Patent ID: 12233635

DETAILED DESCRIPTION

FIG.1is a perspective view of a computing device100that includes a foldable display102, with the foldable display in a partially folded configuration. The device100has the foldable display102mounted so that it folds with the viewable face inward. It is also possible to mount the foldable display102on the opposite side of device100so that the display folds with viewable face outward (not shown).FIG.2is a perspective view of the computing device100, with the display102in a folded configuration. The foldable display102may be, for example, a TFT (Thin-Film-Transistor) OLED (Organic Light Emitting Diode) display, or other appropriate display technology. The foldable display102may comprise appropriate circuitry for driving the display to present graphical and other information to a user.

As shown inFIG.1andFIG.2, the foldable display102can include a first flat rigid section112, a second flat rigid section114, and a third bendable section116. In some implementations, the foldable display102can include more than two flat rigid sections112,114and more than one bendable section116. In some implementations, the foldable display102can include zero, or only one, flat rigid section112,114. For example, when a foldable display102includes zero flat rigid sections, the display102can be continuously bendable, and can be rolled up, as in a scroll. The foldable display102shown inFIG.1andFIG.2has a bendable section116that allows the foldable display to bend about an axis. In other implementations, the foldable display102can include bendable sections that allow the display102to bend about more than one axis.

The bendable section116of the foldable display102allows the display102to bend in an arc that has a radius, and the bendable section can be made to become rigid when the radius of the bendable section reaches a specified minimum radius. This minimum radius may be selected to prevent the display from bending in a radius so small that fragile components of the display would be broken. In some implementations, the minimum radius is greater than or equal to 2.5 millimeters, or greater than or equal to 3.0 millimeters, or greater than or equal to 5 millimeters. Thus, the bendable section can be flexible when bent in a radius greater than the minimum radius and then become rigid when the bend radius is equal to or smaller than the minimum radius.

FIG.3is a schematic diagram of a flexible display device300having a stack of a number of different layers. For example, in some implementations, a flexible organic light-emitting diode (OLED) layer306can be supported by a bend limit layer308, and a backing film310. In some implementations, the bend limit layer308can be between the OLED layer306and the backing film310. In some implementations, the OLED layer306can be between the bend limit layer308and the backing film310An optically clear adhesive layer304can be applied to a front surface of the flexible OLED layer306. A cover window film302can be applied to the optically clear adhesive layer304to protect the device on the front side. As the thickness of each layer of the stack is important to the overall thickness of the device300, it is desirable to have a relatively thin thickness for the layers. For example, in some non-limiting examples, the thickness of the flexible OLED layer306can be on the order of approximately 300 μm; the thickness of the optically clear adhesive layer304can be on the order of approximately 100 μm; and the thickness of the cover window film can be on the order of approximately 200 μm. Thus, the thicknesses of the bend limit layer308and the backing film310are selected so as to maintain an overall thickness of the device300that is not too great, and also should have individual thicknesses that are fractions of a millimeter.

The components of the stack of the device300have different as-fabricated properties, including stresses and strains that exist in the component when the layer is fabricated. Additional stresses and strains can be induced in the layers of the stack when the display is bent into a configuration that is different from the configuration in which the layer was fabricated. For example, if the layer was flat when it was fabricated, then additional strain can be induced by bending the layer, and if the layer was fabricated in a curved configuration, then additional strain can be induced by flattening the layer. If the bend-induced strain exceeds a threshold value characteristic of a component of the stack, the component can be damaged by the strain due to plastic deformation, cracking, buckling, delamination, etc. This characteristic damage threshold strain may be different depending on temperature, humidity, required cycle life, and other use and environmental factors. Brittle inorganic layers of the stack can typically withstand less strain than inorganic layers before they are damaged by the strain. Nevertheless, organic materials in the stack also can be damaged by excessive strain that is induced by bending.

FIG.4is a schematic side view of a fiber reinforced film400. In one implementation, the fiber reinforced film400can be used as the backing film in a flexible display. However, the fiber reinforced film400also can be used in many other implementations, such as providing thin, strong structures. As shown inFIG.4, in some implementations, the fiber reinforced film400can include a plurality of layers402,404,406stacked on top of each other as sheets. However, in other implementations, the fiber reinforced film400may include only a single layer.

The layers402,404,406of the fiber reinforced film400can include strong elongated fibers408that run along the length of the layer and that are embedded in a matrix of polymer material414. For example, as shown inFIG.4, circular cross-sections of four elongated fibers that extend into the page in each of the layers402and406are depicted, and a side view of one fiber412within layer404, which extends across the layer404in a direction perpendicular to the fibers408in layers402and406is depicted. The fiber reinforced film400can be composed of one or more different layers that include unidirectional fibers, where directions of the unidirectional fibers of the different layers are selected depending on the application for which the film400will be used. For example, a layer402,404,406reinforced with unidirectional fibers generally requires greater force to bend in a direction that requires the fibers themselves to stretch or shrink in length, while relatively less force is required to bend the layer along an axis parallel to the direction of the unidirectional fibers. In addition, the density of fibers within a layer affects the strength and stiffness of the layer, where a higher density of fibers generally results in a higher strength and higher stiffness layer. The location of a layer within the stack also affects the force required to bend the film, with layers near the surface generally requiring more force to bend than layers near the center of the stack.

