## 1 Introduction
In this context, additive manufacturing (AM) offers the opportunity to merge the ease of design changes with an agile manufacturing process [32; 33; 34]. Soft tool inserts realised using AM have a lower hardness, lower tensile strength, and modulus of elasticity compared to conventional metal tools, making them suitable for industries and applications with low production rates, frequent prototype refinement, or customisation needs, minimising lead times and costs [35; 36; 37]. In recent years, AM has played a crucial role in biomedical research and diagnostics by enabling the fabrication of biosensors [38; 39], tissue [22; 26; 40] and organ printing [41; 42]. Thanks to recent advancements and capabilities in AM, this technology has emerged as a potential alternative for producing \(\upmu\)IM inserts for biomedical applications. This approach could reduce the time and cost of prototyping devices and accelerate their transition from laboratories to industry. Several rapid prototyping methods are available, such as fused deposition modeling (FDM), material jetting/multi jet moulding (MJ), stereolithography (SLA) and digital light processing (DLP). Among these, SLA and MJ appear to be the most commonly employed AM techniques for producing resin inserts [43]. These technologies are able to realize parts with high resolution having the \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Process** & **Resolution (\(\upmu\))** & **Cycle (\(\min\))** & **Throughput (pcs/mo.)** & **Reference** \\ \hline
3D printing & 1 & 30–60 & 1 & [26] \\ CNC machining & 100 & 5–30 & 300–2000 & [27] \\ Laser machining & 100 & 5–30 & 300–2000 & [27] \\ PDMS casting & \textless{}1 & 30–60 & 150–300 & [28] \\ Hot embossing & \textless{}1 & 10–30 & 300–1000 & [28] \\ Microinjection Moulding & \textless{}1 & 0.15–0.5 & 5000+ & [27] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of manufacturing processes for microfeatured device manufacturing [24]. possibility to print layer 25 \(\upmu\)m height. However, there are limitations in reproducing the microfeatures required for microfluidic chips such as microchannels, pillar arrays, and crossing manifolds, as well as drawbacks related to the material brittleness, which results in decreased tool life during the process, poor accuracy in reproducing structures, and process limitations [44]. Therefore, a careful design of the geometry is essential to optimise the performance of these parts. To enhance the suitability of soft tool inserts for \(\upmu\)IM, researchers are focusing their efforts on improving knowledge on the behaviour of resin inserts and reproducing even smaller features. [PERSON] et al. [45] demonstrated, through theoretical and experimental analysis, that features in MJ inserts do not fail primarily due to bending stress from the injection pressure or the localized pressure. Inserts with pin diameters ranging from 3.63 mm to 7.16 mm and aspect ratios from 1.12 to 1.45 were tested, which showed failures from the sixth shot. [PERSON] et al. [46] instead investigated the thermal expansion and mechanical deformation of MJ resin inserts with three different wall thicknesses (4 mm, 5 mm, and 6 mm), monitoring cavity pressure and strain with sensors and a strain gauge. A progressive accumulation of residual strain with each IM cycle was observed, and a correlation was established between the strain of the insert and cavity pressure. The 6 mm thick insert failed in the ninth cycle, while the 5 mm thick insert fractured in the thirteenth cycle and the 4 mm thick insert succumbed in the twelfth cycle. [PERSON] et al. [47] instead performed an experimental campaign using SLA inserts with features of diverse shapes and sizes ranging from 500 \(\upmu\)m to 1000 \(\upmu\)m to evaluate the thermal conductance and mechanical properties of five photopolymers. The findings suggested that resins with a higher flexural and elongation modulus are more suitable for the \(\upmu\)IM process. Also, [PERSON] et al.