Patent Publication Number: US-11031574-B2

Title: Bendable electronic device modules, articles and methods of making the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application and claims the benefit of priority under 35 U.S.C. § 120 of U.S. application Ser. No. 15/768,313, filed on Apr. 13, 2018, which in turn, claims the benefit of priority under 35 U.S.C. § 371 of International Patent Application Serial No. PCT/US2016/056709, filed on Oct. 13, 2016 and under 35 U.S.C. § 119 of U.S. Provisional Application Ser. No. 62/290,701 filed on Feb. 3, 2016 and U.S. Provisional Application Ser. No. 62/240,879 filed on Oct. 13, 2015, the contents of each of which are relied upon and incorporated herein by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The disclosure generally relates to bendable electronic device modules, articles and methods of making them. More particularly, the disclosure relates to bendable electronic device modules having a glass-containing cover for foldable display device applications. 
     BACKGROUND 
     Flexible versions of products and components that are traditionally rigid in nature are being conceptualized for new applications. For example, flexible electronic devices can provide thin, lightweight and flexible properties that offer opportunities for new applications including curved displays and wearable devices. Many of these flexible electronic devices incorporate flexible substrates for holding and mounting the electronic components of these devices. Metal foils have some advantages including thermal stability and chemical resistance, but suffer from high cost and a lack of optical transparency. Polymeric foils have some advantages including low cost and impact resistance, but suffer from marginal optical transparency, lack of thermal stability, limited hermeticity and cyclic fatigue performance. 
     Some of these electronic devices also can make use of flexible displays. Optical transparency and thermal stability are often important properties for flexible display applications. In addition, flexible displays should have high fatigue and puncture resistance, including resistance to failure at small bend radii, particularly for flexible displays that have touch screen functionality and/or can be folded. Further, flexible displays should be easy to bend and fold by the consumer, depending on the intended application for the display. 
     Some flexible glass and glass-containing materials offer many of the needed properties for flexible and foldable substrate and display applications. However, efforts to harness glass materials for these applications have been largely unsuccessful to date. Generally, glass substrates can be manufactured to very low thickness levels (&lt;25 μm) to achieve smaller and smaller bend radii. These “thin” glass substrates suffer from limited puncture resistance. At the same time, thicker glass substrates (&gt;150 μm) can be fabricated with better puncture resistance, but these substrates lack suitable fatigue resistance and mechanical reliability upon bending. 
     Further, as these flexible glass materials are employed as cover elements in modules that also contain electronic components (e.g., thin film transistors (“TFTs”), touch sensors, etc.), additional layers (e.g., polymeric electronic device panels) and adhesives (e.g., epoxies, optically clear adhesives (“OCAs”)), interactions between these various components and elements can lead to increasingly complex stress states that exist during use of the module within an end product, e.g., an electronic display device. These complex stress states can lead to increased stress levels and/or stress concentration factors experienced by the cover elements. As such, these cover elements can be susceptible to cohesive and/or delamination failure modes within the module. Further, these complex interactions can lead to increased bending forces required to bend and fold the cover element by the consumer. 
     Thus, there is a need for flexible, glass-containing materials and module designs that employ these materials for use in various electronic device applications, particularly for flexible electronic display device applications, and more particularly for foldable display device applications. 
     SUMMARY 
     According to a first aspect of the disclosure, a foldable electronic device module is provided that includes a cover element having a thickness from about 25 μm to about 200 μm and a cover element elastic modulus from about 20 GPa to about 140 GPa. The cover element further includes a component having a glass composition, a first primary surface, and a second primary surface. The module further includes: a stack having a thickness from about 100 μm to about 600 μm; and a first adhesive configured to join the stack to the second primary surface of the cover element, the first adhesive characterized by a shear modulus from about 0.1 MPa to about 100 MPa. The stack further includes a panel having first and second primary surfaces, and a panel elastic modulus from about 300 MPa to about 10 GPa, and an electronic device coupled to the panel. The cover element is further characterized by a puncture resistance of at least 1.5 kgf when the first primary surface of the cover element is loaded with a tungsten carbide ball having a diameter of 1.5 mm. Further, the device module is characterized by a tangential stress at the second primary surface of the cover element of no greater than about 1000 MPa in tension upon bending the module in a two-point configuration to a bend radius from about 20 mm to about 2 mm, for example from about 20 mm to about 3 mm, from about 20 mm to about 4 mm, from about 20 mm to about 5 mm, from about 20 mm to about 6 mm, from about 20 mm to about 7 mm, from about 20 mm to about 8 mm, from about 20 mm to about 9 mm, from about 20 mm to about 10 mm, from about 20 mm to about 11 mm, from about 20 mm to about 12 mm, from about 20 mm to about 13 mm, from about 20 mm to about 14 mm, from about 20 mm to about 15 mm, from about 20 mm to about 16 mm, from about 20 mm to about 17 mm, from about 20 mm to about 18 mm, from about 20 mm, to about 19 mm, from about 19 mm to about 2 mm, from about 18 mm to about 2 mm, from about 17 mm to about 2 mm, from about 16 mm to about 2 mm, from about 15 mm to about 2 mm, from about 14 mm to about 2 mm, from about 13 mm to about 2 mm, from about 12 mm to about 2 mm, from about 11 mm to about 2 mm, from about 10 mm to about 2 mm, from about 10 mm to about 3 mm, from about 9 mm to about 2 mm, from about 8 mm to about 2 mm, from about 7 mm to about 2 mm, from about 6 mm to about 2 mm, from about 5 mm to about 2 mm, from about 4 mm to about 2 mm, from about 3 mm to about 2 mm, from about 19 mm to about 3 mm, from about 18 mm to about 4 mm, from about 17 mm to about 5 mm, from about 16 mm to about 6 mm, from about 15 mm to about 7 mm, from about 14 mm to about 8 mm, from about 13 mm to about 9 mm, from about 12 mm to about 10 mm, such that the first primary surface is in compression and the bend radius is measured from a center point above the first primary surface of the cover element to the second primary surface of the panel. 
     According to a second aspect of the disclosure, a foldable electronic device module is provided that includes a cover element having a thickness from about 25 μm to about 200 μm and a cover element elastic modulus from about 20 GPa to about 140 GPa. The cover element further includes a component having a glass composition, a first primary surface, and a second primary surface. The module further includes: a stack having a thickness from about 100 μm to about 600 μm; and a first adhesive configured to join the stack element to the second primary surface of the cover element, the first adhesive characterized by a shear modulus from about 1 MPa to about 1 GPa. The stack further includes a panel having first and second primary surfaces, and a panel elastic modulus from about 300 MPa and about 10 GPa, an electronic device coupled to the panel, and a stack element having a stack element elastic modulus from about 1 GPa to about 5 GPa, the stack element affixed to the panel with a stack adhesive. The cover element is further characterized by a puncture resistance of at least 1.5 kgf when the first primary surface of the cover element is loaded with a tungsten carbide ball having a diameter of 1.5 mm. Further, the device module is characterized by a tangential stress at the second primary surface of the cover element of no greater than about 1000 MPa in tension upon bending the module in a two-point configuration to a bend radius from about 20 mm to about 2 mm such that the first primary surface is in compression and the bend radius is measured from a center point above the first primary surface of the cover element to the second primary surface of the panel. 
     In certain implementations of the foldable modules, the tangential stress at the second primary surface of the cover element is no greater than about 1000 MPa, for example, 950 MPa, 925 MPa, 900 MPa, 875 MPa, 850 MPa, 825 MPa, 800 MPa, 775 MPa, 750 MPa, 725 MPa, 700 MPa, or any amount between these tangential stress limits, upon bending the module in a two-point configuration to a bend radius from about 20 mm to about 2 mm, for example, 20 mm, 19.75 mm, 19.5 mm, 19.25 mm, 19 mm, 18.5 mm, 17.5 mm, 17 mm, 16.5 mm, 16 mm, 15.5 mm, 15 mm, 14.5 mm, 14 mm, 13.5 mm, and 13 mm, 12.5 mm, 12 mm, 11.5 mm, 11 mm, 10.5 mm, 10 mm, 9.5 mm, 9 mm, 8.5 mm, 7.5 mm, 7 mm, 6.5 mm, 6 mm, 5.5 mm, 5 mm, 4.5 mm, 4 mm, 3.5 mm, 3.25 mm, 3 mm, 2.75 mm, 2.5 mm, 2.25 mm and 2 mm. In certain other aspects of the foldable modules subjected to a bend radius greater than about 20 mm up to about 100 mm, the tangential stress at the second primary surface of the cover element can be substantially reduced through careful selection of the elastic modulus and/or the thickness of the adhesives in the module. 
     In some aspects of the foldable modules, the cover element is further characterized by no cohesive failures upon bending the module, in a two-point configuration, from an un-bent configuration to the bend radius (i.e., ranging from about 20 mm to about 2 mm, for example, 19.75 mm, 19.5 mm, 19.25 mm, 19 mm, 18.5 mm, 17.5 mm, 17 mm, 16.5 mm, 16 mm, 15.5 mm, 15 mm, 14.5 mm, 14 mm, 13.5 mm, and 13 mm, 12.5 mm, 12 mm, 11.5 mm, 11 mm, 10.5 mm, 10 mm, 9.5 mm, 9 mm, 8.5 mm, 7.5 mm, 7 mm, 6.5 mm, 6 mm, 5.5 mm, 5 mm, 4.5 mm, 4 mm, 3.5 mm, 3.25 mm, 3 mm, 2.75 mm, 2.5 mm, and 2.25 mm) for at least 200,000 bend cycles. 
     In other aspects of the foldable modules, the modules can be characterized by a bending force (F bend ) of no greater than 150 Newtons (N) as the module is bent inward or outward to a bend radius of about 3 mm. In certain implementations, the bending force is no greater than about 150 N, 140 N, 130 N, 120 N, 110 N, 100 N, 90 N, 80 N, 70 N, 60 N, 50 N, 40 N, 30 N, 20 N, 10 N, 5 N, or any amount between these bending force upper limits, upon bending of the module to a radius from about 20 mm to about 3 mm (i.e., a plate distance (D) of about 40 mm to about 6 mm), for example, 20 mm, 19.75 mm, 19.5 mm, 19.25 mm, 19 mm, 18.5 mm, 17.5 mm, 17 mm, 16.5 mm, 16 mm, 15.5 mm, 15 mm, 14.5 mm, 14 mm, 13.5 mm, and 13 mm, 12.5 mm, 12 mm, 11.5 mm, 11 mm, 10.5 mm, 10 mm, 9.5 mm, 9 mm, 8.5 mm, 7.5 mm, 7 mm, 6.5 mm, 6 mm, 5.5 mm, 5 mm, 4.5 mm, 4 mm, 3.5 mm, 3.25 mm and 3 mm. 
     According to other aspects of the foldable modules, the cover element is a glass element (e.g., the cover element includes a component having a glass composition) having a cover element elastic modulus from about 20 GPa to about 140 GPa, or any elastic modulus value between these limits, for example, 30 GPa, 35 GPa, 40 GPa, 45 GPa, 50 GPa, 55 GPa, 60 GPa, 65 GPa, 70 GPa, 75 GPa, 80 GPa, 85 GPa, 90 GPa, 95 GPa, 100 GPa, 105 GPa, 110 GPa, 115 GPa, 120 GPa, 125 GPa, 130 GPa, and 135 GPa. In other aspects, the cover element is a glass element having a cover element elastic modulus from about 20 GPa to about 120 GPa, from about 20 GPa to about 100 GPa, from about 20 GPa to about 80 GPa, from about 20 GPa to about 60 GPa, from about 20 GPa to about 40 GPa, from about 40 GPa to about 120 GPa, from about 40 GPa to about 100 GPa, from about 40 GPa to about 80 GPa, from about 40 GPa to about 60 GPa, from about 60 GPa to about 120 GPa, from about 60 GPa to about 100 GPa, from about 60 GPa to about 80 GPa, from about 80 GPa to about 120 GPa, from about 80 GPa to about 100 GPa, and from about 100 GPa to about 120 GPa. In certain implementations, the glass cover element is processed or otherwise configured with strength-enhancing measures that result in the development of one or more compressive stress regions in proximity to one or more primary surfaces of the cover element. 
     In certain aspects of the foldable modules, the first adhesive is characterized by a shear modulus from about 0.1 MPa to about 1 GPa, for example, from about 0.1 MPa to about 800 MPa, from about 0.1 MPa to about 600 MPa, from about 0.1 MPa to about 400 MPa, from about 0.1 MPa to about 200 MPa, from about 0.1 MPa to about 1 MPa, from about 1 MPa to about 800 MPa, from about 1 MPa to about 600 MPa, from about 1 MPa to about 400 MPa, from about 1 MPa to about 200 MPa, from about 200 MPa to about 800 MPa, from about 200 MPa to about 600 MPa, from about 200 MPa to about 400 MPa, from about 400 MPa to about 800 MPa, from about 400 MPa to about 600 MPa, and from about 600 MPa to about 800 MPa. According to an implementation of the first aspect of the foldable module, the first adhesive is characterized by a shear modulus of about 0.1 MPa, 0.2 MPa, 0.3 MPa, 0.4 MPa, 0.5 MPa, 0.6 MPa, 0.7 MPa, 0.8 MPa, 0.9 MPa, 1 MPa, 5 MPa, 10 MPa, 20 MPa, 30 MPa, 40 MPa, 50 MPa, 60 MPa, 70 MPa, 80 MPa, 90 MPa, 100 MPa, or any amount between these shear modulus values. In an implementation of the second aspect of the foldable module, the first adhesive is characterized by a shear modulus of about 1 MPa, 5 MPa, 10 MPa, 20 MPa, 30 MPa, 40 MPa, 50 MPa, 60 MPa, 70 MPa, 80 MPa, 90 MPa, 100 MPa, 200 MPa, 300 MPa, 400 MPa, 500 MPa, 600 MPa, 700 MPa, 800 MPa, 900 MPa, 1000 MPa, or any amount between these shear modulus values. 
     According to some embodiments of the foldable modules of the disclosure, the first adhesive is characterized by a thickness from about 5 μm to about 60 μm, for example, from about 5 μm to about 50 μm, from about 5 μm to about 40 μm, from about 5 μm to about 30 μm, from about 5 μm to about 20 μm, from about 5 μm to about 15 μm, from about 5 μm to about 10 μm, from about 10 μm to about 60 μm, from about 15 μm to about 60 μm, from about 20 μm to about 60 μm, from about 30 μm to about 60 μm, from about 40 μm to about 60 μm, from about 50 μm to about 60 μm, from about 55 μm to about 60 μm, from about 10 μm to about 50 μm, from about 10 μm to about 40 μm, from about 10 μm to about 30 μm, from about 10 μm to about 20 μm, from about 10 μm to about 15 μm, from about 20 μm to about 50 μm, from about 30 μm to about 50 μm, from about 40 μm to about 50 μm, from about 20 μm to about 40 μm, and from about 20 μm to about 30 μm. Other embodiments have a first adhesive characterized by a thickness of about 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, or any thickness between these thickness values. In one aspect, the thickness of the first adhesive is between 10 μm and about 20 μm. 
     In some embodiments of the foldable modules of the disclosure, the first adhesive is further characterized by a Poisson&#39;s ratio from about 0.1 to about 0.5, for example, from about 0.1 to about 0.45, from about 0.1 to about 0.4, from about 0.1 to about 0.35, from about 0.1 to about 0.3, from about 0.1 to about 0.25, from about 0.1 to about 0.2, from about 0.1 to about 0.15, from about 0.2 to about 0.45, from about 0.2 to about 0.4, from about 0.2 to about 0.35, from about 0.2 to about 0.3, from about 0.2 to about 0.25, from about 0.25 to about 0.45, from about 0.25 to about 0.4, from about 0.25 to about 0.35, from about 0.25 to about 0.3, from about 0.3 to about 0.45, from about 0.3 to about 0.4, from about 0.3 to about 0.35, from about 0.35 to about 0.45, from about 0.35 to about 0.4, and from about 0.4 to about 0.45. Other embodiments include a first adhesive characterized by a Poisson&#39;s ratio of about 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, or any Poisson&#39;s ratio between these values. In one aspect, the Poisson&#39;s ratio of the first adhesive is from about 0.1 to about 0.25. 