The thickness of each layer402,404,406can be less than about 50 μm in some implementations. In some implementations, the thickness of each layer402,404,406can be less than about 40 μm. In some implementations, the thickness of each layer402,404,406can be less than about 30 μm. In some implementations, the thickness of each layer402,404,406can be less than about 25 μm. In some implementations, the thickness of each layer402,404,406can be less than about 20 μm. In some implementations, the height of each fiber408in a direction normal to the plane of its layer402(e.g., the diameter of a fiber having a circular cross-section) can be less than 30 μm. In some implementations, the height of each fiber408in a direction normal to the plane of its layer402(e.g., the diameter of a fiber having a circular cross-section) can be less than 20 μm. In some implementations, the height of each fiber408in a direction normal to the plane of its layer402(e.g., the diameter of a fiber having a circular cross-section) can be less than 12 μm. In some implementations, the height of each fiber408in a direction normal to the plane of its layer402(e.g., the diameter of a fiber having a circular cross-section) can be less than 8 μm.

To achieve a thin fiber reinforced film400, the layers402,404,406of the fiber reinforced film400can be created such that each layer includes essentially a monolayer of fibers408. That is, individual fibers408within a layer402can be spaced apart from each other within the layer402, such that different individual fibers408generally do not overlap with each other in a direction normal to the plane of the layer402. Although it is desirable to have each of the fibers408within a layer402perfectly parallel to other fibers within the layer and equally spaced from each other, and in most cases, closely spaced to each other to maximize strength, for thin sheets with small-diameter fibers. limitations in manufacturing techniques may result in a few fibers that overlap each other in a direction normal to the plane of the layer402. For example,FIG.5is a schematic view of a layer500of a unidirectional fiber reinforced film, viewed from a direction normal to the plane of the layer. As shown inFIG.5, two fibers502,504cross each other such that they overlap at locations506,508, causing the local thickness of the layer at locations506,508to be greater than at other locations within the layer.

To reduce the probability of such overlapping fibers, an average lateral spacing410between adjacent fibers can be maintained at a sufficiently large distance. For example, in some implementations, the average lateral spacing410can be greater than two times the height of each fiber408in a direction normal to the plane of its layer402(e.g., the diameter of a fiber having a circular cross-section). In some implementations, the average lateral spacing410can be greater than five times the height of each fiber408in a direction normal to the plane of its layer402.

The fibers408can be made from a variety of different materials. For example, the fibers408can be made of carbon, ceramic, polymer, glass, or metal materials. In some implementations, different layers402,404,406of the film400can include fibers of different materials. For example, the fibers within layer402could be made of ceramic material, while the fibers within layer404could be made of carbon material. In some implementations, an individual layer could include a mix of fibers made of different materials. The fiber material used in particular layer can be selected based on its material properties (e.g., thermal, electrical, mechanical properties) as they may be best suited for a particular application of the film400.

The pattern of the spread fibers408in the layers402,406,408can be made in a number of different ways. For example,FIG.6Ais a schematic diagram of a spreader system600that is configured for spreading a fiber tow602of many individual fibers into an array of laterally separated fibers. The fiber tow602can be a yarn that includes many (e.g., tens, hundreds, or thousands of) individual fibers that are bundled together into the tow. The fiber tow602can be unwound from a spool604by a pair of rollers606. The tension on the fiber tow602can be maintained by the rollers606and a second pair of rollers608that is located between the first pair of rollers606and a take-up drum620. With the use of the two pairs of rollers606,608the tension on the tow602as it moves through the spreader system between the two rollers can be maintained at a predetermined level regardless of the tension of the tow in the spool604.

The spreader system600can include an acoustic speaker610that is powered by an oscillator612and an amplifier614, and acoustic energy output from the speaker610can spread individual fibers of the tow602. The tow602can be routed over and under a series of polished rods616that are located adjacent to the speaker610. As the tow602moves from the first pair of rollers606to the second set of rollers608through the series of polished rollers616, pressure differences in the air through which the tow moves due to the speaker610can cause the tow602to spread into individual, laterally-spaced fibers. The polished rods616hold the spreading tow in its spread form as it is conveyed from the first pair of rollers606to the second set of rollers608. The polished rods616can be driven by a motor to rotate synchronously with the movement of the fiber tow602.FIG.6Bis a schematic top view of a tow of fibers632that is spread into laterally-spaced individual fibers634as the tow moves through the spreading system600.

