# 3D Printing Thermoplastic Vulcanizates: Current Limitations, Innovative Solutions, and Emerging Applications

Abbas Bayati, Mohammad Amin Yousefi, Davood Rahmatabadi, Majid Baniassadi, Karen Abrinia, Mahdi Bodaghi, Daniel George, Mostafa Baghani

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Abbas Bayati, Mohammad Amin Yousefi, Davood Rahmatabadi, Majid Baniassadi, Karen Abrinia, et al.. 3D Printing Thermoplastic Vulcanizates: Current Limitations, Innovative Solutions, and Emerging Applications. Materials Today Advances, In press, 28, pp.100664. 10.1016/j.mtadv.2025.100664. hal-05367987

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Submitted on 24 Nov 2025

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# 3D printing thermoplastic vulcanizates: Current limitations, innovative solutions, and emerging applications

Abbas Bayati<sup>a</sup>, Mohammad Amin Yousefi<sup>a</sup>, Davood Rahmatabadi<sup>a</sup>, Majid Baniassadi<sup>a</sup>, Karen Abrinia<sup>a</sup>, Mahdi Bodaghi<sup>b</sup>, Daniel George<sup>c,**</sup>, Mostafa Baghani<sup>a,*</sup>

## ARTICLE INFO

### Keywords:

Thermoplastic vulcanizates

Elastomers

Additive manufacturing

Dynamic vulcanization

Material extrusion

## ABSTRACT

Thermoplastic vulcanizates (TPVs) are a class of thermoplastic elastomers (TPEs) that achieve a balance between rubber elasticity and thermoplastic processability, which might position them as a sustainable potential replacement for conventional thermoset rubbers. This review covers current state-of-the-art 3D printing TPVs, focusing on the challenges and prospects associated with Material Extrusion (MEX) Methods. Dynamic vulcanization (DV) in processing TPVs increases the dispersion and crosslinking of the elastomeric phase, which might improve both mechanical properties and recyclability. However, the primary obstacles in 3D printing TPVs include filament buckling, phase incompatibility, and high melt viscosity, leading to poor printability and mechanical defects. While there is extensive research on TPVs, these challenges completely prevent their use in additive manufacturing (AM) methods. Various strategies are presented as potential solutions, including adjusting the crosslinking density, incorporating compatibilizers, and using alternative printing methods. While 3D printing of TPVs remains challenging, advancement in material formulation and AM techniques could unlock their full potential in cutting-edge technologies and a greener future. The study highlights the importance of continued research into TPV-based filaments, direct pellet printing, and 4D printing to expand their applications in automotive, biomedical, and soft robotics fields.

## 1. Introduction

Elastomers or rubbers are a unique class of polymers that exhibit both elastic and viscous properties [1]. This distinctive characteristic arises from their molecular structure, composed of long chains of polymer molecules [2]. Elastomers demonstrate a remarkable ability to return to their original shape upon the release of stress, making them highly attractive materials for scientific research [3,4]. This group of polymeric materials holds significant importance and is in high demand worldwide. According to some research, the global elastomers market reached $97.99 billion in 2022 and is expected to surpass approximately $186.54 billion by 2032 [5]. These materials possess unique properties such as high elasticity, softness, and vibration damping, making them ideal for everyday use in automotive tires [6], aircraft [7], seals [8], and vibration absorbers [9], as well as advanced applications in soft robotics [10], stretchable sensors [11], and wearable devices [12]. Additionally,

elastomers typically have a Young's modulus in the range of a few MPa or less, making them an attractive choice for mimicking biological tissues [13]. A typical elastomer can be stretched several times its normal length and return to its original shape upon release of stress [14].

Elastomers are broadly classified into two categories: physically cross-linked or thermoplastic elastomers (TPEs) and chemically cross-linked or thermoset rubbers [15]. Thermoset elastomers, that cannot be melted, are unsuitable for melt-dependent manufacturing methods. These materials burn at elevated temperatures, making them non-recyclable and non-reprocessable by physical methods. As a result, they are either incinerated or landfilled at the end of their lifecycle. Consequently, the use of these elastomers causes severe environmental problems [16]. The disposal of plastic waste has become an urgent global concern [17]. Plastic pollution is a growing issue that threatens the environment and human well-being. This waste can contaminate soil, waterways, and aquatic ecosystems, and even accumulate in the

<footnote><sup>a</sup> School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran</footnote>

<footnote><sup>b</sup> Department of Engineering, School of Science and Technology, Nottingham Trent University, Nottingham, NG11 8NS, UK</footnote>

<footnote><sup>c</sup> University of Strasbourg, CNRS, ICube, 67000, Strasbourg, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>** Corresponding author.</footnote>

<footnote>E-mail addresses: george@unistra.fr (D. George), baghani@ut.ac.ir (M. Baghani).</footnote>
bodies of humans and animals [18]. Plastic recycling is one method that can help reduce the amount of plastic waste that ends up in landfills and oceans [19].