     According to some implementations of the foldable modules of the second aspect of the disclosure, the stack adhesive is characterized by a shear modulus from about 10 kPa to about 100 kPa, for example, from about 10 kPa to about 90 kPa, from about 10 kPa to about 80 kPa, from about 10 kPa to about 70 kPa, from about 10 kPa to about 60 kPa, from about 10 kPa to about 50 kPa, from about 10 kPa to about 40 kPa, from about 10 kPa to about 30 kPa, from about 10 kPa to about 30 kPa, from about 20 kPa to about 90 kPa, from about 20 kPa to about 80 kPa, from about 20 kPa to about 70 kPa, from about 20 kPa to about 60 kPa, from about 20 kPa to about 50 kPa, from about 20 kPa to about 40 kPa, from about 20 kPa to about 30 kPa, from about 30 kPa to about 90 kPa, from about 30 kPa to about 80 kPa, from about 30 kPa to about 70 kPa, from about 30 kPa to about 60 kPa, from about 30 kPa to about 50 kPa, from about 30 kPa to about 40 kPa, from about 40 kPa to about 90 kPa, from about 40 kPa to about 80 kPa, from about 40 kPa to about 70 kPa, from about 40 kPa to about 60 kPa, from about 40 kPa to about 50 kPa, from about 50 kPa to about 90 kPa, from about 50 kPa to about 80 kPa, from about 50 kPa to about 70 kPa, from about 50 kPa to about 60 kPa, from about 60 kPa to about 90 kPa, from about 60 kPa to about 80 kPa, from about 60 kPa to about 70 kPa, from about 70 kPa to about 90 kPa, from about 70 kPa to about 80 kPa, and from about 80 kPa to about 90 kPa. In this aspect, the stack adhesive may also be characterized by a shear modulus of about 10 kPa, 20 kPa, 25 kPa, 30 kPa, 35 kPa, 40 kPa, 45 kPa, 50 kPa, 55 kPa, 60 kPa, 65 kPa, 70 kPa, 75 kPa, 80 kPa, 85 kPa, 90 kPa, 95 kPa, 100 kPa, or any shear modulus value between these values. 
     According to other implementations of the foldable modules of the second aspect of the disclosure, the stack adhesive is characterized by a thickness from about 5 μm to about 60 μm, for example from about 5 μm to about 50 μm, from about 5 μm to about 40 μm, from about 5 μm to about 30 μm, from about 5 μm to about 20 μm, from about 5 μm to about 15 μm, from about 5 μm to about 10 μm, from about 10 μm to about 60 μm, from about 15 μm to about 60 μm, from about 20 μm to about 60 μm, from about 30 μm to about 60 μm, from about 40 μm to about 60 μm, from about 50 μm to about 60 μm, from about 55 μm to about 60 μm, from about 10 μm to about 50 μm, from about 10 μm to about 40 μm, from about 10 μm to about 30 μm, from about 10 μm to about 20 μm, from about 10 μm to about 15 μm, from about 20 μm to about 50 μm, from about 30 μm to about 50 μm, from about 40 μm to about 50 μm, from about 20 μm to about 40 μm, and from about 20 μm to about 30 μm. Other embodiments have a stack adhesive characterized by a thickness of about 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, or any thickness between these thickness values. In one aspect, the thickness of the stack adhesive is from about 30 μm to 60 μm. 
     According to further implementations of the foldable modules of the second aspect of the disclosure, the stack adhesive is further characterized by a Poisson&#39;s ratio from about 0.1 to about 0.5, for example, from about 0.1 to about 0.45, from about 0.1 to about 0.4, from about 0.1 to about 0.35, from about 0.1 to about 0.3, from about 0.1 to about 0.25, from about 0.1 to about 0.2, from about 0.1 to about 0.15, from about 0.2 to about 0.45, from about 0.2 to about 0.4, from about 0.2 to about 0.35, from about 0.2 to about 0.3, from about 0.2 to about 0.25, from about 0.25 to about 0.45, from about 0.25 to about 0.4, from about 0.25 to about 0.35, from about 0.25 to about 0.3, from about 0.3 to about 0.45, from about 0.3 to about 0.4, from about 0.3 to about 0.35, from about 0.35 to about 0.45, from about 0.35 to about 0.4, and from about 0.4 to about 0.45. Other embodiments include a stack adhesive characterized by a Poisson&#39;s ratio of about 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, or any Poisson&#39;s ratio between these values. In one aspect, the Poisson&#39;s ratio of the stack adhesive is from about 0.4 to about 0.5. 
     In a further implementation of the foldable module of the second aspect of the disclosure, the foldable module is configured for a display device application. As such, the stack element includes a touch sensor and a polarizer. In some embodiments, an adhesive is employed between the touch sensor and the polarizer in these configurations. Some embodiments employ a stack adhesive in these modules having a shear modulus from about 10 kPa to 100 kPa, a thickness from about 30 μm to 60 μm, and/or a Poisson&#39;s ratio from about 0.4 to about 0.5. 
     Additional features and advantages will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the embodiments as described herein, including the detailed description which follows, the claims, as well as the appended drawings. 
     It is to be understood that both the foregoing general description and the following detailed description are merely exemplary, and are intended to provide an overview or framework to understanding the nature and character of the claims. The accompanying drawings are included to provide a further understanding, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiments, and together with the description serve to explain principles and operation of the various embodiments. Directional terms as used herein—for example, up, down, right, left, front, back, top, bottom—are made only with reference to the figures as drawn and are not intended to imply absolute orientation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a foldable electronic device module according to an aspect of the disclosure. 
         FIG. 2  is a cross-sectional view of a foldable electronic device module according to a further aspect of the disclosure. 
         FIG. 3  is a cross-sectional view of a foldable electronic device module according to an additional aspect of the disclosure. 
         FIGS. 4A &amp; 4B  depict foldable electronic device modules in an un-bent and a bent configuration, respectively, within a two-point test apparatus according to an aspect of the disclosure. 
         FIG. 5A  is a plot of estimated tangential stress as a function of depth through the thickness of three foldable electronic device modules, each containing a first adhesive with a different shear modulus configured to join a cover element to a stack, according to a further aspect of the disclosure. 
         FIG. 5B  is a plot of estimated tangential stress as a function of depth through the thickness of two foldable electronic device modules, each containing a first adhesive with a different thickness configured to join a cover element to a stack with a different thickness, according to another aspect of the disclosure. 
         FIG. 6  is a plot of estimated tangential stress as a function of depth through the thickness of three foldable electronic device modules having different adhesive configurations, according to a further aspect of the disclosure. 
         FIG. 7  is a schematic plot of estimated bending force as a function of adhesive thickness for three foldable electronic device modules, each configured with adhesives having a distinct shear modulus, according to a further aspect of the disclosure. 
         FIGS. 8A-8C  are plots of estimated bending force as a function of plate distance in a two-point test apparatus for the foldable electronic device modules depicted in  FIGS. 5A, 5B and 6 , respectively, according to another aspect of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to embodiments according to the claims, examples of which are illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts. Ranges can be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another embodiment. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint. 
     Among other features and benefits, the bendable electronic device modules and articles (and the methods of making them) of the disclosure provide mechanical reliability at small bend radii (e.g., in static tension and fatigue) as well as high puncture resistance. Configurations of these device modules and articles are also characterized by relatively low bending forces necessary to fold or otherwise bend these device modules and articles. With regard to mechanical reliability, the bendable modules of the disclosure are configured to avoid cohesive failures in their glass-containing cover elements and delamination-related failures at interfaces between the various components within the modules (e.g., adhesive-cover element interfaces). The small bend radii and puncture resistance capabilities are beneficial when the bendable modules are used in a foldable electronic device display, for example, one wherein one part of the display is folded over on top of another portion of the display. For example, the bendable module may be used as one or more of: a cover on the user-facing portion of a foldable electronic display device, a location in which puncture resistance is particularly important; a substrate module, disposed internally within the device itself, on which electronic components are disposed; or elsewhere in a foldable electronic display device. Alternatively, the bendable modules of the disclosure may be used in a device not having a display, but one in which a glass or glass-containing layer is used for its beneficial properties and is folded or otherwise bent, in a similar manner as in a foldable display, to a tight bend radius. The puncture resistance is particularly beneficial when the bendable module is used on the exterior of the device, at a location in which a user will interact with it. Still further, the relatively low bending forces required to fold or otherwise bend certain configurations of these device modules and articles is particularly beneficial to the user when these modules and articles are employed in applications requiring manual bending (e.g., a foldable, wallet-like flexible display device). 
     More specifically, the foldable electronic device modules in the disclosure can obtain some or all of the foregoing advantages through control of the material properties and thicknesses of each of the adhesives employed within the modules. For example, these foldable modules can exhibit reduced tangential stresses (in tension) at primary surfaces of the cover element (e.g., a glass substrate) through reductions in the thicknesses of the adhesives employed in the modules and/or increases in the shear modulus of the adhesives employed between the cover element and the underlying stack. These lower tensile stresses at the cover element can translate into improved module reliability, bend radius capability and/or a reduced reliance upon other approaches to develop compressive stresses at the primary surfaces of the cover element (e.g., through ion exchange-driven compressive stress region development). As another example, these foldable modules can exhibit reduced tangential stresses (in tension) at the interface between the panel and an adhesive joining the panel to the stack by reducing the shear modulus of this adhesive. These lower tensile stresses can lead to improved module reliability, particular in terms of resistance to delamination between the panel and the stack. In another instance, overall module stiffness (e.g., resistance to the forces required to bend the module) can be reduced through reductions in the shear modulus of any or all of the adhesives employed in the module and/or selecting an optimum range of the thickness of any or all of the adhesives employed in the module. Moreover, the embodiments and concepts in the disclosure provide a framework for those with ordinary skill to design foldable electronic device modules to reduce tangential stresses at the cover element/stack interface, reduce tangential stresses at the panel/stack interface and reduce the resistance of the module to bending, all of which can contribute to the reliability, manufacturability and suitability of these modules for use in various applications requiring differing degrees and quantities of bending and folding evolutions. 
     Referring to  FIG. 1 , a foldable electronic device module  100   a  is depicted according to a first aspect of the disclosure that includes a cover element  50 , first adhesive  10   a , stack  90   a , stack element  75 , electronic devices  102  and panel  60 . Cover element  50  has a thickness  52 , a first primary surface  54  and a second primary surface  56 . Thickness  52  can range from about 25 μm to about 200 μm, for example from about 25 μm to about 175 μm, from about 25 μm to about 150 μm, from about 25 μm to about 125 μm, from about 25 μm to about 100 μm, from about 25 μm to about 75 μm, from about 25 μm to about 50 μm, from about 50 μm to about 175 μm, from about 50 μm to about 150 μm, from about 50 μm to about 125 μm, from about 50 μm to about 100 μm, from about 50 μm to about 75 μm, from about 75 μm to about 175 μm, from about 75 μm to about 150 μm, from about 75 μm to about 125 μm, from about 75 μm to about 100 μm, from about 100 μm to about 175 μm, from about 100 μm to about 150 μm, from about 100 μm to about 125 μm, from about 125 μm to about 175 μm, from about 125 μm to about 150 μm, and from about 150 μm to about 175 μm. In other aspects, thickness  52  can range from about 25 μm to 150 μm, from about 50 μm to 100 μm, or from about 60 μm to 80 μm. The thickness  52  of the cover element  50  can also be set at other thicknesses between the foregoing ranges. 
     The foldable electronic device module  100   a  depicted in  FIG. 1  includes a cover element  50  with a cover element elastic modulus from about 20 GPa to 140 GPa, for example from about 20 GPa to about 120 GPa, from about 20 GPa to about 100 GPa, from about 20 GPa to about 80 GPa, from about 20 GPa to about 60 GPa, from about 20 GPa to about 40 GPa, from about 40 GPa to about 120 GPa, from about 40 GPa to about 100 GPa, from about 40 GPa to about 80 GPa, from about 40 GPa to about 60 GPa, from about 60 GPa to about 120 GPa, from about 60 GPa to about 100 GPa, from about 60 GPa to about 80 GPa, from about 80 GPa to about 120 GPa, from about 80 GPa to about 100 GPa, and from about 100 GPa to about 120 GPa. The cover element  50  may be a component having a glass composition or include at least one component having a glass composition. In the latter case, the cover element  50  can include one or more layers that include glass-containing materials, e.g., element  50  can be a polymer/glass composite configured with second phase glass particles in a polymeric matrix. In one aspect, the cover element  50  is a glass element characterized by an elastic modulus from about 50 GPa to about 100 GPa, or any elastic modulus value between these limits. In other aspects, the cover element elastic modulus is about 20 GPa, 30 GPa, 40 GPa, 50 GPa, 60 GPa, 70 GPa, 80 GPa, 90 GPa, 100 GPa, 110 GPa, 120 GPa, 130 GPa, 140 GPa, or any elastic modulus value between these values. 
     Again referring to  FIG. 1 , the foldable module  100   a  further includes: a stack  90   a  having a thickness  92   a  from about 100 μm to 600 μm; and a first adhesive  10   a  configured to join the stack  90   a  to the second primary surface  56  of the cover element  50 , the first adhesive  10   a  characterized by a thickness  12   a  and a shear modulus from about 0.1 MPa to about 1000 MPa, for example, from about 0.1 MPa to about 800 MPa, from about 0.1 MPa to about 600 MPa, from about 0.1 MPa to about 400 MPa, from about 0.1 MPa to about 200 MPa, from about 0.1 MPa to about 1 MPa, from about 1 MPa to about 800 MPa, from about 1 MPa to about 600 MPa, from about 1 MPa to about 400 MPa, from about 1 MPa to about 200 MPa, from about 200 MPa to about 800 MPa, from about 200 MPa to about 600 MPa, from about 200 MPa to about 400 MPa, from about 400 MPa to about 800 MPa, from about 400 MPa to about 600 MPa, and from about 600 MPa to about 800 MPa. According to an implementation of the first aspect of the foldable module  100   a , the first adhesive  10   a  is characterized by a shear modulus of about 0.1 MPa, 0.2 MPa, 0.3 MPa, 0.4 MPa, 0.5 MPa, 0.6 MPa, 0.7 MPa, 0.8 MPa, 0.9 MPa, 1 MPa, 5 MPa, 10 MPa, 20 MPa, 30 MPa, 40 MPa, 50 MPa, 60 MPa, 70 MPa, 80 MPa, 90 MPa, 100 MPa, 200 MPa, 300 MPa, 400 MPA, 500 MPa, 600 MPa, 700 MPa, 800 MPa, 900 MPa, 1000 MPa, or any amount between these shear modulus values. Aspects of the foldable modules  100   a  incorporate an adhesive  10   a  with a relatively higher shear modulus, e.g., from about 0.1 MPa to about 100 MPa, compared to the shear modulus of conventional adhesives employed in such electronic device applications. The use of such adhesives  10   a  with relatively higher shear modulus values unexpectedly provides a significant decrease in tensile stresses observed at the second primary surface  56  of the cover element  50  upon bending the foldable electronic device module  100   a  in a direction away from the second primary surface  56 —i.e., by bending the module  100   a  such that the second primary surface  56  exhibits a convex shape. 
     Still referring to  FIG. 1 , certain aspects of the foldable module  100   a  can be configured to minimize bending forces associated with bending the entire module. More particularly, the use of a first adhesive  10   a  with a relatively low shear modulus value (e.g., 0.01 MPa to 0.1 MPa) can unexpectedly reduce the overall bending force required to fold or otherwise bend the entire module  100   a  in an upward or downward direction such that the first primary surface  54  exhibits a concave or convex shape, respectively. These bending force reductions associated with certain aspects of the foldable module  100   a  through the use of a first adhesive  10   a  with a relatively low elastic shear modulus value are obtained relative to a foldable module (e.g., foldable module  100   a ) with an adhesive between the cover element and the stack (e.g., first adhesive  10   a ) having a shear modulus that exceeds 0.1 MPa. 