The spreading system600can include a polymerization station618in which the laterally-spaced individual fibers634of the tow632are polymerized to form a layer having a monolayer of fibers. The fibers can be polymerized in the polymerization station618in different ways. For example, polymer material can be sprayed onto the fibers to embed the fibers in the polymer material. In another embodiment, the fibers can be drawn onto a flat surface that is prepared to have a low adhesion to polymer material, and then polymer material can be wetted onto the surface. Once the polymer material has fully or partially cured it can be released from the surface with the fibers embedded in the polymer material.

In another implementation, and individual fiber or a spread tow of a plurality of fibers can be wound onto a mandrel to form a pattern of parallel fibers on the mandrel and the fibers on the mandrel can be polymerized to form a monolayer fiber-reinforced tube on the mandrel. When a plurality of fibers are wound onto the tube, the spacing between adjacent fibers in the tow can be maintained at a predetermined fixed distance, and during the winding, the tow can be moved continuously along the length of the mandrel at a constant speed by the predetermined fixed distance or slightly less than the predetermined distance, so that the fibers do not overlap. Once the polymer material has fully or partially cured, it can be released from the mandrel with the fibers embedded in the material. For example, the polymerized monolayer fiber-reinforced tube can be cut along the length of the tube and then peeled off the mandrel as sheet of material.

In another implementation, oriented chopped fibers can be used to create a fiber-reinforced film. As used herein, chopped fibers are fibers having a median length that is shorter than 5 mm. Chopped fibers can be created in a variety of different ways, for example, by creating long strands of fibers that are then cut, or chopped, to a desired length, or by recycling existing cured materials containing fibers to remove the fibers from the resin in which they are embedded and then cutting the fibers into smaller length pieces.

FIG.7is a schematic top view of a layer of a fiber reinforced film700, in which the fibers include relatively short chopped fibers702. The chopped fibers702can be generally shorter than the elongated fibers408shown inFIG.4. On average, the chopped fibers702can be generally oriented along a preferred direction704in the in the layer, although the orientation of individual chopped fibers may deviate from the general preferred direction. Some chopped fibers702may overlap (e.g., cross) other chopped fibers. In some implementations the fiber reinforced film can include a plurality of individual layers stacked on top of each other. The preferred directions in which chopped fibers702are generally oriented in the different layers can be different. For example, in one implementation, adjacent layers in the film700can include chopped fibers whose preferred orientation directions are perpendicular to each other.

The individual chopped fibers702can be aligned along the preferred direction704with a number of techniques, for example, as described in “Aligned Discontinuous Fibre Composites: A Short History,” Matthew, Such, Carwyn Ward, and Kevin Potter,J. Multifunctional Composites, vol. 3, pp 155-168 (2014), and in U.S. Pat. No. 6,025,285, both of which are incorporated herein by reference in their entirety. For example, in some implementations, individual fibers can be aligned along the preferred direction704by suspending fibers in a carrier fluid that is pumped through a tapered nozzle onto a substrate (e.g., a flat gauze bed, a centrifuge wall, etc.). When passed through the tapered nozzle, the chopped fibers can become aligned along a direction parallel to the axial direction of the nozzle, and this direction can be maintained when the fibers are deposited on the substrate. After deposition on the substrate, the carrier fluid can be removed, and then aligned fibers can be polymerized to form a film of fiber-reinforced material. In another implementation, short length fibers can be chopped from a long strand fiber immediately prior to deposition of the chopped fibers on a moving conveyor belt. The chopped fibers can maintain the direction of the long strand when they are deposited on the belt, and then the deposited fibers can be polymerized to form a film of fiber-reinforced material. Electric, optical, and acoustic fields also can be used to align the fibers. For example, a non-zero charge can be applied to the chopped fibers and then as they are deposited on a substrate an applied static electric field can be used to align the chopped fibers along a preferred direction. In other implementations, standing acoustic or optical waves can be formed at or near the substrate on which the chopped fibers are deposited, and the potential wells of the standing waves can be used to align the chopped fibers that are deposited.

In one implementation, the fiber reinforced film700can be used as the backing film in a flexible display. However, the fiber reinforced film700also can be used in many other implementations, such as providing thin, strong structures. The fiber reinforced film700can include a plurality of layers stacked on top of each other as sheets. However, in other implementations, the fiber reinforced film may include only a single layer.

When implemented as multi-layer film, different layers of the fiber reinforced film700can include strong elongated chopped fibers702that run along the length of the layer and that are embedded in a matrix of polymer material. For example, a layer reinforced with chopped fibers preferentially aligned along axial direction704generally requires greater force to bend in a direction that requires the fibers themselves to bend, while relatively less force is required to bend the layer along an axis parallel to the alignment direction of the chopped fibers. In addition, the density of fibers within a layer affects the strength and stiffness of the layer, where a higher density of fibers generally results in a higher strength and higher stiffness layer.