In recent years, cross-linked elastomers with thermoplastic characterizations, also known as thermoplastic vulcanizates (TPVs), which are a subset of TPEs, have been identified as a highly suitable and sustainable alternative to thermoset rubbers [20]. The sustainability of these eco-friendly materials can address the challenges associated with thermoset rubbers and is essential for a greener future. These materials, also known as green plastics, combine the properties of thermoset rubbers with the processability and recyclability of thermoplastic polymers, attracting significant attention. TPVs have found applications across various fields, including high-temperature fluid transfer hoses in automobiles [21], conductive polymers [22–24], wearable electronic devices [25], and flame-retardant polymeric materials [26]. Like conventional plastic-rubber blends, TPVs are composed of a thermoplastic polymer and an elastomeric material. The thermoplastic or plastic component typically exhibits higher strength and lower elongation, while the elastomeric or rubber component offers higher elongation and lower strength. Combining these two phases allows for tuning the final material properties. Adjusting the plastic-to-rubber ratio can achieve the desired properties for specific applications. For instance, increasing the elastomeric content results in a softer, tougher, and more elastic material. The primary distinction between TPVs and conventional plastic-rubber blends lies in the elastomeric phase’s dynamic vulcanization (DV) during the blending process. DV leads to the cross-linking of the elastomer phase, enhancing its dispersion within the plastic phase, and yielding superior properties and recyclability. Additionally, this type of blending is used to toughen brittle and rigid plastics, such as polylactic acid (PLA).

Major TPV manufacturers include Teknor Apex, ExxonMobil, Mistui Chemicals, Ravago Chemicals, and Kumho Polychem, collectively accounting for about 70% of TPV production. Approximately 50% of TPV is used in the automotive industry, while about 30% is allocated to construction applications [27].

In recent decades, parts made from TPVs, like many other polymers, were predominantly produced using traditional plastic processing techniques such as injection molding, extrusion, and injection blow molding. However, with the recent advent of additive manufacturing (AM) technologies, or 3D printing, many researchers have become interested in fabricating polymeric materials using these innovative methods. The primary reasons for this growing interest include the cost-effectiveness, simplicity, and ability to produce complex shapes and geometries [28]. For instance, AM is used in producing metamaterials [29–31] and biological scaffolds [32], both of which feature intricate structures with high levels of detail. AM is highly effective for producing customized and individual parts, making it superior to traditional methods for rapid prototyping and pre-production processes. Conventional processes require the creation of molds and specialized tooling, along with high initial setup costs, making them unsuitable for producing single or custom-made components. However, with advancements in 3D printing technologies, these processes are now being used to produce final parts, and their applications extend beyond rapid prototyping. For example, customized prosthetics for individual patients [33] or even personalized drug dosages [34] are among the applications of AM, capabilities that are unattainable with traditional technologies.

One of the most well-known and widely used 3D printing techniques is Material Extrusion (MEX), also called Fused Deposition Modeling (FDM). In this method, the thermoplastic material is heated and melted, then deposited layer by layer through the movement of a nozzle across the surface [35]. A computer-aided design (CAD) model and a motion control system control the nozzle’s movement. The most common form of this 3D printing technology is Fused Filament Fabrication (FFF), which involves melting thermoplastic filaments fed into a heating chamber, where they are melted before being extruded [36]. Currently, the FFF market is largely dominated by PLA and ABS filaments, both of

which are rigid and inflexible materials [37]. In recent years, the market has seen an increasing demand for softer filaments. Some flexible elastomers for FFF, such as thermoplastic polyurethane (TPU), are commercially available. For instance, the Ninjatek® brand offers Ninjaflex and Cheetah, which are used in prototyping applications where elasticity or impact resistance is desired. Beyond TPU, the commercial 3D printing of elastomers often relies on more expensive and therefore less popular techniques, such as photopolymerization. Fig. 1 illustrates the benefits of 3D printing and TPVs individually, as well as the possible applications that arise from combining these two technologies.

