Synthetic Material Fabrication in Tracheal Reconstruction

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1		Atanas Iliev Batashki	--	2432	2024-02-15 09:46:46	\|
2	Reference format revised.	Lindsay Dong	Meta information modification	2432	2024-02-18 01:54:02	\|

This entry is adapted from the peer-reviewed paper 10.3390/medicina60010040

The advancements in 3D printing to aid synthetic material fabrication have unveiled promising alternatives to conventional approaches. Achieving successful tracheal reconstruction through this technology demands that the 3D models exhibit biocompatibility with neighbouring tracheal elements by encompassing vasculature, chondral foundation, and immunocompatibility. Tracheal reconstruction has employed grafts and scaffolds, showing a promising beginning in vivo. Concurrently, the integration of resorbable models and stem cell therapy serves to underscore their viability and application in the context of tracheal pathologies.

tracheal reconstruction 3D printing tissue engineering

1. Introduction

The trachea is a U-shaped semiflexible tube measuring a length of 10–13 cm in length, consisting of incomplete rings with hyaline cartilage on anterior and lateral walls and the posterior border layered by trachealis smooth muscle. Beginning inferiorly from the larynx, it extends from C4–C5 to T4–T5, terminating at the tracheal carina before dividing into the left and right main bronchus ^[1]^[2]. The functional physiology of the trachea facilitates crucial roles by providing air conduction between the larynx and bronchi, enabling heat and moisture exchange and particle elimination ^[2]. Tracheal abnormalities are often divided into two categories: congenital and acquired. Congenital tracheomalacia and tracheal stenosis are more commonly associated with infants ^[3], whereas acquired tracheal lesions are associated with malignancy, traumatic injury, complications of tracheostomy tubes, and subglottic stenosis ^[4]. Tracheomalacia and stenosis are fatal conditions for adults and infants, necessitating appropriate intervention according to the severity ^[5]. CPAP through tracheostomy was shown to aid infants with moderate to severe tracheomalacia, but surgery is more often indicated for adults with severe cases of tracheomalacia ^[6]^[7]. When evaluating a surgical approach to dealing with tracheal disorders, resection and anastomosis are specified for segment stenosis with a length of 4–5 cm or less in adults and 2 cm or less in children ^[8].

With the innovative advancements in synthetic material fabrication, a promising new avenue has emerged as a potential alternative to conventional approaches. Successful tracheal reconstruction using such technology requires 3D models to have a biocompatible nature with the adjacent tracheal components including vasculature, chondral foundation, all the way down to a histological cellular level ^[9]. Despite the development of advanced techniques and new source materials, the most complex challenges lie in the complexity of the tracheal anatomy, due to its limited blood supply and its cylindrical framework allowing longitudinal flexibility and lateral stability.

2. Synthetic Material Fabrication in Tracheal Reconstruction

2.1. Tracheal Anatomy

To successfully perform surgical manoeuvres on the trachea, an established understanding of tracheal blood supply is required alongside the positional relationship between the vasculatures. This is vital when avoiding sequalae of tracheal ischemia. The tracheal blood supply is divided into upper (cervical) and lower (thoracic) with the upper being supplied by the inferior thyroid artery whilst the lower tracheal supply is through the first tracheoesophageal arteries and bronchial arteries directly from the aorta. The lateralised location of the arteries along the longitudinal section must be considered. The anteromedial aspect of the descending aorta gives rise to the superior bronchial artery, which is lateral to the carina and posterior to the left main bronchus. The anterior branch of the bronchial artery supplying the anterior carina courses over the proximal left main stem bronchus ^[10]. The structural relationship is important to bear in mind during surgery, with nearby structures at higher risk of iatrogenic injury. With the trachea, the left and right thyroid lobes sit anterolateral to the upper trachea. With the oesophagus beginning at the level of the cricoid cartilage, it passes along the left posterior border of the trachea. The right posterior border of the trachea is positioned anteriorly to the anterior surface of the vertebral bones. During the dissection of the proximal trachea, nerve orientation is key to preserve neurovascular structures. The recurrent laryngeal nerves enter the larynx between the thyroid and cricoid cartilage under the inferior horn of the thyroid cartilage. The left recurrent laryngeal nerve originates beyond the aortic arch, then curves posteriorly, running just lateral to the ligamentum arteriosum. It subsequently loops back and ascends along the left tracheoesophageal groove towards the cricoid cartilage. The right recurrent laryngeal nerve branches from the right vagus nerve shortly after the right subclavian artery, diving posteriorly beneath its origin. It then recurs and ascends within the right tracheoesophageal groove toward the cricoid cartilage. Damage to the recurrent laryngeal nerves, whether due to complete or partial transection, ischemia, contusion, traction thermal injury, or tumour intrusion, can lead to vocal cord weakness or complete paralysis. This may result in hoarseness or even complete loss of voice or airway function, depending on the severity of the injury and the condition of the opposite nerve. Surgeons operating on the proximal trachea must be cautious and mindful of these nerves’ paths during dissection ^[11].