The thickness of each layer of a multi-layer film, or the thickness of an individual layer film, can be less than about 50 μm in some implementations. In some implementations, the thickness of each layer can be less than about 45 μm. In some implementations, the thickness of each layer can be less than about 30 μm. In some implementations, the thickness of each layer can be less than about 25 μm. In some implementations, the thickness of each layer can be less than about 20 μm. In some implementations, the height of each chopped fiber in a direction normal to the plane of its layer (e.g., the diameter of a fiber having a circular cross-section) can be less than 30 μm.

In some implementations, the height of each fiber in a direction normal to the plane of its layer (e.g., the diameter of a fiber having a circular cross-section) can be less than 20 μm. In some implementations, the height of each chopped fiber in a direction normal to the plane of its layer (e.g., the diameter of a fiber having a circular cross-section) can be less than 12 μm. In some implementations, the height of each chopped fiber in a direction normal to the plane of its layer (e.g., the diameter of a fiber having a circular cross-section) can be less than 8 μm.

To achieve a thin film700reinforced with preferentially oriented chopped fibers702, the layers of the fiber reinforced film700can be created such that the probability of overlap between neighboring individual fibers is low (e.g., less than 0.1, meaning that fewer than one of 10 fibers overlaps with a neighboring fiber). Although it is desirable to have each of the chopped fibers702within a layer to be perfectly parallel to other fibers702within the layer, and in most cases, closely spaced to each other to maximize strength, for thin sheets with small-diameter fibers manufacturing techniques may result in a few fibers that overlap each other in a direction normal to the plane of the layer, and overlapping fibers may be more prevalent in the case of a film reinforced with chopped fibers702than in a film reinforced with elongated fibers that individually span the film.

However, the greater density of overlapping fibers in a film of chopped fibers than in a film of long fibers that span the film may be acceptable in some implementations if the manufacturing costs of a chopped fiber reinforced film is sufficiently less than that of a film reinforced with longer fibers that span the film.

A backing film having one or more layers of monolayer preferentially aligned fibers can be used to provide strength and protection to a foldable display.

FIG.8is a schematic diagram of a foldable display800having a bendable section801(the curved portion shown inFIG.8) that is bent around a minimum radius, Renin. The foldable display800can include a display layer802that includes components (e.g., OLED layers, TFT layers, touch screen layers, polarizing layers, encapsulation layers, etc.) that generate images on the display (emitted from the side of the display that faces toward the inside of the bend) and that protect the image generating layers, and a bend limit layer804that limits the radius at which the foldable display800can bend to greater than or equal to the minimum radius, Rmin.

When the display layer802is fabricated in a flat configuration, then bending the display layer802in the absence of the bend limit layer804may cause the bendable section to assume a radius less than the minimum radius, Rminand induce excessive strain within the display layer802. For example, compressive strain will be induced along the inner radius of the bend, Rinner, and tensile strain will be induced along the outer radius of the bend, Router. The display layer802can be approximately characterized by a plane at which no strain is induced when the display layer802is bent. This plane is referred to herein as the “neutral plane”806. If the stack of materials within the layer802is symmetrical about a midplane of the layer, then the neutral plane corresponds to the midplane of the layer. However, different material properties (e.g., thickness, Young's modulus, etc.) of different layers within the display layer802can cause the neutral plane to be displaced above or below the midplane of the layer802. The location of the neutral plane within the layer802, along with the maximum tolerable strain values of the materials within the layer802, determines the minimum bend radius that can be tolerated without causing damage to components within the layer802.

The bend limit layer804can be attached to the display layer802to provide support for the display layer802and also can prevent the display layer from being bent around a radius that is smaller than its minimum tolerable bend radius. A monolayer unidirectional fiber reinforced backing film820of the device having a layer reinforced with fibers can provide strength and support for the device. The fibers in backing film820can have a coefficient of thermal expansion (CTE) that is close to the CTE of the OLED display layer802, so that the fragile components are not unduly stressed by thermal cycling of the device800. For example, while many fiber materials have CTE's that are close to zero or even negative, some ceramic fibers can have CTE's on the order of 8 ppm per Kelvin. Use of such fiber materials can improve thermal expansion matching to a wide range of structures, including OLED display layers. In some implementations, the CTE of the fibers can be within about 50% of the CTE of the OLED display layer820. In some implementations, the CTE of the fibers can be within about 25% of the CTE of the OLED display layer820. In some implementations, the CTE of the fibers can be within about 10% of the CTE of the OLED display layer820.