The combination of AM processes with TPVs offers numerous advantages. For instance, a filament made from TPV based on polypropylene (PP) can not only retain the good flow properties of PP but also address issues associated with PP during 3D printing, such as warping or low elasticity [38,39]. In addition, properties like flexible production, ease of processing, long-term durability, high-temperature resistance, gas impermeability, system cost reduction, weight reduction, sustainability, and recyclability, when compared to materials such as thermoset rubber, can be imparted to the printed part. Although there have been some efforts by researchers to print TPVs using FFF methods, the number of studies in this area remains limited. The primary reasons for this limitation lie in the intrinsic properties of these materials. Awasthi and Banerjee [40], in their review on FDM of thermoplastic elastomeric materials, stated that "commercially available PP/EPDM TPVs are not printable using the current commercial 3D printers based on fused filament fabrication". However, they highlighted that printable 3D TPVs can be developed by leveraging their flow properties.

Fig. 2a shows the number of documents published from 1970 to 2024 that contain ‘additive manufacturing’ or ‘3D printing’ in their abstract, title, or keywords, while Fig. 2b presents similar results for ‘thermoplastic vulcanizate’ based on the Scopus database.

As can be seen from these figures, both concepts have existed in the scientific community for over 50 years. The number of published papers on AM increased from 9 papers in 1970 to 28,142 papers in 2024, demonstrating significant attention from the community on this topic. On the other hand, while TPV is an established topic, it has not been well represented in the literature despite its numerous advantages. In 2024, only 57 papers were published on the topic of TPVs. One of the reasons behind this underdevelopment may be the lack of adaptation of these materials to new manufacturing technologies. This indicates the importance of thoroughly investigating the problems that limit the application of these materials with new manufacturing methods, presenting potential solutions, and indicating the prospects and applications.

Fig. 2c and d show the types of documents published on TPVs. Based on these two figures, 754 documents were research papers in this field, which account for 77.1% of the total documents. It is important to note that reviews constitute only 2.8% of the total documents, and since 1970, only 27 reviews have been published about TPVs. In addition, none of these reviews focus on 3D printing of TPVs.

Since the expansion of new additive printing methods will lead to the increased use of soft materials and plastics, the environmental pollution of thermosets will be extremely challenging, and the use of TPVs with recyclability and mechanical properties close to thermosets can be a suitable solution to this problem. However, the use of 3D printing of TPVs is a nascent topic due to the challenges of designing and manufacturing the material and its printability. To this end, this review paper addresses all the environmental challenges, design, manufacturing, and 3D printing of TPVs and provides suggestions for their improvements.

While numerous review papers have been published on the 3D printing of elastomers [2] and TPEs [40], none have specifically focused on the 3D printing of TPVs and its associated challenges, nor have they proposed solutions to overcome them and propose potential applications of 3D printed TPVs. This review article will examine TPVs and AM methods, particularly MEX, along with the challenges of printing TPVs
<figure><img src="image_3.png" /><figcaption>Fig. 1. Benefits of 3D printing and TPVs with their combined applications.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 2. Trends in publications from 1970 to 2024: (a) Number of documents in additive manufacturing and 3D printing, (b) number of documents in TPVs, (c) number of documents by type in TPVs, and (d) percentage of documents by type in TPVs.</figcaption></figure>

with MEX and the works conducted in this field. We will also discuss potential solutions for printing TPVs and future perspectives in terms of potential applications, future sustainability, and identify research gaps in this area.

## 2. Thermoplastic elastomers (TPEs)

Elastomers exhibit three fundamental characteristics: (i) like other polymers, they are composed of polymer chains, (ii) these chains possess a chemical structure that provides high flexibility and mobility, allowing elastomers to stretch easily and exhibit elasticity, and (iii) these flexible chains are interconnected in a network structure, which is essential for achieving elastomeric properties. This network structure enables the material to recover its original shape after being stretched, a feature

made possible by crosslinking, which prevents irreversible sliding of polymer chains during deformation [41–43].

Based on the type of crosslinking and network formation, elastomers can be classified into two general categories: elastomers with physical crosslinking, commonly referred to as TPEs, and elastomers with chemical crosslinking, known as thermoset elastomers [44].

TPEs can be melted and processed through molding or other manufacturing techniques by increasing the temperature. According to the International Institute of Synthetic Rubber Producers, TPEs are defined as [45]: "Polymers, polymer blends, or compounds that exhibit thermoplastic behavior above their melting temperature that enables them to be shaped into finished products and which, within their design temperature range, exhibit elastomeric behavior without crosslinking during fabrication. This process is reversible, and the product can be
reprocessed and remolded".

The flexible polymer chains in TPEs are connected by weak bonds, including hydrogen bonds, dipole interactions, crystalline regions, and glassy domains. Most TPEs are composed of two distinct phases: hard segments and amorphous segments. The hard segments act as cross-linking points, providing mechanical strength and elasticity recovery, while the amorphous or soft segments provide flexibility. This unique combination of properties allows these sustainable materials to be melted, reshaped, and recycled multiple times while retaining their elastic properties.