     In another embodiment of the foldable module  100   a  depicted in  FIG. 1 , the first adhesive  10   a  is characterized by a thickness  12   a  from about 5 μm to about 60 μm, for example, from about 5 μm to about 50 μm, from about 5 μm to about 40 μm, from about 5 μm to about 30 μm, from about 5 μm to about 20 μm, from about 5 μm to about 15 μm, from about 5 μm to about 10 μm, from about 10 μm to about 60 μm, from about 15 μm to about 60 μm, from about 20 μm to about 60 μm, from about 30 μm to about 60 μm, from about 40 μm to about 60 μm, from about 50 μm to about 60 μm, from about 55 μm to about 60 μm, from about 10 μm to about 50 μm, from about 10 μm to about 40 μm, from about 10 μm to about 30 μm, from about 10 μm to about 20 μm, from about 10 μm to about 15 μm, from about 20 μm to about 50 μm, from about 30 μm to about 50 μm, from about 40 μm to about 50 μm, from about 20 μm to about 40 μm, and from about 20 μm to about 30 μm. Other embodiments have a first adhesive  10   a  characterized by a thickness  12   a  of about 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, or any thickness between these thickness values. In one aspect, the thickness  12   a  of the first adhesive  10   a  is from about 10 μm to 20 μm. Some aspects of the foldable modules  100   a  incorporate an adhesive  10   a  with a relatively lower thickness, e.g., from about 10 μm to about 20 μm, compared to the thicknesses of conventional adhesives employed in such electronic device applications. The use of such adhesives  10   a  with relatively lower thickness values unexpectedly provides a significant decrease in tensile stresses at the second primary surface  56  of the cover element  50  upon bending the foldable electronic device module  100   a  in a direction away from the second primary surface  56 —i.e., by bending the module  100   a  such that the second primary surface  56  exhibits a convex shape. While it is believed that further decreases in the thickness  12   a  of the adhesive  10   a  will result in additional reductions in tensile stresses at the second primary surface  56  of the element  50 , the thickness  12   a  can be limited by the bond strength for joining the element  50  to the underlying stack  90   a , depending on the application requirements for the module  100   a.    
     Still referring to  FIG. 1 , certain aspects of the foldable module  100   a  can be configured to minimize bending forces associated with bending the entire module by controlling the thickness of the first adhesive  10   a . More particularly, the use of a first adhesive  10   a  with a range of thicknesses  12   a  (e.g., from about 10 μm to about 40 μm) can reduce the overall bending force required to fold or otherwise bend the entire module  100   a  in an upward or downward direction such that the first primary surface  54  exhibits a concave or convex shape, respectively. These bending force reductions associated with certain aspects of the foldable module  100   a  through the use of a first adhesive  10   a  within a prescribed range of thicknesses are obtained relative to a foldable module (e.g., foldable module  100   a ) with an adhesive between the cover element and the stack (e.g., first adhesive  10   a ) having a relatively small thickness (e.g. less than 10 μm) or a relatively large thickness (e.g., more than 40 μm). 
     In some embodiments of the foldable module  100   a  depicted in  FIG. 1 , the first adhesive  10   a  is further characterized by a Poisson&#39;s ratio from about 0.1 to about 0.5, for example, from about 0.1 to about 0.45, from about 0.1 to about 0.4, from about 0.1 to about 0.35, from about 0.1 to about 0.3, from about 0.1 to about 0.25, from about 0.1 to about 0.2, from about 0.1 to about 0.15, from about 0.2 to about 0.45, from about 0.2 to about 0.4, from about 0.2 to about 0.35, from about 0.2 to about 0.3, from about 0.2 to about 0.25, from about 0.25 to about 0.45, from about 0.25 to about 0.4, from about 0.25 to about 0.35, from about 0.25 to about 0.3, from about 0.3 to about 0.45, from about 0.3 to about 0.4, from about 0.3 to about 0.35, from about 0.35 to about 0.45, from about 0.35 to about 0.4, and from about 0.4 to about 0.45. Other embodiments include a first adhesive  10   a  characterized by a Poisson&#39;s ratio of about 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, or any Poisson&#39;s ratio between these values. In one aspect, the Poisson&#39;s ratio of the first adhesive  10   a  is from about 0.1 to about 0.25. 
     Referring again to  FIG. 1 , the stack  90   a  of the foldable module  100   a  further includes a panel  60  having first and second primary surfaces  64 ,  66 , and a panel elastic modulus from about 300 MPa to about 10 GPa, for example, from about 300 MPa to 8000 MPa, from about 300 MPa to 6000 MPa, from about 300 MPa to 4000 MPa, from about 300 MPa to 2000 MPa, from about 300 MPa to 1000 MPa, from about 300 MPa to 500 MPa, from about 500 MPa to 8000 MPa, from about 500 MPa to 6000 MPa, from about 500 MPa to 4000 MPa, from about 500 MPa to 2000 MPa, from about 500 MPa to 1000 MPa, from about 1000 MPa to 8000 MPa, from about 1000 MPa to 6000 MPa, from about 1000 MPa to 4000 MPa, from about 1000 MPa to 2000 MPa, from about 2000 MPa to 8000 MPa, from about 2000 MPa to 6000 MPa, from about 2000 MPa to 4000 MPa, from about 4000 MPa to 8000 MPa, from about 4000 MPa to 6000 MPa, and from about 6000 MPa to 8000 MPa. The stack  90   a  also includes one or more electronic devices  102  coupled to the panel  60 . As also depicted in  FIG. 1 , the stack  90   a  can also include a stack element  75 . The stack element  75  can include various features associated with the foldable electronic device module  100   a , depending on its end use application. For example, the stack element  75  may include one or more of a touch sensor, polarizer, other electronic devices, and adhesives or other compounds for joining these features to the panel  60 . 
     In  FIG. 1 , the cover element  50  of the foldable module  100   a  is further characterized by a puncture resistance of at least 1.5 kgf when the first primary surface  54  of the cover element is loaded with a tungsten carbide ball having a diameter of 1.5 mm. Typically, puncture testing according to aspects of this disclosure is performed under displacement control at 0.5 mm/min cross-head speed. In some aspects, the cover element  50  is characterized by a puncture resistance of greater than about 1.5 kgf at a 5% or greater failure probability within a Weibull plot. The cover element  50  can also be characterized by a puncture resistance of greater than about 3 kgf at the Weibull characteristic strength (i.e., a 63.2% or greater). In certain aspects, the cover element  50  of the foldable electronic device module  100   a  can resist puncture at about 2 kgf or greater, 2.5 kgf or greater, 3 kgf or greater, 3.5 kgf or greater, 4 kgf or greater, and even higher ranges. The cover element  50  can also be characterized by a pencil hardness of greater than or equal to 8H. 
     In certain other aspects of the foldable module  100   a , the cover element  50  can be characterized by a puncture resistance according to an alternative test method that employs a stainless steel pin having a flat bottom with a 200 μm diameter (rather than a tungsten carbide ball), performed under displacement control at 0.5 mm/min cross-head speed. In certain aspects, the stainless steel pin is replaced with a new pin after a specified quantity of tests (e.g.,  10  tests) to avoid bias that could result from deformation of the metal pin associated with the testing of materials possessing a higher elastic modulus (e.g., cover element  50 ). In these aspects, the element  50  has a puncture resistance of at least 1.5 kgf when the second primary surface  56  of the element  50  is supported by (i) an approximately 25 μm thick pressure-sensitive adhesive (“PSA”) having an elastic modulus from about 0.01 MPa to about 1 MPa and (ii) an approximately 50 μm thick polyethylene terephthalate layer (“PET”) having an elastic modulus less than about 10 GPa, for example from about 2 GPa to about 4 GPa, and the first primary surface  54  of the element  50  is loaded with a stainless steel pin having a flat bottom with a 200 μm diameter. According to other aspects of the foldable module  100   a , the cover element  50  can be characterized by a puncture resistance according to a test method that employs a tungsten carbide ball having a diameter of 1.5 mm with a PSA/PET support structure, performed under displacement control at 0.5 mm/min cross-head speed. In these aspects, the element  50  has a puncture resistance of at least 1.5 kgf when the second primary surface  56  of the element  50  is supported by (i) an approximately 25 μm thick pressure-sensitive adhesive (“PSA”) having an elastic modulus from about 0.01 MPa to about 1 MPa and (ii) an approximately 50 μm thick polyethylene terephthalate layer (“PET”) having an elastic modulus less than about 10 GPa, for example from about 2 GPa to about 4 GPa, and the first primary surface  54  of the element  50  is loaded with a tungsten carbide ball having a diameter of 1.5 mm. It is also believed that puncture testing according to the foregoing approaches with a stainless steel pin having a flat bottom with a 200 μm diameter will produce results consistent with employing the same approach (e.g., PSA/PET support structure) and test conditions with a tungsten carbide ball having a diameter of 1.5 mm. 
     Referring again to  FIG. 1 , the foldable electronic device module  100   a , according to a first aspect of the disclosure, is characterized by a tangential stress at the second primary surface  56  of the cover element  50  of no greater than 1000 MPa in tension (i.e., at point “T,” as shown in  FIG. 4B ) upon bending the module in a two-point configuration to a bend radius  220  (see  FIG. 4B ) from about 20 mm to about 2 mm such that the first primary surface  54  is in compression (i.e., at point “C,” as shown in  FIG. 4B ) and the bend radius  220  is measured from a center point above the first primary surface  54  of the cover element  50  to the second primary surface  66  of the panel  60 . In certain implementations, the tangential stress (in tension) at the second primary surface  56  of the cover element  50  is no greater than about 1000 MPa, 950 MPa, 925 MPa, 900 MPa, 875 MPa, 850 MPa, 825 MPa, 800 MPa, 775 MPa, 750 MPa, 725 MPa, 700 MPa, or any amount between these tangential stress upper limits, upon bending of the module to a radius from about 20 mm to about 2 mm in a two-point configuration, for example, 20 mm, 19.75 mm, 19.5 mm, 19.25 mm, 19 mm, 18.5 mm, 17.5 mm, 17 mm, 16.5 mm, 16 mm, 15.5 mm, 15 mm, 14.5 mm, 14 mm, 13.5 mm, and 13 mm, 12.5 mm, 12 mm, 11.5 mm, 11 mm, 10.5 mm, 10 mm, 9.5 mm, 9 mm, 8.5 mm, 7.5 mm, 7 mm, 6.5 mm, 6 mm, 5.5 mm, 5 mm, 4.5 mm, 4 mm, 3.5 mm, 3.25 mm, 3 mm, 2.75 mm, 2.5 mm, 2.25 mm and 2 mm, or for example from about 20 mm to about 3 mm, from about 20 mm to about 4 mm, from about 20 mm to about 5 mm, from about 20 mm to about 6 mm, from about 20 mm to about 7 mm, from about 20 mm to about 8 mm, from about 20 mm to about 9 mm, from about 20 mm to about 10 mm, from about 20 mm to about 11 mm, from about 20 mm to about 12 mm, from about 20 mm to about 13 mm, from about 20 mm to about 14 mm, from about 20 mm to about 15 mm, from about 20 mm to about 16 mm, from about 20 mm to about 17 mm, from about 20 mm to about 18 mm, from about 20 mm, to about 19 mm, from about 19 mm to about 2 mm, from about 18 mm to about 2 mm, from about 17 mm to about 2 mm, from about 16 mm to about 2 mm, from about 15 mm to about 2 mm, from about 14 mm to about 2 mm, from about 13 mm to about 2 mm, from about 12 mm to about 2 mm, from about 11 mm to about 2 mm, from about 10 mm to about 2 mm, from about 10 mm to about 3 mm, from about 9 mm to about 2 mm, from about 8 mm to about 2 mm, from about 7 mm to about 2 mm, from about 6 mm to about 2 mm, from about 5 mm to about 2 mm, from about 4 mm to about 2 mm, from about 3 mm to about 2 mm, from about 19 mm to about 3 mm, from about 18 mm to about 4 mm, from about 17 mm to about 5 mm, from about 16 mm to about 6 mm, from about 15 mm to about 7 mm, from about 14 mm to about 8 mm, from about 13 mm to about 9 mm, from about 12 mm to about 10 mm. In certain other aspects of the foldable modules subjected to a bend radius greater than about 20 mm up to about 100 mm in a two-point configuration, the tangential stress at the second primary surface of the cover element can be substantially reduced through careful selection of the elastic modulus and/or the thickness of one or more of the adhesives in the module. 
     Still referring to  FIG. 1 , the foldable electronic device module  100   a , according to another implementation, can be characterized by a bending force (F bend ) of no greater than 150 Newtons (N) as the module is bent inward by a test apparatus to a bend radius  220 , the bend radius being approximately half the distance (D) between two test plates  250  (see  FIGS. 4A &amp; 4B ). In certain implementations, the bending force is no greater than about 150 N, 140 N, 130 N, 120 N, 110 N, 100 N, 90 N, 80 N, 70 N, 60 N, 50 N, 40 N, 30 N, 20 N, 10 N, 5 N, or any amount between these bending force upper limits, upon bending of the module to a radius from about 20 mm to about 3 mm (i.e., a plate distance (D) of about 40 to about 6 mm), for example, 20 mm, 19.75 mm, 19.5 mm, 19.25 mm, 19 mm, 18.5 mm, 17.5 mm, 17 mm, 16.5 mm, 16 mm, 15.5 mm, 15 mm, 14.5 mm, 14 mm, 13.5 mm, and 13 mm, 12.5 mm, 12 mm, 11.5 mm, 11 mm, 10.5 mm, 10 mm, 9.5 mm, 9 mm, 8.5 mm, 7.5 mm, 7 mm, 6.5 mm, 6 mm, 5.5 mm, 5 mm, 4.5 mm, 4 mm, 3.5 mm, 3.25 mm and 3 mm. As outlined earlier, these relatively low bending forces can be obtained in the foldable electronic device module  100   a  through tailoring of the material properties and/or thickness of the first adhesive  10   a.    
     In other aspects of the foldable electronic device module  100   a  depicted in  FIG. 1 , the cover element  50  can characterized by an absence of failure when the element is held at the bend radius  220  (see  FIG. 4B ) from about 2 mm to 20 mm for at least 60 minutes at about 25° C. and about 50% relative humidity. As used herein, the terms “fail,” “failure” and the like refer to breakage, destruction, delamination, crack propagation or other mechanisms that leave the foldable modules, assemblies and articles of this disclosure unsuitable for their intended purpose. When the cover element  50  is held at the bend radius  220  under these conditions (i.e., by virtue of a bending applied to the module  100   a ), bending forces are applied to the ends of the element  50 . In most (if not all) aspects of the foldable electronic device modules  100   a , tensile stresses are generated at the second primary surface  56  of the element  50  and compressive stresses are generated at the first primary surface  54  during the application of bending forces to the foldable module  100   a  such that the first primary surface  54  is bent upwards into a concave shape (see  FIG. 4B ). In other aspects, the bend radius  220  can be set to a range from about 5 mm to 7 mm without causing a failure in the cover element  50 . Without being bound by theory, it is believed that the cover element  50  also can be characterized, in certain aspects of the disclosure, by an absence of failure when the element  50  (including the entire foldable module  100   a ) is held at a bend radius  220  from about 3 mm to about 10 mm for at least 120 hours at about 25° C. and about 50% relative humidity. It should also be understood that bend testing results associated with the foldable electronic device modules  100   a  depicted in  FIG. 1  can vary under testing conditions with temperatures and/or humidity levels that differ from the foregoing test parameters. 
     In some aspects of the foldable module  100   a , the cover element  50  is characterized by a high-cycle fatigue stress resistance. In particular, the cover element  50  can be characterized by no cohesive failures upon bending the module, in a two-point configuration, from an un-bent configuration to a constant, defined bend radius  220  (see  FIGS. 4A &amp; 4B ) (i.e., ranging from 20 mm to about 2 mm) for at least 200,000 bend cycles. In other aspects, the cover element  50  is characterized by no cohesive failures upon bending the module, in a two-point configuration, from an un-bent configuration to a bend radius  220  that ranges from about 20 mm to about 2 mm for about 100,000 cycles, 110,000 cycles, 120,000 cycles, 130,000 cycles, 140,000 cycles, 150,000 cycles, 160,000 cycles, 170,000 cycles, 180,000 cycles, 190,000 cycles, 200,000 cycles, or any amount of bend cycles between these values. In certain other aspects of the foldable module  100   a  subjected to a bend radius  220  greater than about 20 mm up to about 100 mm, the high-cycle fatigue stress resistance of the cover element can be substantially reduced through careful selection of the elastic modulus and/or the thickness of the adhesives in the module. 