The bend limit layer804can be relatively flexible when it bent in radii greater than Rminand then can become stiff and inflexible when the radius of the bend approaches, or matches, Rmin. Stiffness can be parameterized by the change in bend radius per unit of applied force that causes the foldable display800to bend. For example, inFIG.8, when the display is folded in half around a 180 degree bend, twice the radius of the bend is shown by the parameter, x, when a force, F, is applied to bend the foldable display. The stiffness of the foldable display800then can be parameterized by the derivative, k=dF/dx. The strength of the foldable display can be characterized as the maximum force, F, that the foldable display800can withstand before failure of the display occurs.

When the foldable display800is laid flat in its folded configuration it can be maintained in its folded configuration by the force of gravity on the upper folded portion of the display, such that zero additional force is needed to be applied to the upper folded portion to maintain the foldable display in its flat folded configuration. In this configuration the radius of the bend can be defined as the limit radius, Rmin, i.e., the radius at which the bend limit layer804limits the further bending of the foldable display unless additional external force is applied. To bend the foldable display further from this configuration requires additional external force to overcome the stiffness of the bend limit layer. Thus, an example stiffness curve for a foldable display in which the limit radius is reached with the foldable display is folded 180 degrees, showing stiffness as a function of x is shown inFIG.9.

It can be advantageous to have a foldable display with a stiffness curve that exhibits a relatively sharp increase in stiffness once the limit radius is reached, such that the foldable display can be easily folded into its folded configuration in which Rlimitis close to Rmin, and then the foldable display will become quite stiff, such that it remains in this configuration despite forces pressing it toward a radius smaller than Rlimit.

The bend limit layer804is shown on the outside of the bend inFIG.8, but also can be on the inside of the bend, for example, as shown inFIG.10, in which case the content displayed by the display is on the outside of the bend and the monolayer unidirectional fiber reinforced backing film1020is on the inside of the bend.

FIG.11is a schematic diagram of an example implementation of a bend limit layer1100. The bend limit layer1100can include a plurality of adjacent segments1102that are each separated from neighboring segments for R>Rlimitand that are in contact with neighboring segments when R≤Rlimit. Each segment1102can have a base portion1104that is attached to a thin film1106and a head portion1108that is wider in a direction parallel to the plane of the bend limit layer1106than the base portion1104. For example, the thin film1106can be bent in radii of less than 3 mm. The thin film can be reinforced by elongated fibers that are preferentially aligned along a particular direction, and the fibers can be different lengths in different implementations. The alignment of the fibers in the film can allow the film to have different stiffnesses and strengths when folded in different directions with respect to the preferential direction.

The thin film1106can have a thickness that is small compared with the height of the segments1102in a direction perpendicular to the thin film1106. The stiffness of the thin film1106is low, so that the bend limit layer1106is easily bent for radii R≥Rlimit. The thin film1106can be bent in radii small enough to accommodate the design parameters of the bend limit layer1100. In one non-limiting example, the thin film1106can have a thickness of about 50 μm and when bend into a radius of 2.5 mm can experience a 1% strain. Of course, the thickness of the material can be adjusted to trade off advantages between different parameters, for example, the minimum radius of the thin film can be bent into, the strength of the thin film, and the stiffness of the thin film.

In the example implementation shown inFIG.11, the bond line between the base portions1104and the thin film1106covers 50% of one surface of the thin film1106. In other words, half of the surface of the thin film1106is attached to base portions1104of adjacent segments1102, and half of the surface is unattached. Other configurations are also possible, in which the bond line coverage is more or less than 50%. The portion of the thin film1106that is bonded to the adjacent segments1102is much stiffer than the portions that are not bonded. This increases the stain in the unbonded portions of thin film1106, and this increase must be accounted for in the materials and geometry of the bend limit layer1100.

The head portion1108of each segment1102can have vertical sides1110that, when the bend limit film1106is flat, are not perfectly perpendicular to the thin film1106, but rather that are angled toward each other as they extend away from the thin film1106. Then, when the bend limit layer1106is bent into a radius equal to Rlimit, the vertical sides1110of adjacent segments1102become in intimate contact with, and parallel to, each other, so that they form a rigid, rugged layer of material that has a high stiffness for R≤Rlimit. Some means of fabricating the head portion1108of each segment1102may not have perfectly flat sides, but instead have other surface geometries that also allow both faces of adjacent segments1102to come into intimate contact with each other, so that they form a rigid, rugged layer of material that has a high stiffness for R≤Rlimit.

The segments1102can be formed from a number of different materials including, for example, metals, polymers, glasses, and ceramics. Individual blocks can be molded, machined, drawn (e.g., through a shaped wire) and then attached to the thin film1106at the correct spacing. In another implementation, a plurality of adjacent segments1102can be formed simultaneously and then attached to the thin film1106. For example, as shown inFIG.12, a plurality of adjacent segments1202can be formed on a substrate1204, for example, by a single- or multi-step molding process, and then, after the segments1202are bonded to the thin film1206, the substrate can be broken, dissolved, or otherwise removed from the segments1202. In another implementation, the plurality of adjacent segments1202can be formed on a substrate1204, for example, by a lithography and etching process, and then, after the segments1202are bonded to the thin film1206, the substrate can be broken, dissolved, or otherwise removed from the segments1202.

