## 1 Introduction  
Nowadays, the use of pieces with reduced dimensions has increased in several commercial sectors, especially in electronic and medicine fields. Some examples of these devices are microchips, nanoneedles or pumps for microfluidics [1; 2; 3]. The wide range of applications and the high resolution required for the final performance have enhanced the interest towards the use of polymers for these purposes due to their versatility. However, the technology to produce them in a large scale is still under development. There are some techniques to mold polymers in a microsize scale as hot embossing, thermoforming or microinjection [4; 5]. The former ones are mainly employed in a laboratory scale, limiting their applicability for a large-scale processing. On the other hand, microinjection equipment has been introduced in the market and is widely used to get microsamples in a commercial way. However, production cost is high due to the procedure itself and consequently microinjection application is somehow restricted. Microinjection equipment is a simple adaptation of conventional injection equipment where a microsized mold is connected by an adapted pipe. For this reason, the dosage of the material is really poor, the feeding is excessive, and the amount of non-useful material is huge, in some cases even more than 90%. Moreover, the produced molten material is more than required leading to an elevated energy cost and a large cycle time process [6; 7]. In the search of new technologies to obtain small pieces, some studies come up with the development of processing techniques based on the use of ultrasound as a heating source. This technique has been applied for over 40 years as standard assembly procedure [8] where ultrasonic welding is based on a local melting in a thermoplastic surface. The polymer heating is generated from inside to outside the material as a result of internal polymer chain friction. This heating technique is highly efficient and accurate; moreover, ultrasound vibration increases the motion of molecular chain dropping the polymer viscosity and facilitating the molten polymer flow [9]. Some prototypes have been launched based on this heating concept combined with direct injection to plasticize material [10]. This process also offers a highly efficient dosage system, delivering just the amount of material required. The potential applications of this methodology are clear, being up to now different studies conducted to improve knowledge about the effect of ultrasound in the melting process of some commodities, such as polyethylene or polystyrene [11; 12]. In addition, this technology is being considered for biodegradable polymers with biomedical applications, such as polylactide (PLA) [7]. PLA is an aliphatic polyester (i.e., a poly \(\alpha\)-hydroxyacid), which can be obtained from renewal sources, such as sugar cane and starch. PLA is a biocompatible polymer which can be biodegradable into lactic acid, has low immunologic toxicity and good mechanical properties. Furthermore, PLA has been approved by the food drug administration (FDA) for clinical assays [13] and therefore its use in the biomedical field is widely extended. Specifically, PLA is employed in medical implants such as screws, pins or meshes [14; 15], bioabsorbable sutures [16; 17], tissue engineering applications [18; 19; 20] and drug-delivery systems [21; 22]. One of the potential applications of ultrasonic micromodling is the possibility to incorporate drugs inside the polymer matrix and perform the molding process in a single step. Considering the prolonged cycle time and the mechanical stress suffered by the polymer, the conventional injection process is not suitable for the direct loading of thermally unstable drugs. In most cases, drugs degrade and become biologically inactive. On the contrary, ultrasonic micromodling can reduce the time at which the sample should be exposed to the power source (e.g., 1-2 s in our case as will then be explained).