     In certain aspects of the foldable module  100   a , the cover element  50  can include a glass layer. In other aspects, the cover element  50  can include two or more glass layers. As such, the thickness  52  reflects the sum of the thicknesses of the individual glass layers making up the cover element  50 . In those aspects in which the cover element  50  includes two or more individual glass layers, the thickness of each of the individual glass layers is no less than 1 μm. For example, the cover element  50  employed in the module  100   a  can include three glass layers, each having a thickness of about 8 μm, such that the thickness  52  of the cover element  50  is about 24 μm. It should also be understood, however, that the cover element  50  could include other non-glass layers (e.g., compliant polymer layers) sandwiched between multiple glass layers. In other implementations of the module  100   a , the cover element  50  can include one or more layers that include glass-containing materials, e.g., element  50  can be a polymer/glass composite configured with second phase glass particles in a polymeric matrix. 
     In  FIG. 1 , a foldable electronic device module  100   a  including a cover element  50  comprising a glass material can be fabricated from alkali-free aluminosilicate, borosilicate, boroaluminosilicate, and silicate glass compositions. The cover element  50  can also be fabricated from alkali-containing aluminosilicate, borosilicate, boroaluminosilicate, and silicate glass compositions. In certain aspects, alkaline earth modifiers can be added to any of the foregoing compositions for the cover element  50 . In one exemplary aspect, glass compositions according to the following are suitable for a cover element  50  having one or more glass layers: SiO 2  at 64 to 69% (by mol %); Al 2 O 3  at 5 to 12%; B 2 O 3  at 8 to 23%; MgO at 0.5 to 2.5%; CaO at 1 to 9%; SrO at 0 to 5%; BaO at 0 to 5%; SnO 2  at 0.1 to 0.4%; ZrO 2  at 0 to 0.1%; and Na 2 O at 0 to 1%. In another exemplary aspect, the following composition is suitable for the glass layer  50   a : SiO 2  at ˜67.4% (by mol %); Al 2 O 3  at ˜12.7%; B 2 O 3  at ˜3.7%; MgO at ˜2.4%; CaO at 0%; SrO at 0%; SnO 2  at ˜0.1%; and Na 2 O at ˜13.7%. In a further exemplary aspect, the following composition is also suitable for a glass layer employed in the cover element  50 : SiO 2  at 68.9% (by mol %); Al 2 O 3  at 10.3%; Na 2 O at 15.2%; MgO at 5.4%; and SnO 2  at 0.2%. Various criteria can be used to select the composition for a cover element  50  comprising a glass material, including but not limited to ease of manufacturing to low thickness levels while minimizing the incorporation of flaws; ease of development of a compressive stress region to offset tensile stresses generated during bending, optical transparency; and corrosion resistance. 
     The cover element  50  employed in the foldable module  100   a  can adopt a variety of physical forms and shapes. From a cross-sectional perspective, the element  50 , as a single layer or multiple layers, can be flat or planar. In some aspects, the element  50  can be fabricated in non-rectilinear, sheet-like forms depending on the final application. As an example, a mobile display device having an elliptical display and bezel could employ a cover element  50  having a generally elliptical, sheet-like form. 
     Still referring to  FIG. 1 , the cover element  50  of the foldable electronic device module  100   a  can, in certain aspects of the disclosure, comprise a glass layer or component with one or more compressive stress regions (not shown) that extend from the first and/or second primary surfaces  54 ,  56  to a selected depth in the cover element  50 . Further, in certain aspects of the module  100   a , edge compressive stress regions (not shown) that extend from edges of the element  50  (e.g., as normal or substantially normal to primary surfaces  54 ,  56 ) to a selected depth can also be developed. For example, the compressive stress region or regions (and/or edge compressive stress regions) contained in a glass cover element  50  can be formed with an ion-exchange (“IOX”) process. As another example, a glass cover element  50  can comprise various tailored glass layers and/or regions that can be employed to develop one or more such compressive stress regions through a mismatch in coefficients of thermal expansion (“CTE”) associated with the layers and/or regions. 
     In those aspects of the device module  100   a  with a cover element  50  having one or more compressive stress regions formed with an IOX process, the compressive stress region(s) can include a plurality of ion-exchangeable metal ions and a plurality of ion-exchanged metal ions, the ion-exchanged metal ions selected so as to produce compressive stress in the compressive stress region(s). In some aspects of the module  100   a  containing compressive stress region(s), the ion-exchanged metal ions have an atomic radius larger than the atomic radius of the ion-exchangeable metal ions. The ion-exchangeable ions (e.g., Na +  ions) are present in the glass cover element  50  before being subjected to the ion exchange process. Ion-exchanging ions (e.g., K +  ions) can be incorporated into the glass cover element  50 , replacing some of the ion-exchangeable ions within region(s) within the element  50  that ultimately become the compressive stress region(s). The incorporation of ion-exchanging ions, for example, K +  ions, into the cover element  50  can be effected by submersing the element  50  (e.g., prior to formation of the complete module  100   a ) in a molten salt bath containing ion-exchanging ions (e.g., molten KNO 3  salt). In this example, the K +  ions have a larger atomic radius than the Na +  ions and tend to generate local compressive stress in the glass cover element  50  wherever present, e.g., in the compressive stress region(s). 
     Depending on the ion-exchanging process conditions employed for the cover element  50  employed in the foldable electronic device module  100   a  depicted in  FIG. 1 , the ion-exchanging ions can be imparted from the first primary surface  54  of the cover element  50  down to a first ion exchange depth (not shown), establishing an ion exchange depth-of-layer (“DOL”). Similarly, a second compressive stress region can be developed in the element  50  from the second primary surface  56  down to a second ion exchange depth. Compressive stress levels within the DOL that far exceed 100 MPa can be achieved with such IOX processes, up to as high as 2000 MPa. The compressive stress levels in the compressive stress region(s) within the cover element  50  can serve to offset tensile stresses generated in the cover element  50  upon bending of the foldable electronic device module  100   a.    
     Referring again to  FIG. 1 , the foldable electronic device module  100   a  can, in some implementations, include one or more edge compressive stress regions in the cover element  50  at edges that are normal to the first and second primary surfaces  54 ,  56 , each defined by a compressive stress of at least 100 MPa. It should be understood that such edge compressive stress regions can be developed in the cover element  50  at any of its edges or surfaces distinct from its primary surfaces, depending on the shape or form of element  50 . For example, in an implementation of foldable module  100   a  having an elliptical-shaped cover element  50 , edge compressive stress regions can be developed inward from the outer edge of the element that is normal (or substantially normal) from the primary surfaces of the element. IOX processes that are similar in nature to those employed to generate the compressive stress region(s) in proximity to the primary surfaces  54 ,  56  can be deployed to produce these edge compressive stress regions. More specifically, any such edge compressive stress regions in the cover element  50  can be used to offset tensile stresses generated at the edges of the element through, for example, bending of the cover element  50  (and module  100   a ) across any of its edges and/or non-uniform bending of the cover element  50  at its primary surfaces  54 ,  46 . Alternatively, or as an addition thereto, without being bound by theory, any such edge compressive stress regions employed in the cover element  50  may offset adverse effects from an impact or abrasion event at or to the edges of the element  50  within the module  100   a.    
     Referring again to  FIG. 1 , in those aspects of the device module  100   a  with a cover element  50  having one or more compressive stress regions formed through a mismatch in CTE of regions or layers within the element  50 , these compressive stress regions are developed by tailoring of the structure of the element  50 . For example, CTE differences within the element  50  can produce one or more compressive stress regions within the element. In one example, the cover element  50  can comprise a core region or layer that is sandwiched by clad regions or layers, each substantially parallel to the primary surfaces  54 ,  56  of the element. Further, the core layer is tailored to a CTE that is greater than the CTE of the clad regions or layers (e.g., by compositional control of the core and clad layers or regions). After the cover element  50  is cooled from its fabrication processes, the CTE differences between the core region or layer and the clad regions or layers cause uneven volumetric contraction upon cooling, leading to the development of residual stress in the cover element  50  manifested in the development of compressive stress regions below the primary surfaces  54 ,  56  within the clad region or layers. Put another way, the core region or layer and the clad regions or layers are brought into intimate contact with one another at high temperatures; and these layers or regions are then cooled to a low temperature such that the greater volume change of the high CTE core region (or layer) relative to the low CTE clad regions (or layers) creates the compressive stress regions in the clad regions or layers within the cover element  50 . 
     Still referring to the cover element  50  in the module  100   a  that is depicted in  FIG. 1  with CTE-developed compressive stress regions, the CTE-related compressive stress regions reach from the first primary surface  54  down to a first CTE region depth and the second primary surface  56  to a second CTE region depth, respectively, thus establishing CTE-related DOLs for each of the compressive stress regions associated with the respective primary surfaces  54 ,  56  and within the clad layer or regions. In some aspects, the compressive stress levels in these compressive stress regions can exceed 150 MPa. Maximizing the difference in CTE values between the core region (or layer) and the clad regions (or layers) can increase the magnitude of the compressive stress developed in the compressive stress regions upon cooling of the element  50  after fabrication. In certain implementations of the foldable electronic device module  100   a  with a cover element  50  having such CTE-related compressive stress regions, the cover element  50  employs a core region and clad regions with a thickness ratio of greater than or equal to 3 for the core region thickness divided by the sum of the clad region thicknesses. As such, maximizing the size of the core region and/or its CTE relative to the size and/or CTE of the clad regions can serve to increase the magnitude of the compressive stress levels observed in the compressive stress regions of the foldable module  100   a.    
     Among other advantages, the compressive stress regions (e.g., as developed through the IOX- or CTE-related approaches outlined in the foregoing paragraphs) can be employed within the cover element  50  to offset tensile stresses generated in the element upon bending of the foldable module  100   a , particularly tensile stresses that reach a maximum on one of the primary surfaces  54 ,  56 , depending on the direction of the bend. In certain aspects, the compressive stress region can include a compressive stress of at least about 100 MPa at the primary surfaces  54 ,  56  of the cover element  50 . In some aspects, the compressive stress at the primary surfaces is from about 600 MPa to about 1000 MPa. In other aspects, the compressive stress can exceed 1000 MPa at the primary surfaces, up to 2000 MPa, depending on the process employed to produce the compressive stress in the cover element  50 . The compressive stress can also range from about 100 MPa to about 600 MPa at the primary surfaces of the element  50  in other aspects of this disclosure. In additional aspect, the compressive stress region (or regions) within the cover element  50  of the module  100   a  can exhibit a compressive stress from about 100 MPa to about 2000 MPa, for example, from about 100 MPa to about 1500 MPa, from about 100 MPa to about 1000 MPa, from about 100 MPa to about 800 MPa, from about 100 MPa to about 600 MPa, from about 100 MPa to about 400 MPa, from about 100 MPa to about 200 MPa, from about 200 MPa to about 1500 MPa, from about 200 MPa to about 1000 MPa, from about 200 MPa to about 800 MPa, from about 200 MPa to about 600 MPa, from about 200 MPa to about 400 MPa, from about 400 MPa to about 1500 MPa, from about 400 MPa to about 1000 MPa, from about 400 MPa to about 800 MPa, from about 400 MPa to about 600 MPa, from about 600 MPa to about 1500 MPa, from about 600 MPa to about 1000 MPa, from about 600 MPa to about 800 MPa, from about 800 MPa to about 1500 MPa, from about 800 MPa to about 1000 MPa, and from about 1000 MPa to about 1500 MPa. 
     Within such a compressive stress region employed in the cover element  50  of a foldable electronic device module  100   a , the compressive stress can stay constant, decrease or increase as a function of depth from the primary surfaces down to one or more selected depths. As such, various compressive stress profiles can be employed in the compressive stress region. Further, the depth of each of the compressive stress regions can be set at approximately 15 μm or less from the primary surfaces  54 ,  56  of the cover element  50 . In other aspects, the depth of the compressive stress region(s) can be set such that they are approximately ⅓ of the thickness  52  of the cover element  50  or less, or 20% of the thickness  52  of the cover element  50  or less, from the first and/or second primary surfaces  54 ,  56 . 
     Referring again to  FIG. 1 , the foldable electronic device module  100   a  can include a cover element  50  comprising a glass material having one or more compressive stress regions with a maximum flaw size of 5 μm or less at the first and/or second primary surfaces  54 ,  56 . The maximum flaw size can also be held to about 2.5 μm or less, 2 μm or less, 1.5 μm or less, 0.5 μm or less, 0.4 μm or less, or even smaller flaw size ranges. Reducing the flaw size in the compressive stress region of a glass cover element  50  can further reduce the propensity of the element  50  to fail by crack propagation upon the application of tensile stresses by virtue of bending forces to the foldable module  100   a  (see  FIG. 4B ). In addition, some aspects of the foldable device module  100   a  can include a surface region with a controlled flaw size distribution (e.g., flaw sizes of 0.5 μm or less at the first and/or second primary surfaces  54 ,  56 ) without employing one or more compressive stress regions. 
     Referring to  FIGS. 1 and 4A , bending forces, F bend , applied to the foldable electronic device module  100   a  can result in tensile stresses at the second primary surface  56  of the cover element  50 , e.g. at point “T” shown in  FIG. 4A . Tighter (i.e., smaller) bending radii  220  lead to higher tensile stresses. Further, tighter bending radii  220  also require increasingly higher bending forces, F bend , to bend or otherwise fold the module  100   a  to the desired radii  220 . Equation (1) below can be used to estimate the maximum tensile stresses in the cover element  50 , particularly at the second primary surface  56  of the cover element  50 , subjected to bending with a constant bend radius  220 . Equation (1) is given by: 
                     σ   max     =       E     1   -     v   2         ⁢     h   2     ⁢     1   R               (   1   )               
where E is the Young&#39;s modulus of the glass cover element  50 , v is the Poisson&#39;s ratio of the cover element  50  (typically v is ˜0.2-0.3 for most glass compositions), h is reflective of the thickness  52  of the cover element, and R is the bend radius of curvature (comparable to bend radius  220 ). Using Equation (1), it is apparent that maximum bending stresses are linearly dependent on the thickness  52  of the glass cover element  50  and elastic modulus, and inversely dependent on the bend radius  220  of curvature of the glass cover element  50 .
 
