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and high-resolution TVs. Their huge commercial prospect has been partially confirmed, accompanied by a
booming display market worth tens of billions of dollars.

In contrast to the tremendous progress of using planar Micro-LEDs for conventional rigid displays, the
development of flexible Micro-LED counterparts is currently actively explored, evidenced by intensive
research interest from both academia and industry [9-18] . Benefiting from their extraordinary luminescent
property, deformable displays made from inorganic Micro-LEDs can not only significantly improve visual
experience of users but also potentially provide more convenience, better portability, and easier connectivity
with their surroundings. Furthermore, they allow more versatile design with various forms that are difficult
to achieve using conventional planar devices. Examples of such innovations have led to the recent
[10]
[19]
demonstration of distortion-free displays , foldable three-dimensional (3D) displays , etc.
Apart from high-resolution deformable displays, some emerging applications, such as optogenetics [20-28] and
smart contact lenses [29-33] , also drive the development of highly efficient, biocompatible light sources that can
[34]
be conformally attachable to skins or implantable inside the human body for healthcare . Micro-LEDs in
flexible/stretchable formats are well suited for these purposes. Here, we highlight the recent technology
advancements in developing such flexible emitters and then discuss their potential applications for
uncommon displays and healthcare. Finally, future research trends of flexible inorganic emitters are also
given.

STRATEGIES TOWARD DEFORMABLE MICRO-LEDS
Conventional inorganic semiconductors used for Micro-LEDs have higher conductivities and mobilities,
leading to higher brightness and better operation stability. However, high-performance inorganic
semiconductors are usually grown on rigid, planar substrates, which means they are not intrinsically
flexible. In order to render Micro-LEDs with flexibility, several strategies must be considered, including
substrate removal, chip transfer printing, and certain mechanical design for enhanced stretchability
[Figure 1].

Substrate removal
Thickness reduction by substrate removal can impart certain flexibility to Micro-LEDs. Two major
strategies have been developed to take off the growth substrate: laser lift-off (LLO) [6,35,36] and epitaxial lift-off
(ELO) [37-43] . LLO exploits the laser energy absorption at the LED layer/substrate interface, which leads to the
release of Micro-LEDs from the substrate due to the high-temperature induced material decomposition at
[35]
the interface [Figure 1A]. Prior to LLO, the LED wafer is temporarily bonded to a supporting carrier.
High-energy laser scanning leads to the transfer of a thin membrane to the supporting substrate. ELO, on
the other hand, is a technique based on a modified epitaxial structure, where a sacrificial layer is
incorporated in the epi-stack to assist the release of Micro-LEDs grown on the sacrificial layer . Depending
[38]
on specific release layers, two ELO principles have been established: chemical lift-off (CLO) [43-46] and
mechanical lift-off (MLO) [37,38] . In the case of CLO, the sacrificial layer can be removed by chemical solvents,
resulting in released thin film Micro-LEDs [Figure 1B]. For MLO, a handling carrier (for instance, a tape) is
used to peel off the epi-stack grown on the release layer directly due to the significantly weakened bonding
of the epi-layer to the substrate [Figure 1C]. Two-dimensional (2D) material-assisted epitaxy or remote
epitaxy has emerged to fulfill this purpose [39-42,47-49] . Growth on voids or porous templates can also lead to
successful MLO [50,51] . However, MLO is usually done manually, making it difficult to accurately control the
process with good reproducibility. By contrast, CLO is more controllable, which has witnessed commercial
success, especially for the fabrication of solar cell modules [52,53] .