Mitigating drilling fluid loss, a major contributor to non-productive time and cost in well construction, remains a

global challenge due to the limitations of conventional loss circulation material (LCM), especially their lack of

size responsiveness. This study introduces a novel solution: bidirectional shape memory poly(acrylamideacrylonitrile) hydrogels with an upper critical solution temperature (UCST), synthesized via free radical polymerization. We characterized the structure and temperature-triggered size responsiveness of these hydrogels.

Optimal reversible expansion, featuring a swelling rate (SR) of 24.06 % within a 64.61–97.84?C responsive

range, was attained using AAm/AN molar ratios of 0.5–0.65 and 1.5–3.5 % MBA crosslinker, tested under 20℃-

90℃ cycling. The mechanism involves swelling upon heating due to dominant hydrophilic interactions and

contraction upon cooling driven by enhanced hydrophobic interactions and water expulsion. Based on this

hydrogel, a shape memory LCM was developed. Its plugging mechanism leverages this reversible expansion to

effectively seal loss zones, significantly reducing fluid filtration and improving filter cake quality. Hightemperature and high-pressure (HTHP) experiments validate its strong sealing performance and permeability

reduction, demonstrating its potential for substantially improving loss prevention and plugging in drilling

operations.