2.2. Aliphatic Polymers

The potential personalising ability of scaffolded materiel could provide major benefits with enhanced mechanical strength and excellent biocompatibility ^[12]. An increase in material availability with 3D bioprinting provides a greater variety in material durability and compatibility ^[13]. When evaluating biomaterials, it is key for them to have similar anatomical and physiological properties to the tracheal cartilage. A group of aliphatic polymers including poly-caprolactone (PCL), poly-glycolic acid (PGA), poly-lactic acid (PLA), and poly-lactic-co-glycolic acid (PLGA) have been extensively researched in their potential application and integration in tracheal reconstruction through 3D printing.

2.3. Tracheal Grafting and Scaffolding

PCL is at the forefront of tracheal biomaterial research, providing superb results in maintaining airway patency, tracheal wall integration, granulation tissue prevention, and involvement in mucosalisation in respiratory epithelium when used to recreate a partial tracheal defect ^[14]. Using an electrospun patch, including randomly layered PCL nanofibers enveloping 3D printed PCL rings, an airtight, bio-resorbable graft was created and tested for airway support in sheep with all surviving for at least 10 weeks ^[15]. When used in rabbits, none showed signs of respiratory distress and electron microscopy confirmed the regeneration of cilia with its beating frequency identical to natural tracheal cilia ^[16]. The challenge of vascularisation was approached with a PCL/vacuum-assisted decellularised trachea, which exhibited angiogenesis in vivo alongside low immunogenicity ^[17]. It has an ideal non-toxic degradation alongside low porosity, which stimulates chondrogenesis, thus strengthening the cartilage ^[18]. All this, coupled with its low boiling point, allows PCL to be easily printed with 3D rendering machines, making its use economically viable ^[19]. PGA and PLA were trialled in laryngotracheal reconstruction in rabbits with results showing stable airtight airways and re-epithelialisation seen alongside minimal scarring and no stenosis, although its stability in larger animals has yet to be seen ^[20].

The integration of 3D printed PCL microfibres with an artificially designed trachea was shown to enhance tracheal cartilage and mucosa regeneration through the combination of human induced pluripotent derived stem cells and a two-layer tubular layer scaffold. Micro-CT analysis showed effective neocartilage formation at defect sites ^[21]. When a 3D printed (3DP) PCL scaffold was coated in mesenchymal stem cells (MSC’s) seeded in fibrin, the half-pipe tracheal graft was able to restore shape and function in rabbits without any graft rejection ^[16]. Similarly, when 3DP poly-L-Lactic acid scaffolds were seeded with chondrocytes in rabbits via end-to-end anastomosis, there was successful tracheal segment regeneration alongside neo-angiogenesis, which facilitated the survival of the chondrocytes ^[22]. With thermoplastic polymers being ideal due to their boiling points and cost of printing, a blend of thermoplastic polyurethane and PLA have ideal properties due to their biocompatibility and absorption/degradation rate ^[23]. When the trachea is based on a 3DP scaffold, it acts as the baseline foundation, but when regenerative stem cells were incorporated into the scaffold, it transforms its applicational ability due to its implantable nature allowing it to carry out the anatomo-physiological function while regenerating the trachea and adapting to the surrounding tracheal structures. In an independent study, porcine derived small intestine submucous extracellular matrix patches were wrapped around PCL supports and lined the inside and outside of the graft, which allowed the animals to remain clinically stable for two weeks with no respiratory distress ^[24].