TPEs can be broadly divided into two main groups: rubber-plastic blends and block copolymers [46]. Rubber-plastic blends are mixtures of semi-crystalline and amorphous polymers, where the semi-crystalline portion (e.g. PP) acts as the strong, stiff component, and the amorphous portion (e.g. EPDM) serves as the soft, flexible part. In contrast, block copolymers consist of distinct crystalline and amorphous domains within a single polymer chain. These two categories are illustrated in Fig. 3.

Crystalline domains, also known as hard segments, have an organized structure that imparts thermoplastic properties to TPEs [47]. The crystalline phase forms a compact structure with intermolecular forces that hold the polymer chains together, creating a molecular architecture resembling a crosslink. On the other hand, the amorphous domains, referred to as soft segments, possess a random structure that contributes to the elastic behavior of the material [48]. This is due to the greater free volume in their molecular structure, which allows for more molecular movement.

Block copolymer-based TPEs consist of polymers with hard and soft segments along their main chain. When these polymers solidify from their molten state, the crystalline domains consolidate into hard segments, exhibiting thermoplastic behavior with a glass transition temperature (Tg) above room temperature. Meanwhile, the amorphous domains form elastomeric bridges or bonding molecules with a Tg below room temperature, which gives TPEs their elastic properties.

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Two main groups of TPEs. TPEs can be categorized into six subgroups: styrenic block copolymers (TPS), thermoplastic polyolefins (TPO), thermoplastic vulcanizates (TPV), thermoplastic copolyesters (TPC), thermoplastic polyamides (TPA), and thermoplastic polyurethanes (TPU). TPOs and TPVs are produced through polymer blends, while the remaining TPEs are block copolymers.</figcaption></figure>

### 3. Thermoplastic vulcanizates (TPVs) and processing methods of TPVs

As previously mentioned, TPVs are created from a polymer blend comprising a thermoplastic material and an elastomer. In this respect, there is little distinction between TPVs and other TPEs. However, TPVs consist of crosslinked rubber as the dispersed phase and a thermoplastic material as the continuous phase, which are produced through static vulcanization or dynamic vulcanization [49,50]. These two processing methods will be discussed in detail below.

Researchers often prefer DV as a preparation process because it is simple and efficient. This method allows for the quick formation of a uniform three-dimensional rubber network structure since the rubber phase is cured under shear force while being mixed with thermoplastics during the melt-mixing process [51]. In TPVs, the rubber domains are well-dispersed within the continuous thermoplastic matrix [52]. Although the rubber phase is chemically crosslinked through vulcanization, the thermoplastic phase remains melt-processable, allowing the material to flow at processing temperatures and reversibly solidify upon cooling to be a sustainable material.

Several new types of TPVs with different applications and diverse properties can be developed through DV by carefully selecting appropriate combinations of rubbers and thermoplastics. The elastic, thermal, mechanical, and flow characteristics of TPVs can be tailored by controlling the degree of curing, adjusting the concentrations of curing agents and other additives. The key advantages of TPVs include enhanced tensile strength, higher elongation at break, improved melt strength, superior extrusion characteristics, increased resistance to cracking, extended high-temperature applications, and greater resistance to corrosive fluids [21,53,54]. The most important properties of all TPVs, including mechanical properties, elasticity, and rheological characteristics, are governed by their microstructure [55–57]. To achieve high elasticity in TPVs, a rubber phase with a high degree of crosslinking is required. The ratio of rubber to plastic in these materials also plays a critical role in determining other properties. However, to ensure good processability and easy recyclability of TPVs, which is a crucial aspect of their eco-friendliness, a continuous plastic phase is needed as the matrix. On the other hand, to achieve good mechanical properties, a finely dispersed rubber phase is essential. Even though crosslinked rubber domains form the dispersed phase in TPVs, they typically exhibit excellent elasticity like vulcanized rubbers.