FIG.13is a schematic diagram of a rotating mold that can be used in an example molding process for forming the adjacent segments1102. For example, slides 1, 2, 3, etc. can be inserted radially into position with respect to a core pin, and then material can be injected into the voids between the slides and the core pin to simultaneously form the segments1102and the thin film1106. As segments1102are formed, the assembly can be rotated counter-clockwise and the slides removed in numerical order to free segments from the counter-clockwise-most position inFIG.13while new segments are formed in positions clockwise from the counter-clockwise-most position. By using transparent tooling and an ultra-violet (UV) rapid-curing molding compound, high production throughput can be achieved.

FIG.14is a schematic diagram of a mold1402that can be used for forming adjacent segments1102of a bend limit layer1404. The shape of the mold1402can correspond to the shape of the bend limit layer1404, when the bend limit layer is in its designed limit radius configuration. Then, the adjacent segments1102of the bend limit layer1404can be formed as a unified part within the mold1402, however, with imperfections along the designed boundaries between adjacent segments1102. The imperfections then can allow the unified part to be cracked along the designed boundaries between the adjacent segments, so that after the bend limit layer1404is removed from the mold and flattened the bend limit layer1404has the separated adjacent segments1102shown inFIG.11, but when the bend limit layer1404is bent to its limit radius, the adjacent segments form strong, rugged contacts to their adjacent segments.

FIG.15is a schematic diagram of another implementation of the foldable display1500, in which a bend limit layer1502is coupled to a display layer1504. The bend limit layers1502can include a plurality of sublayers. The sublayers can include, for example an outer layer1506, a middle layer1508, and an inner layer1510. The inner layer1510can include one or more fingers1512that extends outward toward the outer layer1506and that, when the bend limit layer1502is in a relaxed, un-bent configuration, are each horizontally separated by a gap1514in the plane of the layers from a portion of the middle layer1508that is closest to the middle of the bend into which the bend limit layer1502can be bent. Two fingers1512and gaps1514are shown inFIG.15, but any number of fingers and corresponding gaps is possible.

FIG.16is a schematic diagram of the foldable display1500when it is in a bent configuration. As shown inFIG.16, compressive strain on the inner layer at the apex of the bend due to the bending of the foldable display causes the gaps1514between the fingers1512of the inner layer and the middle layer to be closed. Thus, the sections of the inner layer1510can act as leaves that move across the inner layer in response to the compressive strain and that pull their corresponding fingers with them. When the gaps1514are closed, the stiffness of the bend limit layer1502increases, so that further bending of the foldable display is restricted.

FIG.17is a schematic diagram of another implementation of the display1700in which a bend limit layer1702is coupled to a display layer1704and to a monolayer unidirectional fiber reinforced backing film1720. The bend limit layer1702can include a plurality of sublayers. The sublayers can include, for example, an outer skin layer1706, a middle layer1708, and an inner skin layer1710. The layers can be made of different materials. In one implementation, the inner and outer layers1710,1706can be made of very thin layer of a material with very high elongation (e.g. Nitinol film), and the middle layer1708can be made of material whose stiffness changes as a function of the bend radius of the foldable display1700.

FIG.18is a schematic diagram of the foldable display1700ofFIG.17when it is in a bent configuration. As shown in FIG.18, compressive strain on the inner layer1708due to the bending of the foldable display causes the stiffness of the middle layer1708to increase. This can occur in a number of different ways. In one implementation, the compressive strain on the inner layer1710and the middle layer1708causes the layers1710,1708to deform inward toward the center of the bend, and the deformation of the material can increase the stiffness of the materials in the layers. In another implementation, the compressive strain on the inner layer1710and the middle layer1708causes a changes of state of an electromechanical device (e.g., a piezoelectric device)1712within at least one of the layers1710,1708and a signal due to the change of state can be used to cause a change in the stiffness of the middle layer1708. For example, an electrical signal from the electromechanical device1712in response to the bend-induced strain can trigger an electrical current or a voltage to be applied to the materials in the middle layer, which, in turn, can cause a change of state and stiffness of the material in the middle layer. For example, the material can change from a liquid to a solid in response to the applied current or voltage, or material can be pumped into the bent portion of the middle layer, or the orientation of particles of material can be rearranged in response to the applied current or voltage to increase the stiffness of the bent portion.

