**1. Introduction**

Tracheal injury can result from several conditions, including cancer, infection, trauma, or congenital anomalies. The conventional indication for therapy in severely injured tracheas of any aetiology is partial or full reconstruction, which necessitates the substitution of a graft or scaffold at the site of the lesion [1,2]. Unfortunately, even though there are a few treatment options available such as natural grafts or synthetic replacement, no optimal material has met the criteria. The limitations for natural grafts include the availability of donors and the unmatched size of donor grafts. According to the Organ Procurement & Transplantation Network, United States Department of Health and Services, the number of patients on the national transplant waiting list until July 2019 for all organ types has increased to more than 100,000. Out of this number, two-thirds are above the age of 50, while almost 2000 are below the age of eighteen and only one-third of the total numbers received organ transplantation [3]. In addition, the natural grafts derived from donors are challenged by the possibility of severe immune-rejection risks and complications caused by infection or disease from the donor-to-patient [4]. On the other hand, synthetic scaffolds are commonly associated with the biocompatibility of the scaffold material, inadequate mechanical properties, and biodegradability over time. In addition, other problems (such

**Citation:** Abdul Samat, A.; Abdul Hamid, Z.A.; Jaafar, M.; Yahaya, B.H. Mechanical Properties and In Vitro Evaluation of Thermoplastic Polyurethane and Polylactic Acid Blend for Fabrication of 3D Filaments for Tracheal Tissue Engineering. *Polymers* **2021**, *13*, 3087. https:// doi.org/10.3390/polym13183087

Academic Editors: Swee Leong Sing and Wai Yee Yeong

Received: 15 August 2021 Accepted: 7 September 2021 Published: 13 September 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

as availability for mass production with easy fabrication and the need for the size of the trachea should be custom-made to patients) persist [5–7].

Synthetic materials such as biodegradable polymers are gaining attention as materials in tissue engineering due to their broad processability window where the macro- and microstructures, mechanical properties, and degradation time can be easily manipulated and controlled. Scaffolds are fabricated and manipulated through various techniques to produce high precision, which suits the application. Some of the techniques used in the fabrication of tracheal scaffold using biodegradable materials that have been tested in animal models are electrospinning [8], thermally induced phase separation [9], and three dimensional (3D) printed technology (additive manufacturing) [10,11]. However, the choice of material and design for tracheal scaffold fabrication remains a challenge. To meet the selection criteria, many requirements must be fulfilled. For example, the scaffolds must create a suitable 3D niche for the cells to grow, proliferate, and differentiate, and should not elicit an immune reaction that can trigger a severe inflammatory response that might reduce healing or cause rejection by the body [12]. In vivo, the scaffold functions as a temporary framework that degrades over time which is eventually replaced by the body's cells. Therefore, the degradation rate should match the rate at which the cells produce their cellular matrix, while the by-products released should be innocuous and eliminated safely through the body system [13]. Similarly, sufficient mechanical integrity of the implanted scaffold is required to allow for physiological functionality starting from implantation until the completion of the remodelling process.

Additive manufacturing (AM), also known as three-dimensional (3D) printing, has been used to fabricate tissue-engineered constructs. According to the ISO/ASTM standard, AM is defined as the "process of joining materials to make parts from 3D model data, usually layer upon layer" [14]. The AM is significantly different from traditional formative or subtractive manufacturing. It is the closest to 'bottom up' manufacturing, in which a structure can be built into its intended shape using a layer-by-layer technique. This layer-by-layer manufacturing technique enables unprecedented precision and control for constructing complex, composite, and hybrid structures. The four key components in AM include a digital model of the object, materials that are consolidated from the smallest possible form, a machine for laying materials, and a digital control system for the machine to lay the materials layer-by-layer to form a complex structure with customizable shape, size, and internal architecture [15–17]. The 3D fabrication of a tracheal scaffold has been reported in several preclinical studies using different types of polymeric materials such as polylactic acid [18] and polycaprolactone [19,20].

Thermoplastic polyurethane (TPU) is a polymeric material that can be manipulated, moulded, and produced through heating in various industrial processes. Polyurethane is composed of three materials; a diisocyanate, a chain extender and a macrodiol (or polyol) which are linked to form linear, segmented copolymers consisting of alternating hard and soft segments. The soft and flexible segment is derived from polyols such as polyester, while the rigid and hard segment is formed from the diisocyanate and chain extender [21]. TPU exhibits a broad range of mechanical properties across a wide range of temperatures due to the various ratios of soft to hard segments. As a result of its excellent physical properties and biocompatibility, it is widely used in biomedical applications, particularly in flexible uses such as blood vessels [22–24], catheters [25,26], and cartilage [27,28].