     The bending forces, F bend , applied to the foldable module  100   a  and, particularly the cover element  50 , could also result in the potential for crack propagation leading to instantaneous or slower, fatigue failure mechanisms within the element  50 . The presence of flaws at the second primary surface  56 , or just beneath the surface, of the element  50  can contribute to these potential failure modes. Using Equation (2) below, it is possible to estimate the stress intensity factor in a glass cover element  50  subjected to bending forces, F bend . Equation (2) is given by: 
                   K   =       Y   ⁢           ⁢   σ   ⁢       π   ⁢           ⁢   a         =       YE     1   -     v   2         ⁢     h   2     ⁢     1   R     ⁢       π   ⁢           ⁢   a                   (   2   )               
where a is the flaw size, Y is a geometry factor (generally assumed to be 1.12 for cracks emanating from a glass edge, a typical failure mode), and σ is the bending stress associated with the bending forces, F bend , as estimated using Equation (1). Equation (2) assumes that the stress along the crack face is constant, which is a reasonable assumption when the flaw size is small (e.g., &lt;1 μm). When the stress intensity factor K reaches the fracture toughness of the glass cover element  50 , K IC , instantaneous failure will occur. For most compositions suitable for use in glass cover element  50 , K IC  is ˜0.7 MPa√m. Similarly, when K reaches a level at or above a fatigue threshold, K threshold , failure can also occur via slow, cyclic fatigue loading conditions. A reasonable assumption for K threshold  is ˜0.2 MPa√m. However, K threshold  can be experimentally determined and is dependent upon the overall application requirements (e.g., a higher fatigue life for a given application can increase K threshold ). In view of Equation (2), the stress intensity factor can be reduced by reducing the overall tensile stress level and/or the flaw size at the primary surfaces of the glass cover element  50 , particularly at those surfaces likely subject to high tensile stresses upon bending.
 