2.4. Resorbing Models

In a patient with paratracheal tumour resection alongside tracheal reconstruction, a resorbing synthetic mesh was successfully placed with a myocutaneous pedicled pectoralis flap. After a 9-month follow-up, the patient showed a well-healed flap with no evidence of tracheomalacia, and no breathing complaints 12 months after the procedure ^[25]. Tian et al. has previously incorporated 3DP to produce a custom titanium mesh for patients with thyroid cartilage reconstruction ^[26]. A similar concept can be applied to the use of a nickel-titanium shape memory alloy mesh, which produced promising results in patients, where a woman with left vocal fold paralysis and subglottic atresia had the memory alloy mesh surgically implanted as an extraluminal tracheal scaffold and showed no signs of dyspnoea or discomfort upon exertion ^[27]. Marlex mesh has been used in conjunction with pericardium to manipulate tracheal reconstruction in 13 patients suffering from malignancy. With the results of the mesh insertion being successful, expansion on its use can be trialled ^[28]. Given the polypropylene foundation of a Marlex mesh ^[29], its potential for 3DP holds major value for the future of mesh-based tracheal reconstruction.

2.5. Stem-Cell Biology and Epithelial Reconstruction

The continuity of the tracheal epithelium and mucosa is breached in congenital tracheal anomalies, which compromises the structure and functionality. With synthetic and allogeneic materials studied, a reliable tracheal tissue replacement is missing, fuelling expanded research into tissue engineering. By isolation, respiratory epithelial cells from the nasal turbinate are incorporated into calcium chloride polymerised blood plasma to produce human tissue respiratory epithelial construct (HTREC). After gene expression analysis, Ki67 (a proliferative marker) and MUC5AC (a mucin-secreting marker) both had significant increases in gene expression levels after four days, highlighting the viability of HTREC as an option for respiratory epithelial reconstruction ^[30]. When autologous nasal epithelial sheets were incorporated into a partially decellularised scaffold in rabbits, the cell sheets enhanced epithelial regeneration and the transplanted patches were wholly incorporated into the trachea after two months ^[31].

2.6. Bioink Usage for Structural Biomimicking

Through the utilisation of photocrosslinkable tissue specific bioinks, a novel strategy was developed, incorporating 3DP to produce a cartilage vascularised fibrous tissue integrated trachea. The physiological importance of maintaining the tracheal architecture assists in mimicking tracheal function. Sun et al. previously used an O-shaped tracheal design but, to comply with functionality, a C shaped biomimetic 3DP trachea was manufactured and its mechanical adaptability and physiological regeneration tested. The C shape effectively dissipated anisotropic forces, thereby enabling a natural dynamic movement of the trachea. Cytocompatibility testing of the GC-Gel and GE-Gel showed no cytotoxicity reinforcing the photocrosslinkable nature of these hydrogels.

2.7. Material Summary

Table 1 included a summary of the materials with their individual analysis to provide a clear and distinguished outlook on each one to better assess their uses.

Table 1. Summary of synthetic materials used for tracheal reconstruction.

