## 1 Introduction
[23] investigated re-entrant honeycomb auxetic structures through theoretical calculations, computer simulations, and experimental analyses of Poisson's ratio. This study determined the ideal parameters for these structures in 3D-printed samples using thermoplastic polyurethane (TPU). During low-cycle compression tests, the auxetic hexagonal cell-based structure showcased nearly 1.75 times greater endurance against very low cycle fatigue compared to a similar non-auxetic structure in the experiments. [PERSON] et al. [24] modified a re-entrant auxetic structure using ABS was that was 3D-printed. In contrast to a traditional re-entrant auxetic structure, they examined the deformation mode, stress-strain response, and energy absorption properties of an optimal re-entrant auxetic honeycomb. When compared to the standard design, the redesigned auxetic structure showed a significant increase of 36% in its specific energy absorption capacity. Multiple studies concentrated on modeling and designing auxetic structures for enhanced performance. However, they mostly employed pure polymers in fabricating these structures via 3D printing. Although pure polymers possess diverse applications, transitioning to polymer composites offers the advantages of improved mechanical and thermal properties [25; 26]. Additionally, this change makes it possible to alter functional characteristics, including electromagnetic characteristics, to cater to certain uses [27; 28]. Towards developing polymer composites by adding metal fillers to polymer matrix, investigations have very rarely been conducted during recent years. In a research work by [PERSON] et al. [29] copper/ABS and iron/ABS composites were developed to find the effect of different metal contents on the thermo-mechanical properties of new materials. The results indicated that the thermal conductivity was increased by 41% when the copper content increased from 0 to 50 wt%. The tensile strength of ABS was decreased by 42% and 21% with adding 30 wt% of copper and 40 wt% of iron, respectively. [PERSON] et al. [30] utilized the material extrusion method to additively manufacture PLA composite scaffolds, with 10 vol% of two distinct iron-based powders: stainless steel 316L and pure iron. Their assessment encompassed the scaffolds' geometric, thermal, and mechanical characteristics, as well as their in vitro biodegradability, wettability, and cytocompatibility. By modifying the printing conditions, they were able to create scaffolds with pore sizes of 0.80 \(\pm\) 0.08 mm, which guaranteed a uniform distribution of iron-based powders in the PLA matrix. Notably, both the iron/PLA and 316L/PLA scaffolds had a decreased linear coefficient of thermal expansion, which led to a decrease in surface roughness and an increase in strut width. Moreover, significant increases in compressive strength were obtained by adding 10 vol% iron-based powders. The compressive strength of the iron/PLA and 316L/PLA scaffolds rose by almost 100% and 166%, respectively, in comparison to clean PLA. Iron did not, however, considerably increase PLA's flexural strength or elongation. The main objective of this research was to investigate how adding recycled iron powders to biodegradable PLA impacts the mechanical properties of 3D-printed auxetic structures, specifically to enhance fracture and deformation behavior. By incorporating iron at 5 wt%, 10 wt%, and 15 wt% through a solution casting process, followed by filament extrusion and 3D printing, this study uniquely merges material development with auxetic structure design. Unlike prior research that focuses separately on material innovation or auxetic design, our work integrates these areas. Notably, this study successfully fabricates re-entrant auxetic structures using an iron/PLA composite, focusing on how varying iron content enhances compressive strength and energy absorption capabilities--key factors for creating durable, resilient materials in demanding applications.