FIG.19is a schematic diagram of another implementation of the foldable display1900in which a bend limit layer1902is coupled to a display layer1904. The content of the display can be displayed on a surface of the display that is on the opposite side of the foldable display1900from the bend limit layer1902(e.g., content side facing down, as shown inFIG.19). The bend limit layer1902can include a plurality of threads or fibers1908arranged across the layer1902in a plane and that, when the bend limit layer1902is in a flat configuration, are arranged in a serpentine configuration, so that the length of each fibers is longer than the straight end-to-end distance in the plane between the ends of each fiber. The fibers1908can be made of strong, low-stretch material, such as, for example, fibers made from glass, Kevlar®, graphite, carbon fiber, ceramics, etc. and can be laid down in a low modulus substrate. For example, the fibers1908can be laid down via a spread tow technique in the desired pattern using specialized manufacturing equipment. In some implementations, the fibers1908can be pinned at locations1906along their lengths to a layer of the foldable display, e.g., to a substrate in the bend limit layer1902or to an surface at interface between the bend limit layer1902and the display layer1904. For example, the fibers can be pinned at nodes of the serpentine configuration of the fibers. The pinning can be performed by a number of different techniques. For example, a laser heating process may bond the fibers at the pinning sites to the layer, or the fibers can be mechanically bonded at the sites.

The fibers can limit the bend radius of the foldable display1900when the display is bent, when the bend limit layer1902is on the outside of the bend and the display layer1904is on the inside of the bend, because the fibers can become straight and limit the bend radius of the foldable display when the desired minimum bend radius is reached. In other words, the resistance of the bend limit layer1902to tensile strain in the layer is very low while the fibers are unstretched and then becomes high when the fibers are stretched to their full lengths. With the fibers bonded to material in the bend limit layer1902at the pinning sites, a sudden increase in stiffness of the bend limit layer occurs when the bending of the bend limit layer1902causes the fibers to become straight between adjacent pinning sites1906.

FIG.20is a schematic diagram of the foldable display1900when the display is in a bent configuration with the bend limit layer1902on the outside of the bend and with the display layer1904on the inside of the bend. In this configuration, when the bend limit layer is under tensile strain, the fibers can be become straight in the curved plane of the bend limit layer1902, and the end-to-end distance, within the curved plane, of each fiber segment between adjacent pinning sites1906can be close to, or the same as, the length of each fiber between the adjacent pinning sites1906. In this configuration the strong, low-stretch fibers resist the tensile strain on the bend limit layer, and thereby limit the bend radius of the foldable display1900.

FIG.21is a schematic diagram of another implementation of the foldable display2100, in which a bend limit layer2102and a monolayer unidirectional fiber reinforced backing film2120are coupled to a display layer2104. The bend limit layers2102can include a patterned structure of materials that can have a non-linear stiffness response to compressive forces caused by bending of the foldable display2100.

In one implementation, the patterned structure can include an array of ribs2106that extend away from the display layer2104. As shown inFIG.21, the ribs2106can be rectangular shaped, but other shapes are also possible. The ribs2106can be relatively rigid, in that they have a high bulk modulus and a high shear modulus. Therefore, the ribs2106retain their shape when the foldable display2100is bent. The ribs can include a variety of different rigid materials, including, for example, metals (e.g., aluminum, copper, steel, etc.) ceramic materials, glass materials, etc.

Gaps or trenches2108between adjacent ribs2106can be partially or fully filled with a second material that has a non-linear stiffness response to compressive forces caused by bending of the foldable display2100. The material can include a foam (e.g., and open cell foam), a gel, or other material whose bulk modulus changes as a function of the compressive forces on the material.

When the bend limit layer2102is in a relaxed, unbent configuration, as shown inFIG.21, the material in the gaps2108between the ribs2106can exert a relatively low force on the ribs holding in place in the gaps, for example, because in the unbent configuration the material in the gaps2108is in an undeformed state and therefore the material exerts little force due to its compressibility. The distance between adjacent ribs at the distal ends of the ribs (i.e., away from the display layer2104) can be approximately equal to the distance between adjacent ribs2106at the proximate ends of the ribs (i.e. closest to the display layer2104).

FIG.22is a schematic diagram of the foldable display2100ofFIG.21when the display is in a bent configuration. As shown inFIG.22, compressive strain in the bend limit layer2102layer can cause the distance between adjacent ribs2106at the proximate ends of the ribs to be less than when the bend limit layer2102is in its relaxed, unbent configuration. In addition, because of the bend of the bend limit layer2102and the non-zero length of the ribs the distance between adjacent ribs at the distal ends of the ribs2106is even shorter when the bend limit layer2102is in the bent configuration than when in the unbent configuration. Consequently, the material in the in gaps or trenches2108between the ribs2106is squeezed when the layer2102is bent. The squeezing of the material can cause a sudden increase in the stiffness of the material when the radius of the bend becomes less than a threshold radius. For example, in the case of an open cell foam material in the gaps2108between the ribs2106, air can be squeezed of the cells when the material is compressed, and when a critical amount of air has been squeezed from the material when the radius reaches the threshold radius, then the stiffness of the material can suddenly increase.