     According to some aspects of foldable electronic device module  100   a , the tensile stress and stress intensity factor estimated through Equations (1) and (2) can be minimized through the control of the stress distribution at the second primary surface  56  of the glass cover element  50 . In particular, a compressive stress profile (e.g., through one or more of the CTE- or IOX-related compressive stress regions outlined in the foregoing paragraphs) at and below the second primary surface  56  is subtracted from the bending stress calculated in Equation (1). As such, overall bending stress levels are beneficially reduced which, in turn, also reduces the stress intensity factors that can be estimated through Equation (2). 
     Again referring to  FIG. 1 , other implementations of the foldable electronic device module  100   a  can include a cover element  50  comprising a glass material subjected to various etching processes that are tailored to reduce the flaw sizes and/or improve the flaw distribution within the element  50 . These etching processes can be employed to control the flaw distributions within the cover element  50  in close proximity to its primary surfaces  54 ,  56 , and/or along its edges (not shown). For example, an etching solution containing about 15 vol % HF and 15 vol % HCl can be employed to lightly etch the surfaces of a cover element  50  having a glass composition. The time and temperature of the light etching can be set, as understood by those with ordinary skill, according to the composition of the element  50  and the desired level of material removal from the surfaces of the cover element  50 . It should also be understood that some surfaces of the element  50  can be left in an un-etched state by employing masking layers or the like to such surfaces during the etching procedure. More particularly, this light etching can beneficially improve the strength of the cover element  50 . In particular, cutting or singling processes employed to section the glass structure that is ultimately employed as the cover element  50  can leave flaws and other defects within the surfaces of the element  50 . These flaws and defects can propagate and cause glass breakage during the application of stresses to the module  100   a  containing the element  50  from the application environment and usage. The selective etching process, by virtue of lightly etching one or more edges of the element  50 , can remove at least some of the flaws and defects, thereby increasing the strength and/or fracture resistance of the lightly-etched surfaces, e.g., as demonstrated in the foregoing paragraphs in view of Equation (1) and (2). 
     It should also be understood that the cover element  50  employed in the foldable module  100   a  depicted in  FIG. 1  can include any one or more of the foregoing strength-enhancing features: (a) IOX-related compressive stress regions; (b) CTE-related compressive stress regions; and (c) etched surfaces with smaller defect sizes. These strength-enhancing features can be used to offset or partially offset tensile stresses generated at the surfaces of the cover element  50  associated with the application environment, usage and processing of the foldable electronic device module  100   a.    
     As outlined above, the foldable electronic device module  100   a  depicted in  FIG. 1  includes an adhesive  10   a  with certain material properties (e.g., a shear modulus from about 0.1 MPa to 100 MPa). Example adhesives that can be employed as the adhesive  10   a  in the module  100   a  include optically clear adhesives (“OCAs”) (e.g., Henkel Corporation LOCTITE® liquid OCAs), epoxies, and other joining materials as understood by those with ordinary skill in the field that are suitable to join the stack  90   a  to the second primary surface  56  of the cover element  50 . In some aspects of the module  100   a , the adhesive  10   a  will also possess a high thermal resistance such that its material properties experience little to no change upon being subjected to various temperatures (e.g., 500 hours at −40° C. and about +85° C.), humidity and high temperature (e.g., 500 hours at +65° C. at 95% R.H.), and temperature gradients (e.g., 200 thermal shock cycles, each cycle given by one hour at −40° C. followed one hour at +85° C.) in the application environment, including those generated by friction from bending of the foldable electronic device module  100   a . Further, the adhesive  10   a  may have high resistance to ultraviolet light exposure and high peel adhesion properties comparable to those exhibited by  3 M Company  8211 ,  8212 ,  8213 ,  8214  and  8215  OCAs. 
     As also outlined above, the foldable electronic device module  100   a  depicted in  FIG. 1  includes a panel  60  having a panel elastic modulus from about 300 MPa to about 10 GPa, for example, from 300 MPa to about 5000 MPa, from 300 MPa to about 2500 MPa, from 300 MPa to about 1000 MPa, from 300 MPa to about 750 MPa, from 300 MPa to about 500 MPa, from 500 MPa to about 5000 MPa, from 500 MPa to about 2500 MPa, from 500 MPa to about 1000 MPa, from 500 MPa to about 750 MPa, from 750 MPa to about 5000 MPa, from 750 MPa to about 2500 MPa, from 750 MPa to about 1000 MPa, from 1000 MPa to about 5000 MPa, from 1000 MPa to about 2500 MPa, and from 2500 MPa to about 5000 MPa. In some aspects, the panel elastic modulus of the panel  60  is about 350 MPa, 400 MPa, 450 MPa, 500 MPa, 550 MPa, 600 MPa, 650 MPa, 700 MPa, 750 MPa, 800 MPa, 850 MPa, 900 MPa, 950 MPa, 1000 MPa, 2 GPa, 3 GPa, 4 GPa, 5 GPa, 6 GPa, 7 GPa, 8 GPa, 9 GPa, 10 GPa, or any elastic modulus value between these values. Suitable materials that can be employed as the panel  60  in the module  100   a  include various thermoset and thermoplastic materials, e.g., polyimides, suitable for mounting electronic devices  102  and possessing high mechanical integrity and flexibility when subjected to the bending associated with the foldable electronic device module  100   a . For example, panel  60  may be an organic light emitting diode (“OLED”) display panel. The material selected for the panel  60  may also exhibit a high thermal stability to resist material property changes and/or degradation associated with the application environment for the module  100   a  and/or its processing conditions. 
     In some implementations, the foldable electronic device module  100   a  depicted in  FIG. 1  can be employed in a display, printed circuit board, housing or other features associated with an end product electronic device. For example, the foldable module  100   a  can be employed in an electronic display device containing numerous thin film transistors (“TFTs”) or in an LCD or OLED device containing a low-temperature polysilicon (“LTPS”) backplane. When the foldable module  100   a  is employed in a display, for example, the module  100   a  can be substantially transparent. Further, the module  100   a  can have pencil hardness, bend radius, puncture resistance and/or optimized bending force capabilities as described in the foregoing paragraphs. In one exemplary implementation, the foldable electronic device module  100   a  is employed in a wearable electronic device, for example, a watch, wallet or bracelet. As defined herein, “foldable” includes complete folding, partial folding, bending, flexing, discrete bends, and multiple-fold capabilities. 
     Referring now to  FIG. 2 , a foldable electronic device module  100   b  is provided with many features in common with the foldable electronic device module  100   a  (see  FIG. 1 ). Unless otherwise noted, any features in common between the modules  100   a  and  100   b  (i.e., with the same element numbers) have the same or similar construction, features and properties. As shown in  FIG. 2 , the module  100   b  includes a cover element  50  having a thickness from about 25 μm to about 200 μm and a cover element elastic modulus from about 20 GPa to about 140 GPa. The cover element  50  further includes a glass composition or a component having a glass composition, a first primary surface  54 , and a second primary surface  56 . 
     The module  100   b  depicted in  FIG. 2  further includes: a stack  90   b  having a thickness  92   b  from about 100 μm to about 600 μm; and a first adhesive  10   a  configured to join the stack element  75  to the second primary surface  56  of the cover element  50 . In the module  100   b , the first adhesive  10   a  is characterized by a shear modulus between about 1 MPa and about 1 GPa, for example, from about 0.1 MPa to about 800 MPa, from about 0.1 MPa to about 600 MPa, from about 0.1 MPa to about 400 MPa, from about 0.1 MPa to about 200 MPa, from about 0.1 MPa to about 1 MPa, from about 1 MPa to about 800 MPa, from about 1 MPa to about 600 MPa, from about 1 MPa to about 400 MPa, from about 1 MPa to about 200 MPa, from about 200 MPa to about 800 MPa, from about 200 MPa to about 600 MPa, from about 200 MPa to about 400 MPa, from about 400 MPa to about 800 MPa, from about 400 MPa to about 600 MPa, and from about 600 MPa to about 800 MPa. In some aspects of the module  100   b , the first adhesive  10   a  is characterized by a shear modulus of 0.1 MPa, 0.2 MPa, 0.3 MPa, 0.4 MPa, 0.5 MPa, 0.6 MPa, 0.7 MPa, 0.8 MPa, 0.9 MPa, 1 MPa, 5 MPa, 10 MPa, 20 MPa, 30 MPa, 40 MPa, 50 MPa, 60 MPa, 70 MPa, 80 MPa, 90 MPa, 100 MPa, 200 MPa, 300 MPa, 400 MPa, 500 MPa, 600 MPa, 700 MPa, 800 MPa, 900 MPa, 1000 MPa, or any amount between these shear modulus values. Aspects of the foldable module  100   b  incorporate an adhesive  10   a  with a relatively higher shear modulus, e.g., from about 1 MPa to about 1000 MPa (i.e., 1 GPa), compared to the shear modulus of conventional adhesives typically employed in such electronic device applications. The use of such adhesives  10   a  with relatively higher shear modulus values unexpectedly provides a significant decrease in tensile stresses observed at the second primary surface  56  of the cover element  50  upon bending the foldable electronic device module  100   b  in a direction away from the second primary surface  56 —i.e., by bending the module  100   b  such that the second primary surface  56  exhibits a convex shape. 
     Still referring to  FIG. 2 , certain aspects of the foldable module  100   b  can be configured to minimize bending forces associated with bending the entire module by controlling the shear modulus of one or more of the adhesives employed within the module  100   b . More particularly, the use of a first adhesive  10   a  with a relatively low shear modulus value (e.g., from about 0.01 MPa to about 0.1 MPa) can reduce the overall bending force required to fold or otherwise bend the entire module  100   b  in an upward or downward direction such that the first primary surface  54  exhibits a concave or convex shape, respectively. These bending force reductions associated with certain aspects of the foldable module  100   b  through the use of a first adhesive  10   a  with a relatively low elastic shear modulus value are obtained relative to a foldable module (e.g., foldable module  100   b ) with an adhesive between the cover element and the stack (e.g., first adhesive  10   a ) having a shear modulus that exceeds 0.1 MPa. 
     Referring again to the foldable electronic device module  100   b  depicted in  FIG. 2 , the stack  90   b  further includes a panel  60  having first and second primary surfaces  64 ,  66 , and a panel elastic modulus from about 300 MPa to 10 GPa. The stack  90   b  also includes one or more electronic devices  102  coupled to or within the panel  60 , and a stack element  75  having a stack element elastic modulus from about 1 GPa to about 5 GPa, with the stack element being affixed to the panel  60  with a stack adhesive  10   b . As outlined earlier in connection with the module  100   a  (see  FIG. 1 ), the stack element  75  can include various components, including but not limited to a touch sensor, polarizer, touch sensor components (e.g., electrode layers), thin film transistors, driving circuits, sources, drains, doped regions, and other electronic device and electronic device components, other adhesives, and joining materials. Collectively, these features possess an elastic modulus between about 1 GPa and about 10 GPa within the foldable electronic device module  100   b . It should also be understood that the relationship between the panel  60 , stack element  75  and electronic devices  102  (e.g., as located within the panel  60 ) is depicted schematically in  FIG. 2 . Depending on the application for the device module  100   b , these elements may have different orientations with regard to one another. For example, panel  60  can be an LCD panel or an OLED display in which the electronic devices  102  are sandwiched within the panel  60  (e.g., as shown schematically in  FIG. 2 ) by two glass layers (not shown), or a polymeric substrate encapsulated by a glass sealing layer, for instance. In another example, as schematically shown in  FIG. 3  and discussed further below, the electronic devices  102  can be aspects of a touch sensor (e.g., electronic trace lines in a transparent conductor such as indium tin oxide, silver nanowires, etc.) located at a higher vertical position within the stack  75 , above the panel  60  and stack adhesive  10   b.    
     With regard to the stack adhesive  10   b  employed in the foldable electronic device module  100   b , its composition can be selected to join the stack element  75  to the panel  60  with a bond strength suitable for the application employing the module  100   b . According to some implementations of the foldable modules  100   b  of the second aspect of the disclosure, the stack adhesive  10   b  is characterized by a shear modulus from about 10 kPa to about 100 kPa, for example, from about 10 kPa to about 90 kPa, from about 10 kPa to about 80 kPa, from about 10 kPa to about 70 kPa, from about 10 kPa to about 60 kPa, from about 10 kPa to about 50 kPa, from about 10 kPa to about 40 kPa, from about 10 kPa to about 30 kPa, from about 10 kPa to about 30 kPa, from about 20 kPa to about 90 kPa, from about 20 kPa to about 80 kPa, from about 20 kPa to about 70 kPa, from about 20 kPa to about 60 kPa, from about 20 kPa to about 50 kPa, from about 20 kPa to about 40 kPa, from about 20 kPa to about 30 kPa, from about 30 kPa to about 90 kPa, from about 30 kPa to about 80 kPa, from about 30 kPa to about 70 kPa, from about 30 kPa to about 60 kPa, from about 30 kPa to about 50 kPa, from about 30 kPa to about 40 kPa, from about 40 kPa to about 90 kPa, from about 40 kPa to about 80 kPa, from about 40 kPa to about 70 kPa, from about 40 kPa to about 60 kPa, from about 40 kPa to about 50 kPa, from about 50 kPa to about 90 kPa, from about 50 kPa to about 80 kPa, from about 50 kPa to about 70 kPa, from about 50 kPa to about 60 kPa, from about 60 kPa to about 90 kPa, from about 60 kPa to about 80 kPa, from about 60 kPa to about 70 kPa, from about 70 kPa to about 90 kPa, from about 70 kPa to about 80 kPa, and from about 80 kPa to about 90 kPa. In this aspect, the stack adhesive  10   b  may also be characterized by a shear modulus of about 10 kPa, 20 kPa, 25 kPa, 30 kPa, 35 kPa, 40 kPa, 45 kPa, 50 kPa, 55 kPa, 60 kPa, 65 kPa, 70 kPa, 75 kPa, 80 kPa, 85 kPa, 90 kPa, 95 kPa, 100 kPa, or any shear modulus value between these values. Aspects of the foldable modules  100   b  incorporate a stack adhesive  10   b  with a relatively lower shear modulus, e.g., from about 10 kPa to about 100 kPa, compared to the shear modulus of conventional adhesives typically employed in such electronic device applications. The use of such adhesives  10   b  with relatively lower shear modulus values unexpectedly provides a significant decrease in tensile stresses observed at the first primary surface  64  of the panel  60  upon bending the foldable electronic device module  100   b  in a direction away from the second primary surface  66 —i.e., by bending the module  100   b  such that the second primary surface  66  exhibits a convex shape. 
     Referring again to  FIG. 2 , certain aspects of the foldable module  100   b  can be configured to minimize bending forces associated with bending the entire module by controlling the shear modulus of one or more of the adhesives employed within the module  100   b . For example, the use of stack adhesive  10   b  with a relatively low shear modulus value (e.g., from about 0.01 MPa to about 0.1 MPa) can unexpectedly reduce the overall bending force required to fold or otherwise bend the entire module  100   b  in an upward or downward direction such that the first primary surface  54  exhibits a concave or convex shape, respectively. Moreover, other aspects of the foldable module  100   b  can be configured to minimize bending forces associated with bending the entire module by controlling the shear modulus of the first adhesive  10   a  and the shear modulus of the stack adhesive  10   b  (e.g., both adhesives having a shear modulus from about 0.01 MPa to about 0.1 MPa). These bending force reductions associated with certain aspects of the foldable module  100   b  through the use of a first adhesive  10   a  and/or a stack adhesive  10   b  with a relatively low elastic shear modulus value are obtained relative to a foldable module (e.g., foldable module  100   b ) with one or more adhesives (e.g., adhesives  10   a ,  10   b ) having a shear modulus that exceeds 0.1 MPa. 
     According to other implementations of the foldable modules  100   b  (see  FIG. 2 ) of the second aspect of the disclosure, the stack adhesive  10   b  is characterized by a thickness  12   b  from about 5 μm to about 60 μm, for example, from about 5 μm to about 50 μm, from about 5 μm to about 40 μm, from about 5 μm to about 30 μm, from about 5 μm to about 20 μm, from about 5 μm to about 15 μm, from about 5 μm to about 10 μm, from about 10 μm to about 60 μm, from about 15 μm to about 60 μm, from about 20 μm to about 60 μm, from about 30 μm to about 60 μm, from about 40 μm to about 60 μm, from about 50 μm to about 60 μm, from about 55 μm to about 60 μm, from about 10 μm to about 50 μm, from about 10 μm to about 40 μm, from about 10 μm to about 30 μm, from about 10 μm to about 20 μm, from about 10 μm to about 15 μm, from about 20 μm to about 50 μm, from about 30 μm to about 50 μm, from about 40 μm to about 50 μm, from about 20 μm to about 40 μm, and from about 20 μm to about 30 μm. Other embodiments have a stack adhesive  10   b  characterized by a thickness  12   b  of about 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, or any thickness between these thickness values. In one aspect, the thickness  12   b  of the stack adhesive  10   b  is from about 30 μm to about 60 μm. The use of such adhesives  10   b  with relatively higher thickness values unexpectedly provides a significant decrease in tensile stresses observed at the first primary surface  64  of the panel  60  upon bending the foldable electronic device module  100   b  in a direction away from the second primary surface  66  of the panel. While it is believed that further increases in the thickness  12   b  of the adhesive  10   b  will result in additional reductions in tensile stresses observed at the first primary surface  64  of the panel  60 , the thickness  12   b  can be limited by application requirements aimed at minimizing the overall thickness  92   b  of the stack  90   b.    
     Still referring to  FIG. 2 , certain aspects of the foldable module  100   b  can be configured to minimize bending forces associated with bending the entire module by controlling the thickness of the first adhesive  10   a  and/or the stack adhesive  10   b . More particularly, the use of a first adhesive  10   a  with a range of thicknesses  12   a  (e.g., from about 10 μm to about 40 μm) and/or the stack adhesive  10   b  with a range of thicknesses  12   b  (e.g., from about 10 μm to 40 μm) can reduce the overall bending force required to fold or otherwise bend the entire module  100   b  in an upward or downward direction such that the first primary surface  54  exhibits a concave or convex shape, respectively. These bending force reductions associated with certain aspects of the foldable module  100   b  through the use of a first adhesive  10   a  and/or a stack adhesive  10   b  within a prescribed range of thicknesses are obtained relative to a foldable module (e.g., foldable module  100   b ) with one or more adhesives (e.g., first adhesive  10   a  and/or a stack adhesive  10   b ) having a relatively small thickness (e.g. less than 10 μm) or a relatively large thickness (e.g., more than 40 μm). 