Polylactic acid (PLA) is a semi-crystalline polymer that belongs to the α-hydroxy acid family, derived from renewable sources such as corn, potatoes, sugarcane, and beets. It is classified as an aliphatic polyester because of the ester bonds that connect the monomer units, the lactic acids [29,30]. PLA and its copolymers have become one of the most attentively studied components in the biomedical field because of their excellent biological and mechanical properties, biodegradability and processability. Hence, it has wide applications such as medical implants, sutures [31], bone fixation screws [32], and drug delivery systems [33]. However, biodegradable PLA exhibits little to no elastic behaviour and is not favoured for applications requiring high flexibility or deformation in situ. Furthermore, the inherent hydrophobicity and slow degradability of PLA slightly impede its application in biological systems [34].

Blending two or more polymers is a common physical modification approach to enhance the existing properties of both materials to customize the desired properties for a particular application [34–36]. The blending technique has been utilized to overcome the limitations of the physical properties of polymers and has resulted in materials with novel properties such as shape memory and morphology that are not present in the parent polymers. These materials can be moulded into various structures, including films, porous scaffolds, fibres, filaments, and particles, depending on the intended application, with properties tuned for use in a variety of biomedical applications. Polymer blending facilitates the efficient and cost-effective modification or improvement of a polymer's properties, thereby minimizing the significant costs and efforts associated with research and development of new polymers or copolymers. The blending techniques used are melt extrusion, foaming, electrospinning, and compression moulding [34,35].

Several biodegradable polymers that have been used to fabricate a tracheal scaffold are polylactic acid (PLA), polyglycolic acid (PGA), polylactide-co-glycolide acid (PLGA) [37–40], polypropylene [41,42], polyethylene terephthalate [43], high-density polyethylene (HDPE) [44], and polycaprolactone (PCL) [45–48]. Most of these polymers are used in combination with other synthetic or natural materials to enhance their properties.

Despite a large amount of research into various biodegradable polymers, clinical performance has yet to satisfy theoretical expectations. As a result, there is currently no clinically feasible solution for patients with long segmental airway problems. Therefore, an ideal synthetic scaffold that is biocompatible, timely degraded, and eliminated by the body system with appropriate and physical-mechanical qualities that can be easily replicated when needed (and is maybe individually custom-made to prevent prosthesis failure) is required. This study investigated the physical blending of two materials to obtain the optimum mechanical properties while retaining the material's superior inherent properties. Additionally, it aimed to evaluate the physical and mechanical properties of a series of TPU and PLA blends, which will be used to produce filaments for 3D printing for tracheal tissue engineering. The TPU/PLA blended matrix, a combination of soft material TPU and rigid material PLA, is expected to act as an artificial ECM by possessing suitable mechanical strength and flexibility between the TPU and PLA.

#### **2. Experimental**

#### *2.1. Materials*

TPU Estane 58,311 NAT 028 (Brussel, Belgium); PLA NatureWorks, 2002D was purchased from NatureWorks LLC (Minnesota, MN, USA) with a specific gravity of 1.24 and a melt index of 5.0–7.0 g/10 min (2.16 kg loads at 210 ◦C).

#### *2.2. Methods*

## 2.2.1. Fabrication of Polymer Blends' Filaments via Melt Extrusion Technique

Prior to extrusion, the pellets of both polymers were dried in a 60 ◦C oven for 12 h. Then, the extrusion of fibres was performed using a Brabender (Duisburg, Germany) single screw extruder with a 1.75 mm die, operated according to the manufacturer's instructions. A total weight of 100 g was used for each composition based on their weight percentage ratio (TPU: PLA) and coded as 100/0, 90/10, 80/20, 70/30, 60/40 and 0/100, respectively. Next, both materials were manually premixed via tumbling in a plastic zip-lock bag before melt-compounding. Once optimised, the temperature of the single screw extruder was set at 170◦ to 205 ◦C (±5 ◦C), the rotation speed was at 40 (±5) rpm, and the mixture was fed for melt compounding. Finally, the filaments were pelletised and hot-pressed into dumbbell shapes and 10 mm × 10 mm square samples, allowing for various characterisation methods. The TPU filament was produced using TPU pellets only in the same manner as other blends.