Although rectangular ribs2106are illustrated inFIGS.21and22, and rectangular gaps2108between the ribs2106are shown inFIG.21, other shapes of both the ribs and the material in the gaps between the ribs are possible. For example, as shown inFIG.23A, ribs2302can be generally T-shaped profile. In another example, as shown inFIG.23B, ribs2304can have a generally trapezoid-shaped profile. In another example, as shown inFIG.23C, ribs2306can have a profile that is narrower in the middle than at the top and the bottom of the ribs. In another example, as shown inFIG.23D, ribs2308can have a custom shaped profile that is configured, in conjunction with the type and shape of the material in the gaps between the ribs to accomplish a desired stiffness vs. bend radius response.

Correspondingly, the shape of the materials in the gaps between the ribs, which materials have a non-linear stiffness response to the radius of curvature of the bend limit film, can have different shapes. For example,FIGS.24A,24B,24C, and24Dshow rectangular gaps between rectangular ribs2402, but with the materials in the gaps having different shapes in the different figures. For example, as shown inFIG.24A, the rectangular gaps can be filled with non-linear stiffness response material2405that bulges above the tops of the gaps when the bend limit layer is in its relaxed configuration. In another example, as shown inFIG.24B, the rectangular gaps can be filled with non-linear stiffness response material2406that precisely fills the rectangular gaps when the bend limit layer is in its relaxed configuration. In another example, as shown inFIG.24C, the rectangular gaps can be filled with non-linear stiffness response material2408that descends below the tops of the gaps when the bend limit layer is in its relaxed configuration. In another example, as shown inFIG.24D, the rectangular gaps can be filled with non-linear stiffness response material2410along the sides and bottom of the gaps, but not on in the central portion of the gaps. The type and shape of the material in the gaps between the ribs can be selected to accomplish a desired stiffness response to the bend radius response of the bend limit layer.

FIG.25is a schematic diagram of a foldable display2500having a bendable section2501) that is bent around a minimum radius, Rmin. The display2500can also include one or more straight or non-bendable sections. The foldable display2500can include a display layer2502that includes components (e.g., OLED layers, TFT layers, touch screen layers, polarizing layers, etc.) that generate images on the foldable display and a bend limit layer2504that limits the radius at which the foldable display2500can bend to greater than or equal to the minimum radius, Rmin. The bend limit layer2504is coupled to the display layer2502by a coupling layer2503. The coupling layer2503can include, for example, an adhesive material or a bonding material on respective surfaces that touch the display layer2502and the bend limit layer2504. The display2500also includes and a monolayer unidirectional fiber reinforced backing film2520coupled to a display layer2504.

As described above, when the display layer2502is fabricated in a flat configuration, bending the display layer2502induces compressive strain along the inner radius of the bend, and tensile strain is induced along the outer radius of the bend. It is desirable to keep the neutral plane2506of the assembly, at which no stain occurs in response to the bending, at, or close to, the plane in which fragile and sensitive components of the assembly (e.g., TFTs) exist. Thus, the coupling layer2503can include low modulus material (e.g., rubber, gel, etc.), so that little strain within the planes of the layers is transmitted between the display layer2502and the bend limit layer2504. In some implementations, the display2500can include an additional layer2510on the opposite side of the display layer2502from the bend limit layer2504and that functions to maintain the neutral plane close to its designed location within the display layer2502when the bend limit layer2504acts to limit the bend radius of the display2500. For example the additional layer2510can have a stiffness that compensates for the effect of the stiffness of the bend limit layer on the position of the neutral plane, so that the neutral plane does not shift from its designed location in the display layer2502when the display layer2502is coupled to the bend limit layer2504.

It will also be understood that when an element is referred to as being on, connected to, electrically connected to, coupled to, or electrically coupled to another element, it may be directly on, connected or coupled to the other element, or one or more intervening elements may be present. In contrast, when an element is referred to as being directly on, directly connected to or directly coupled to another element, there are no intervening elements present. Although the terms directly on, directly connected to, or directly coupled to may not be used throughout the detailed description, elements that are shown as being directly on, directly connected or directly coupled can be referred to as such. The claims of the application may be amended to recite exemplary relationships described in the specification or shown in the figures.

While certain features of the described implementations have been illustrated as described herein, many modifications, substitutions, changes and equivalents will now occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the scope of the implementations. It should be understood that they have been presented by way of example only, not limitation, and various changes in form and details may be made. Any portion of the apparatus and/or methods described herein may be combined in any combination, except mutually exclusive combinations. The implementations described herein can include various combinations and/or sub-combinations of the functions, components and/or features of the different implementations described.

In addition, the logic flows depicted in the figures do not require the particular order shown, or sequential order, to achieve desirable results. In addition, other steps may be provided, or steps may be eliminated, from the described flows, and other components may be added to, or removed from, the described systems. Accordingly, other embodiments are within the scope of the following claims.