     Referring again to  FIG. 2 , the foldable electronic device module  100   b , according to another implementation, can be characterized by a bending force (F bend ) of no greater than 150 Newtons (N) as the module is bent inward by a test apparatus to a bend radius  220 , the bend radius being approximately half the distance (D) between two test plates  250  (see  FIGS. 4A &amp; 4B ). In certain implementations, the bending force is no greater than about 150 N, 140 N, 130 N, 120 N, 110 N, 100 N, 90 N, 80 N, 70 N, 60 N, 50 N, 40 N, 30 N, 20 N, 10 N, 5 N, or any amount between these bending force upper limits, upon bending of the module to a radius from about 20 mm to about 3 mm (i.e., a plate distance (D) of about 40 to about 6 mm), for example, 20 mm, 19.75 mm, 19.5 mm, 19.25 mm, 19 mm, 18.5 mm, 17.5 mm, 17 mm, 16.5 mm, 16 mm, 15.5 mm, 15 mm, 14.5 mm, 14 mm, 13.5 mm, and 13 mm, 12.5 mm, 12 mm, 11.5 mm, 11 mm, 10.5 mm, 10 mm, 9.5 mm, 9 mm, 8.5 mm, 7.5 mm, 7 mm, 6.5 mm, 6 mm, 5.5 mm, 5 mm, 4.5 mm, 4 mm, 3.5 mm, 3.25 mm and 3 mm. As outlined earlier, these relatively low bending forces can be obtained in the foldable electronic device module  100   b  through tailoring of the material properties and/or thicknesses of the first adhesive  10   a  and/or the stack adhesive  10   b.    
     In some embodiments of the foldable module  100   b  depicted in  FIG. 2 , the stack adhesive  10   b  is further characterized by a Poisson&#39;s ratio from about 0.1 to about 0.5, for example, from about 0.1 to about 0.45, from about 0.1 to about 0.4, from about 0.1 to about 0.35, from about 0.1 to about 0.3, from about 0.1 to about 0.25, from about 0.1 to about 0.2, from about 0.1 to about 0.15, from about 0.2 to about 0.45, from about 0.2 to about 0.4, from about 0.2 to about 0.35, from about 0.2 to about 0.3, from about 0.2 to about 0.25, from about 0.25 to about 0.45, from about 0.25 to about 0.4, from about 0.25 to about 0.35, from about 0.25 to about 0.3, from about 0.3 to about 0.45, from about 0.3 to about 0.4, from about 0.3 to about 0.35, from about 0.35 to about 0.45, from about 0.35 to about 0.4, and from about 0.4 to about 0.45. Other embodiments include a stack adhesive  10   b  characterized by a Poisson&#39;s ratio of about 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, or any Poisson&#39;s ratio between these values. In one aspect, the Poisson&#39;s ratio of the stack adhesive  10   b  is from about 0.4 to about 0.5. 
     As outlined above, the foldable electronic device module  100   b  depicted in  FIG. 2  can include a stack adhesive  10   b  with certain material properties (e.g., a shear modulus from about 10 kPa to about 100 kPa). Example adhesives that can be employed as the stack adhesive  10   b  in the module  100   b  are generally the same or similar to those suitable for the first adhesive  10   a . Thus, the stack adhesive  10   b  can include OCAs, epoxies, and other joining materials as understood by those with ordinary skill in the field that are suitable to join the stack element  75  to the first primary surface  64  of the panel  60 . In some aspects of the module  100   b , the stack adhesive  10   b  will also possess a high thermal resistance such that its material properties experience little to no change upon being subjected to various temperatures and temperature gradients in the application environment, including those generated by friction from bending of the foldable electronic device module  100   b.    
     Referring again to  FIG. 2 , the cover element  50  of the foldable electronic device module  100   b  is further characterized by a puncture resistance of at least 1.5 kgf when the first primary surface  54  of the cover element is loaded with a tungsten carbide ball having a diameter of 1.5 mm. Further, the device module  100   b  is characterized by a tangential stress at the second primary surface  56  of the cover element  50  of no greater than about 1000 MPa in tension upon bending the module in a two-point configuration to a bend radius from about 10 mm to about 3 mm such that the first primary surface  54  is in compression and the bend radius is measured from a center point above the first primary surface  54  of the cover element  50  to the second primary surface  66  of the panel  60  (see  FIG. 4B ). These performance characteristics associated with the foldable electronic device module  100   b  ( FIG. 2 ) are comparable to those demonstrated by the foldable electronic device module  100   a  ( FIG. 1 ). More particularly, these reduced tensile stress levels at the second primary surface  56  of the cover element  50  are achieved through tailoring of the material properties of the first adhesive  10   a  (e.g., shear modulus and/or Poisson&#39;s ratio) and/or the thickness  12   a  of the first adhesive  10   a . Thus, some aspects of the disclosure provide a foldable electronic device module with improved mechanical reliability, particularly at its cover element, through the control of the material properties and/or thickness of the adhesive joining the cover element to the stack within the module. 
     Referring to  FIG. 3 , a foldable electronic device module  100   c  is provided with most of its features in common with the foldable electronic device module  100   b  (see  FIG. 2 ), including performance characteristics (i.e., high puncture resistance and minimal tangential stresses (in tension) at the second primary surface of the cover element). Unless otherwise noted, any features in common between the modules  100   b  and  100   c  (i.e., with the same element numbers) have the same or similar construction, features and properties. As shown in  FIG. 3 , the module  100   c  also includes a cover element  50  having a thickness  52  from about 25 μm to about 200 μm and a cover element elastic modulus from about 20 GPa to about 140 GPa. 
     The module  100   c  depicted in  FIG. 3  further includes: a stack  90   c  having a thickness  92   c  from about 100 μm to about 600 μm; and a first adhesive  10   a  configured to join the stack element  75   c  to the second primary surface  56  of the cover element  50 . The stack  90   c  further includes a panel  60  having first and second primary surfaces  64 ,  66 , and a panel elastic modulus from about 300 MPa to about 10 GPa. The stack  90   c  also includes one or more electronic devices  102  (e.g., touch sensor electrode lines, and other electronic device and electronic device components) coupled to the panel  60  or touch sensor  80  (e.g., as shown schematically in  FIG. 3 ), and a stack element  75   c  having a stack element elastic modulus from about 1 GPa to about 5 GPa, with the stack element being affixed to the panel  60  with a stack adhesive  10   b . It should also be understood that the relationship between the panel  60 , stack element  75   c  and electronic devices  102  (e.g., as coupled to the touch sensor  80  depicted in  FIG. 3 ) is depicted in exemplary, schematic form in  FIG. 3 . Depending on the application for the device module  100   c , these elements may have different orientations with regard to one another. For example, panel  60  can be an LCD panel or an OLED display in which the electronic devices  102  are sandwiched within the panel  60  by two glass layers, or a polymeric substrate encapsulated by a glass sealing layer, for instance. See  FIG. 2 . In another example (as depicted in  FIG. 3 ), the electronic devices  102  can be aspects of a touch sensor (e.g., electronic trace lines in a transparent conductor such as indium tin oxide, silver nanowires, etc.) located at a higher vertical position within the stack  75   c , above the panel  60  and stack adhesive  10   b , and coupled to the sensor  80 . Depending on the application for the module  100   c , it is also envisioned that some electronic devices  102  could be located within or on panel  60  and others coupled to touch sensor  80 . 
     In some aspects of the module  100   c  depicted in  FIG. 3 , the stack element  75   c  exhibits a stack element elastic modulus from about 1 GPa to about 5 GPa, for example, from about 1 GPa to about 4.5 GPa, from about 1 GPa to about 4 GPa, from about 1 GPa to about 3.5 GPa, from about 1 GPa to about 3 GPa, from about 1 GPa to about 2.5 GPa, from about 1 GPa to about 2 GPa, from about 1 GPa to about 1.5 GPa, from about 1.5 GPa to about 4.5 GPa, from about 1.5 GPa to about 4 GPa, from about 1.5 GPa to about 3.5 GPa, from about 1.5 GPa to about 3 GPa, from about 1.5 GPa to about 2.5 GPa, from about 1.5 GPa to about 2 GPa, from about 2 GPa to about 4.5 GPa, from about 2 GPa to about 4 GPa, from about 2 GPa to about 3.5 GPa, from about 2 GPa to about 3 GPa, from about 2 GPa to about 2.5 GPa, from about 2.5 GPa to about 4.5 GPa, from about 2.5 GPa to about 4 GPa, from about 2.5 GPa to about 3.5 GPa, from about 2.5 GPa to about 3 GPa, from about 3 GPa to about 4.5 GPa, from about 3 GPa to about 4 GPa, from about 3 GPa to about 3.5 GPa, from about 3.5 GPa to about 4.5 GPa, from about 3.5 GPa to about 4 GPa, and from about 4 GPa to about 4.5 GPa. 
     In the foldable electronic device module  100   c  depicted in  FIG. 3 , the stack element  75   c  includes a touch sensor  80 , a polarizer  70 , and an adhesive  10   c  that joins the touch sensor  80  to the polarizer  70 . In general, the composition and thickness of the adhesive  10   c  is comparable to those employed in the first adhesive  10   a  and the stack adhesive  10   b . To the extent that the adhesives  10   a  and  10   b  possess different material properties and/or thicknesses, the adhesive  10   c  can be selected to match the properties and/or thicknesses of the first adhesive  10   a  or stack adhesive  10   b.    
     Still referring to  FIG. 3 , certain aspects of the foldable module  100   c  can be configured to minimize bending forces associated with bending the entire module by controlling the shear modulus of one or more of the adhesives employed within the module  100   c . More particularly, the use of a first adhesive  10   a , stack adhesive  10   b  and/or an adhesive  10   c  with a relatively lower shear modulus value (e.g., 0.01 MPa to 0.1 MPa) can unexpectedly reduce the overall bending force required to fold or otherwise bend the entire module  100   c  in an upward or downward direction such that the first primary surface  54  exhibits a concave or convex shape, respectively. These bending force reductions associated with certain aspects of the foldable module  100   c  through the use of a first adhesive  10   a , stack adhesive  10   b  and/or adhesive  10   c  with a relatively low elastic shear modulus value are obtained relative to a foldable module (e.g., foldable module  100   c ) with one or more adhesives (e.g., adhesives  10   a ,  10   b  and  10   c ) having a shear modulus that exceeds 0.1 MPa. Further, certain aspects of the foldable module  100   c  can be configured to minimize bending forces associated with bending the entire module by controlling the thickness of the first adhesive  10   a , stack adhesive  10   b  and/or adhesive  10   c . More particularly, the use of a first adhesive  10   a  with a range of thicknesses  12   a  (e.g., from about 10 μm to about 40 μm), stack adhesive  10   b  with a range of thicknesses  12   b  (e.g., from about 10 μm to about 40 μm) and/or adhesive  10   c  with a range of thicknesses (e.g., from about 10 μm to 40 μm) can reduce the overall bending force required to fold or otherwise bend the entire module  100   c  in an upward or downward direction such that the first primary surface  54  exhibits a concave or convex shape, respectively. These bending force reductions associated with certain aspects of the foldable module  100   c  through the use of a first adhesive  10   a , stack adhesive  10   b  and/or adhesive  10   c  within a prescribed range of thicknesses are obtained relative to a foldable module (e.g., foldable module  100   c ) with one or more adhesives (e.g., first adhesive  10   a , stack adhesive  10   b  and/or adhesive  10   c ) having a relatively small thickness (e.g. less than 10 μm) or a relatively large thickness (e.g., more than 40 μm). 
     Referring again to  FIG. 3 , the foldable electronic device module  100   c  can be characterized by a bending force (F bend ) of no greater than 150 Newtons (N) as the module is bent inward by a test apparatus to a bend radius  220 , the bend radius being approximately half the distance (D) between two test plates  250  (see  FIGS. 4A &amp; 4B ). In certain implementations, the bending force is no greater than about 150 N, 140 N, 130 N, 120 N, 110 N, 100 N, 90 N, 80 N, 70 N, 60 N, 50 N, 40 N, 30 N, 20 N, 10 N, 5 N, or any amount between these bending force upper limits, upon bending of the module to a radius from about 20 mm to about 3 mm (i.e., a plate distance (D) of about 40 to about 6 mm), for example, 20 mm, 19.75 mm, 19.5 mm, 19.25 mm, 19 mm, 18.5 mm, 17.5 mm, 17 mm, 16.5 mm, 16 mm, 15.5 mm, 15 mm, 14.5 mm, 14 mm, 13.5 mm, and 13 mm, 12.5 mm, 12 mm, 11.5 mm, 11 mm, 10.5 mm, 10 mm, 9.5 mm, 9 mm, 8.5 mm, 7.5 mm, 7 mm, 6.5 mm, 6 mm, 5.5 mm, 5 mm, 4.5 mm, 4 mm, 3.5 mm, 3.25 mm and 3 mm. As outlined earlier, these relatively low bending forces can be obtained in the foldable electronic device module  100   c  through tailoring of the material properties and/or thicknesses of the first adhesive  10   a , stack adhesive  10   b  and/or adhesive  10   c.    
     As also depicted in  FIG. 3 , the foldable electronic device module  100   c  containing three adhesives and multiple layers exhibits performance characteristics comparable to those demonstrated by the foldable modules  100   a  and  100   b  (see  FIGS. 1 and 2 ). In particular, reduced tensile stress levels at the second primary surface  56  of the cover element  50  are achieved through tailoring of the material properties of the first adhesive  10   a  (e.g., shear modulus and/or Poisson&#39;s ratio) and/or the thickness  12   a  (see  FIG. 2 ) of the first adhesive  10   a . In general, the disclosure provides a foldable electronic device module  100   c  with improved mechanical reliability, particularly at its cover element through the control of the material properties and/or thickness of the adhesive joining the cover element to the stack within the module. The foldable electronic device module  100   c  also demonstrates high mechanical reliability indicative of low tensile stresses at the first primary surface  64  of the panel  60  through the control of the material properties and/or thickness of the stack adhesive  10   b  joining the panel to the stack element  75   c.    
     Referring to  FIGS. 4A &amp; 4B , the foldable electronic device modules  100   a - c  (see  FIGS. 1-3 ) are depicted in an un-bent and a bent configuration, respectively, within a two-point test apparatus  200  according to an aspect of the disclosure. It should be understood that some of the features associated with the foldable electronic device modules  100   a - c  are not depicted in  FIGS. 4A and 4B  for purposes of clarity. 
     In  FIG. 4A , the modules  100   a - c  are depicted in an un-bent configuration within the two-point test apparatus  200  (see  FIG. 4B , showing the test apparatus  200 ). Two vertical plates  250  are pressed inward against the module  100   a ,  100   b  or  100   c  during a bending test with a constant force, F bend . Fixtures (not shown) associated with the test apparatus  200  ensure that the modules are bent in an upward direction as the F bend  forces are applied to the modules via the plates  250 . 
     Referring to  FIG. 4B , the plates  250  are moved together in unison until a particular bend radius  220  is achieved. In general, the bend radius  220  is about half the distance, D, between the plates  250 . As outlined earlier, the foldable electronic device modules  100   a - c  are characterized by a tangential stress at the second primary surface  56  (see  FIGS. 1-3 ) of the cover element  50  of no greater than 1000 MPa in tension (i.e., at point “T”) upon bending the module in a two-point apparatus  200  to a bend radius  220  from about 20 mm to about 2 mm such that the first primary surface  54  is in compression (i.e., at point “C”). As shown in  FIG. 4B , the bend radius  220  is measured from a center point above the first primary surface  54  of the cover element  50  to the second primary surface  66  of the panel  60 . This center point is located on a line of symmetry  210  associated with the modules  100   a - c . In certain implementations, the tangential stress (in tension) at the second primary surface  56  (see  FIGS. 1-3 ) of the cover element  50  is no greater than about 1000 MPa, 950 MPa, 925 MPa, 900 MPa, 875 MPa, 850 MPa, 825 MPa, 800 MPa, 775 MPa, 750 MPa, 725 MPa, 700 MPa, or any amount between these tangential stress limits (in tension). Further, in other implementations of the disclosure, the modules  100   a ,  100   b  and  100   c , can be characterized by a bending force (F bend ) of no greater than 150 Newtons (N) as the module is bent inward by the test apparatus  220  employing plates  250  (see  FIGS. 4A &amp; 4B ). In certain implementations, the bending force is no greater than about 150 N, 140 N, 130 N, 120 N, 110 N, 100 N, 90 N, 80 N, 70 N, 60 N, 50 N, 40 N, 30 N, 20 N, 10 N, 5 N, or any amount between these bending force upper limits, upon bending of the module to a radius from about 20 mm to about 3 mm (i.e., a plate distance (D) of about 40 to about 6 mm). 
     Through careful study and analysis of foldable modules comparable in configuration to the foldable modules  100   a ,  100   b  and  100   c , an understanding of the importance of controlling the material properties and/or thicknesses of the adhesives employed within the modules was developed. These studies included the development of simple two-layer models based on conventional composite beam theory and equations, with one layer corresponding to the cover element and the other layer corresponding to a stack (e.g., as envisioned to include a panel, electronic devices and other components). In addition, more sophisticated non-linear finite element analysis (“FEA”) models (i.e., employing conventional FEA software packages) contributed to aspects of the disclosure. In particular, the FEA models were used to simultaneously assess stresses that could lead to cohesive failures of the cover element, delamination effects, and potential buckling issues within the foldable modules. 
     The output of these non-linear FEA models included the plots depicted in  FIGS. 5A, 5B and 6 . Each of these figures includes a plot of estimated tangential stress (MPa) as a function of depth (mm) through the thickness of foldable electronic device modules comparable in design to the modules contained in the disclosure, e.g., modules  100   a - c . The foldable electronic device modules were subjected to a bend radius of 3 mm (e.g., bend radius  220 , as shown in  FIG. 4B ) within the FEA model. Table 1 below provides a listing of elements employed in the FEA model, including assumed material properties for each of them. Further, the FEA model was conducted with the following additional assumptions: (a) the entire module was assumed to have a non-linear geometric response; (b) the adhesives were assumed to be incompressible, hyper-elastic materials; (c) the cover elements and other non-adhesive features in the model were assumed to have elastic material properties; and (d) the bending was conducted at room temperature. 
     
