg surface in the pattern of a Frequency Selective Surface, and may be sintered during the curing process.

Following the sin-cur process, the Frequency Selective Surface functions as expected, to filter the undesired electromagnetic wavelengths.

Claims (7)

47274/1L/23 What is claimed is:

1. A method of embedding an electronic device in a composite structure, comprising: placing the electronic device on one or more prepreg layer(s) of the composite structure; printing a trace of conductive ink on the prepreg layer, wherein, following the placing and printing steps, the electronic device and the trace are in contact with each other; simultaneously curing the composite structure, sintering the ink, and forming an electrical connection between the sintered ink and the electronic device.

2. The method of claim 1, wherein the placing step comprises forming the electronic device in an additive manufacturing process.

3. The method of claim 1, further comprising placing one or more additional prepreg layers above the conductive ink and electronic device, and wherein the curing step comprises curing the additional layers together with the first prepreg layer.

4. The method of claim 1, wherein the curing step comprises applying heat at a temperature between 130°C and 180 °C.

5. The method of claim 1, wherein the conductive ink is a copper-based ink.

6. The method of claim 1, wherein the electronic device is a strain gauge, a capacitative sensor, or a piezoelectric sensor.

7. The method of claim I, wherein the step of simultaneously curing, sintering, and forming further comprises: setting values of temperature, pressure, 47274/1L/23 vacuum, and dwell time, so as to control a completeness of the sintering of the ink and a degree of release of volatiles from the ink and prepreg into the composite structure.