Type of Material	Fabrication	Degradation	Biocompatibility	Mechanical Properties	Study Type	Surgery Type	Outcome
Poly-caprolactone (PCL) ^[12]^[14]^[15]^[32]^[33]	1. Patch 2. Splint 3. Stent	2–3 years	1. Nontoxic 2. Maintains Airway patency 3. Compatible integration into tracheal wall 4. Minimal granulation tissue formation 5. Enables mucosalization with respiratory epithelium	1. Low melting point for shaping 2. Good flexibility at room temperature 3. Semi—crystalline polymer	Patch—In Vivo with Sheep Splint—Clinical Trial with paediatric patient Stent—Clinical Trial with paediatric patient	Patch—Patch tracheoplasty Splint—Bronchoplasty Stent— Esophagotracheoplasty	Patch—Sheep survived up to 10 weeks. Splint—Patient survival Stent—Patient survival
Poly-glycolic acid (PGA) ^[12]^[20]	Mini plate	6–12 months	1. Nontoxic 2. Maintains airway patency 3. Complete resorption and integration into tracheal wall 4. Good re-epithelialization with some rabbits exhibiting neocartilage formation 5. Some rabbits exhibited mild inflammation focussed on the plate implant.	1. Rigid with a poor flexibility 2. Semi—crystalline polymer	In vivo with Rabbits	Laryngotracheoplasty	All rabbits survived.
Poly-lactic acid (PLA) ^[12]^[23]	Splint	1–2 years	1. Nontoxic 2. Good biocompatibility and integration into the tracheal wall. 3. Demonstrated high water absorption like regenerative process of native tissue 4. Maintenance of airway patency	1. Semi—crystalline polymer 2. Does not exhibit an elastic nature which makes it inflexible	Ex Vivo	No surgical intervention as study was performed Ex Vivo	The model demonstrated that tracheomalacia can be successfully treated with a PLA based 3DP splint.
Poly-lactic-co-glycolic acid (PLGA) ^[12]^[34]	Microplate	1–2 years	1. Low toxicity if resorbed below 2 years 2. Can induce an acute and chronic inflammatory response which could lead to scarring and fibrous capsule formation 3. Maintains airway patency	1. Has a high tensile strength 2. Has a high flexural and bending strength. 3. Low melting point for shaping	Clinical Trial with paediatric patients	Microplate partial tracheoplasty	All patients survived.
Nickel-titanium alloy ^[12]^[27]	Mesh	Does not degrade	1. Can be toxic at high levels 2. Alloy has a high biocompatibility with human tissue 2. Small granulomas from chronic inflammation but did not impede airflow	1. Shape memory alloy 2. Woven mesh reinforced fibrous tissue to increase strength.	In Vivo with Dogs Clinical trial with one patient.	Mesh partial tracheoplasty	1 dog died 5 days after the operation and the remaining survived during the observation period. Patient in a clinical trial survived.
Polypropylene ^[12]^[35]	Mesh	Does not degrade	1. Nontoxic 2. Minimal signs of stenosis 3. Maintenance of airway patency 4. No granulation tissue found in lumen	1. The mesh provided stiffness against compression 2. Mechanical resistance against compression matched the native trachea	In Vivo with Dogs	End-to-End Anastomosis	All dogs survived and had an uneventful postoperative course.

2.8. Roadblocks Impacting Innovation

A major drawback of tissue engineering constructs lies in the time frame. With mucosalisation and epithelialisation taking significant time, the process is only hindered when applied to longer tracheal segments. With epithelial failure during neo-tracheal migration, an incomplete process results in cicatrix formation and without an epithelial barrier as protection, it opens a risk of bacterial infection, re-granulation, and potential restenosis ^[36]. With 3DP PCL rings, there were fibrotic deposits surrounding the luminal areas of the ring, which were implanted as a patch due to the acute and chronic inflammation resulting from its initial implantation. If significant, this could cause stenosis and require a re-operation. A necrotic process could follow an untreated fibrotic accumulation, leading to further infection and a substantial reduction in the cellular presence within the patch ^[15]. With chondrocyte-seeded fibrin/hyaluronan producing a successful partial reconstruction, there was a lack of neo-cartilage formation, which inevitably impacted the mechanics and structural integrity of the trachea leading to future collapse ^[37]. Inadequate mechanical strength alongside a short absorption time has made PLGA difficult to incorporate for tracheal reconstruction. Despite their high porosity allowing cell infiltration and neovascularization ^[38]^[39], PLGA is not considered for a long-term therapeutic effect. Due to the trachea being primarily sourced by the inferior thyroid artery and superior bronchial artery, there is a lack of arteriovenous vessels for anastomosis, which prevents the graft from utilising the tracheal blood supply. Without a supply of endothelial progenitor cells for neovascularization, graft necrosis from vascular ischemia and cell death will result ^[17]. Long-segment tracheal reconstruction faces the biggest challenge, with many animals dying from respiratory distress caused by airway stenosis ^[27]. Within paediatric situations, growth distortion can occur due to cartilage immaturity, which creates a growth disparity between the trachea and the graft, predisposing tracheal collapse.

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