       
         
           
               
               
               
               
             
               
                 TABLE ONE 
               
               
                   
               
               
                   
                   
                 Elastic modulus, E 
                 Poisson&#39;s 
               
               
                 Element 
                 Thickness (μm) 
                 (GPa) 
                 ratio, v 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Glass cover element 
                 67 
                 71 
                 0.22 
               
               
                 Touch sensor - PET 
                 170 
                 2.8 
                 0.37 
               
               
                 Polarizer - PET 
                 60 
                 2.8 
                 0.37 
               
               
                 Panel - polyimide 
                 95 
                 2.5 
                 0.34 
               
               
                 Adhesive 
                 Variable (10 to 
                 Variable (shear 
                 0.499 
               
               
                   
                 36) 
                 modulus) 
               
               
                   
               
            
           
         
       
     
     Referring to  FIG. 5A , a plot of estimated tangential stress as a function of depth through the thickness of three foldable electronic device modules is provided. In this plot, each of the three bendable modules contains adhesives (e.g., comparable to first adhesive  10   a  and stack adhesive  10   b  employed in the bendable module  100   c  shown in  FIG. 3 ) configured to join a cover element to a stack, and a stack to a panel, each with a different shear modulus, 10 kPa, 100 kPa and 1000 kPa, respectively. In particular, each of the adhesives employed in a given module was assumed to possess the same shear modulus, 10 kPa, 100 kPa or 1000 kPa. As the plot makes clear, the tangential stresses observed at the interface between the cover element and the first adhesive (e.g., at the second primary surface  56  of the cover element  50 ) are reduced by about 400 MPa (in tension) with an increase in the shear modulus of the adhesives contained in the module from 10 kPa to 1000 kPa. That is,  FIG. 5A  demonstrates that increasing the shear modulus of all of the adhesives within a given bendable electronic device module can beneficially reduce the tensile stresses at the second primary surface of the cover element. 
     Also referring to  FIG. 5A , the tensile stresses observed at the interface between the panel and an adhesive joining the panel to a stack element (e.g., stack adhesive  10   b  employed in the foldable module  100   c  shown in  FIG. 3 ) are reduced by about 200 MPa with a decrease in the shear modulus of the adhesives contained in the module from 1000 kPa to 10 kPa. That is,  FIG. 5A  demonstrates that decreasing the shear modulus of all of the adhesives within a given bendable electronic device module can beneficially reduce the tensile stresses at the first primary surface of the panel employed in the device module. 
     Referring to  FIG. 5B , a plot of estimated tangential stress as a function of depth through the thickness of two foldable electronic device modules is provided. In this plot, each of the bendable modules contain adhesives (e.g., comparable to first adhesive  10   a  and stack adhesive  10   b  employed in the bendable module  100   c  shown in  FIG. 3 ) configured to join a cover element to a stack, and to join a stack to a panel, with a shear modulus of 10 kPa. In one of the modules, the thickness of each of the adhesives employed in the module was set at 10 μm. In the other module, the thickness of each of the adhesives employed in the module was set at 36 μm. As the plot makes clear, the tensile stresses observed at the interface between the cover element and the first adhesive (e.g., at the second primary surface  56  of the cover element  50 ) are reduced by about 80 MPa with a decrease in the thickness of the adhesives contained in the module from 36 μm to 10 μm. That is,  FIG. 5B  demonstrates that decreasing the thickness of all of the adhesives within a given bendable electronic device module can beneficially reduce the tensile stresses at the second primary surface of the cover element. 
     Referring to  FIG. 6 , a plot of estimated tangential stress as a function of depth through the thickness of three foldable electronic device modules is provided. In this plot, “Case (1)” corresponds to a bendable module with all of its adhesives exhibiting a shear modulus of 10 kPa and having a thickness of 36 μm. “Case (2)” corresponds to a bendable module with the same configuration as Case (1), except that the shear modulus of the adhesive adjacent to the cover element was increased to 1000 kPa. “Case (3)” corresponds to a bendable module with the same configuration as Case (2), except that the thickness of the adhesive adjacent to the cover element is reduced to 12 μm. As the plot makes clear, the tensile stresses observed at the interface between the cover element and the first adhesive (e.g., at the second primary surface  56  of the cover element  50 ) are reduced by about 240 MPa with an increase in the shear modulus of the first adhesive adjacent to the cover element from 10 kPa to 1000 kPa (i.e., from Case (1) to Case (2)). Further, another 48 MPa reduction in tensile stress is observed with a decrease in the thickness of the first adhesive adjacent to the cover element from 36 μm to 12 μm (i.e., from Case (2) to Case (3)). That is,  FIG. 6  demonstrates that decreasing the thickness and increasing the shear modulus of the adhesive joining the cover element to the stack within a given bendable electronic device module can beneficially reduce the tensile stresses at the second primary surface of the cover element. 
     Referring to  FIG. 7 , a schematic plot of estimated bending force (N) as a function of adhesive thickness (μm) is provided for three foldable electronic device modules configured in an arrangement comparable to modules  100   c . More particularly, each of the three modules is configured with three adhesives (e.g., a first adhesive  10   a , a stack adhesive  10   b  and an adhesive  10   c ). Further, the three adhesives in each of the modules all have a single, distinct shear modulus; consequently, the adhesives in the first module have a shear modulus of “E PSA   1 ,” the adhesives in the second module have a shear modulus of “E PSA   2 ” and the adhesives in the third module have a shear modulus of “E PSA   3 .” As shown in  FIG. 7 , E PSA   1 &gt;E PSA   2 &gt;E PSA   3 . It is evident from  FIG. 7  that a reduction in the shear modulus of the adhesives employed in these foldable electronic device modules results in a significant decrease in the bending forces required to fold or otherwise bend these modules (e.g., as in a two-point test configuration depicted in  FIGS. 4A &amp; 4B ). It is also apparent from  FIG. 7  that optimum reductions in bending forces (N) occur for these electronic device modules for a certain range of thicknesses, i.e., between “t PSA   1 ” and “t PSA   2 .” Some aspects of the electronic device modules exhibit their lowest bending forces in a thickness range from about 10 μm to about 30 μm, corresponding to respective t PSA   1  and t PSA   2  thicknesses, as depicted in  FIG. 7 . In contrast, adhesive thicknesses (μm) greater than t PSA   2  and thicknesses lower than t PSA   1  tend to result in increasing bending forces. 
     Referring to  FIG. 8A , a plot of estimated bending force, F bend  (N), as a function of plate distance, D (mm), in a two-point test apparatus is provided for the foldable electronic device modules depicted in  FIG. 5A . That is, each of the three bendable modules depicted in  FIG. 8A  contains adhesives (e.g., comparable to first adhesive  10   a , stack adhesive  10   b  and adhesive  10   c  employed in the bendable module  100   c  shown in  FIG. 3 ) configured to join a cover element to a stack, and a stack to a panel, each with a different shear modulus, 10 kPa, 100 kPa and 1000 kPa, respectively. In particular, each of the adhesives employed in a given module was assumed to possess the same shear modulus, 10 kPa, 100 kPa or 1000 kPa. As shown in  FIG. 8A , the bending force for a module as a function of plate distance is sensitive to the shear modulus of the adhesives employed within the module. For example, at a plate distance of 6 mm (i.e., bend radius of about 3 mm), the device module with adhesives exhibiting a shear modulus of 1000 kPa experienced a bending force of about 140 N and the device module with adhesives exhibiting a shear modulus of 10 kPa experienced a bending force of about 30N. Accordingly, foldable electronic device modules can be optimized to reduce bending forces by employing adhesives with a relatively low shear modulus. Depending on the application for the module, however, any reduction in bending force through control of adhesive shear modulus can be offset or otherwise optimized in view of the decreases in tangential stress between the cover element and first adhesive that can be obtained through increasing the shear modulus of the adhesives within the module, as outlined earlier in connection with  FIG. 5A . 
     Referring to  FIG. 8B , a plot of estimated bending force, F bend  (N), as a function of plate distance, D (mm), in a two-point test apparatus is provided for the two foldable electronic device modules depicted in  FIG. 5B . That is, each of the bendable modules contain adhesives (e.g., comparable to first adhesive  10   a , stack adhesive  10   b  and adhesive  10   c  employed in the bendable module  100   c  shown in  FIG. 3 ) configured to join a cover element to a stack, and to join a stack to a panel, with a shear modulus of 10 kPa. In one of the modules, the thickness of each of the adhesives employed in the module was set at 10 μm. In the other module, the thickness of each of the adhesives employed in the module was set at 36 μm. As shown in  FIG. 8B , the bending force for a module as a function of plate distance is fairly insensitive to the thickness of the adhesives employed within the module when the thickness is between about 10 μm and about 36 μm. For example, at a plate distance of 6 mm (i.e., bend radius of about 3 mm), both device modules experienced about the same bending force, between about 35 N and about 40 N. Nevertheless, it is also evident from  FIG. 7  that adhesive thickness levels farther above 36 μm and below 10 μm can lead to increasing amounts of bending forces experienced by the modules. 
     Referring to  FIG. 8C , a plot of estimated bending force, F bend  (N), as a function of plate distance, D (mm), in a two-point test apparatus for the three foldable electronic device modules depicted in  FIG. 6 . As noted earlier, “Case (1)” corresponds to a bendable module with all of its adhesives exhibiting a shear modulus of 10 kPa and having a thickness of 36 μm. “Case (2)” corresponds to a bendable module with the same configuration as Case (1), except that the shear modulus of the adhesive adjacent to the cover element was increased to 1000 kPa. Hence, in Case (2) the shear modulus values of the other adhesives in the module not adjacent to the cover element are set at 10 kPa. “Case (3)” corresponds to a bendable module with the same configuration as Case (2), except that the thickness of the adhesive adjacent to the cover element is reduced to 12 μm. That is, in Case (3) the thicknesses of the other adhesives in the module not adjacent to the cover element are set at 36 μm and the adhesive adjacent to the cover element had a shear modulus of 1000 kPa, and a thickness of 12 μm. 
     As shown in  FIG. 8C , the bending force for a plate distance of 6 mm is at a minimum for Case (1) at about 40 N, which corresponds to an electronic module with all of its adhesives having a thickness of 36 μm and a shear modulus of 10 kPa. For the Case (3) condition, however, a modest increase in bending force of about 40 N is realized by adjusting the thickness and shear modulus of the first adhesive (i.e., without any change to the shear modulus or thickness of the other adhesives in the module) to 12 μm and 1000 kPa, respectively. The Case (3) condition with a modest increase of about 40 N in the bending force stands in contrast to the approximate 110 N increase in bending force that results from increasing the shear modulus of all of the adhesives in the module as shown in  FIG. 8A . Moreover, as demonstrated earlier in  FIG. 6 , the Case (3) condition is particularly advantageous in providing a 288 MPa reduction in tangential stress between the glass cover element and the first adhesive. Hence, a significant decrease in tangential stress can be realized in the module by increasing the shear modulus and reducing the thickness of the first adhesive, i.e., the adhesive adjacent to the glass cover element, with only modest increases to the bending force. 
     Advantageously, the foldable electronic device modules in the disclosure are configured for high mechanical reliability and puncture resistance. In particular, these foldable modules exhibit reduced tangential stresses (in tension) at primary surfaces of the cover element and/or panel through control of the material properties and/or thicknesses of the adhesives employed in the modules. These lower tensile stresses translate into better reliability and/or smaller bend radius capability. Moreover, these lower tensile stresses can provide an improved design margin for electronic devices employing these foldable modules. In view of the reductions of tensile stress in the foldable modules associated with aspects of the disclosure, the need for compressive stress regions and/or other strength-enhancing measures that produce high, residual compressive stresses in the cover element can be reduced. Accordingly, compressive stress region-related processing costs associated with the cover element can be reduced in view of the concepts set forth in the disclosure. Further, the beneficial effects of reducing the thickness of the first adhesive in these bendable modules in terms of tensile stress reductions can additionally provide an overall reduction in the thickness of the module. Such module thickness reductions can be advantageous for many end product applications for these modules having a low profile. 
     Also advantageously, the foldable electronic device modules in the disclosure can be configured to minimize the bending forces required of the user to bend or otherwise fold the module. In particular, bending forces experienced by these modules can be reduced by decreasing the shear modulus and/or optimizing the thickness of the adhesives employed in the module. Further, certain exemplary foldable electronic device modules can be optimized for mechanical reliability, puncture resistance and bend force reductions by utilizing a relatively high shear modulus adhesive at the glass cover element and relatively low shear modulus adhesives in other locations within the module. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the foldable electronic device modules of the disclosure without departing from the spirit or scope of the claims.