Historically, Gessler [58] first introduced the concept of DV in 1962, and TPVs were officially developed in 1972. In 1973, Fischer [50] produced a TPV composed of ethylene-propylene-diene monomer rubber (EPDM)/PP, where the cured EPDM phase retained processability. In 1978, Coran et al. [59] conducted extensive studies on EPDM/PP TPVs that were fully vulcanized under dynamic shear, resulting in a highly crosslinked rubber phase and significantly improved TPV properties. By the early 1980s, they expanded their research on TPVs based on different constituent components [60]. These efforts led to the commercialization of the first dynamically vulcanized thermoplastic product, based on EPDM/PP and marketed under the name *Santoprene*, by Monsanto (now ExxonMobil AES) in 1981 [61]. Later, various new families of general-purpose TPVs were explored, including ethylene-octene copolymer (EOC)/PP blends [62,63], natural rubber (NR)/PP [64,65], and epoxidized natural rubber (ENR)/PP [66]. Meanwhile, various types of special-purpose TPVs, such as nitrile butadiene rubber (NBR)/PP [67,68] and isobutylene-isoprene rubber (IIR)/PP [69], garnered significant attention due to their unique properties, such as excellent oil resistance, gas barrier performance, or high-temperature resistance, which made them suitable for specialized uses [70–74]. In recent years, there has been growing interest in developing bio-friendly TPVs from natural sources, which offer biocompatibility and biodegradability. These materials are particularly appealing for applications in the medical and tissue engineering fields due to their environmentally friendly and bio-safe characteristics.
The processing of TPVs, like other elastomers, involves two primary stages: formulation design and production. The first stage includes creating a specific compound tailored for a particular application, while the second stage focuses on converting the rubber compound into the final product through a manufacturing process [75]. During the processing phase, various chemicals, blend components, additives, and fillers can be added to the elastomer. Typically, this process is carried out using one of two main types of mixers: a two-roll mill or an internal mixer. Both types of mixers are batch mixers, meaning they process small quantities of materials at a time. To improve the efficiency of this process, efforts have been made since World War II to develop continuous mixing techniques that can process larger quantities of material simultaneously [76].

The method used to prepare TPVs and the sequence of processing steps significantly impact their properties. Generally, two main methods, as discussed before, are followed for TPV preparation: static vulcanization and DV. The schematic representations of these methods

are shown in Fig. 4a [53].

As shown in Fig. 4a, in static vulcanized TPVs, the rubber phase is cured in a compression process or via a two-roll mill during a specified curing time, where it is mixed with a suitable curing agent. The semi-cured rubber is then blended with thermoplastics at processing temperatures to create TPVs. In this case, the crosslinking of the rubber phase occurs rapidly due to the pre-mixing of the curing agents with the rubber, while the melt mixing time between the rubber and plastic is relatively short. This method is used less frequently by researchers for preparing TPVs.

Dynamic vulcanization is a specialized polymer reaction-mixing technique and is a process in which the soft rubber phase is vulcanized or cured in the presence of molten thermoplastic under shear forces [77]. In this process, the rubber phase is simultaneously mixed and vulcanized during the melt-blending with the thermoplastic material. DV can be carried out using two primary methods, each leading to distinct microstructures and mechanical properties. Both methods

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** TPV preparation process: (a) static and dynamic vulcanization for processing TPVs (Inspired by Ref. [53]), (b) phase inversion (Reproduced with permission from Ref. [40]), and (c) torque-time curve and microstructure evaluation in method 2 of DV.</figcaption></figure>
involve the initial mixing of rubber and plastic through melt mixing and DV, as shown in Fig. 4a [78].

Typically, a high proportion of the rubber and a smaller amount of the plastic phases are mixed together at high temperatures and high shear stress [79]. In this state, the rubber forms the continuous phase, while the plastic can either form the dispersed phase or the continuous phase, depending on the composition ratio and the viscosity ratio of the rubber (R) and plastic (P) (R/P). Curing agents and other additives are subsequently added to these pre-blended mixtures under the same processing conditions to enhance the DV of the rubber phase and further mix the various components [80]. Throughout the DV process, the crosslinking and breakage of the rubber phase occur simultaneously, leading to phase inversion. Also, the phase inversion of the rubber, from a continuous to a dispersed phase, is crucial for the formation of the final TPV structure [81,82]. This phase inversion is driven by increased rubber viscosity during crosslinking, causing it to change from the matrix phase to the dispersed phase [55]. The process of phase inversion is shown schematically in Fig. 4b.

The main difference between methods 1 and 2 in DV is when the curing agents are added. In method 2, curing agents are added to the melt while the thermoplastic and elastomer are being mixed. This addition causes phase inversion during the mixing process. The success of this phase inversion can be monitored using torque-time curves from the mixer, as torque is an indicator of the mixture's viscosity. In Fig. 4c, a schematic torque-time curve and microstructure evaluation of TPVs for this method are presented. At the starting point, when the thermoplastic is added to the mixer, a peak in torque is observed at point A due to the high viscosity of solid thermoplastics. After a few seconds, as the thermoplastic melts (point B), both torque and viscosity decrease. At point C, when uncured rubber is added, another peak in torque occurs. After a few minutes, at point D, the thermoplastic-elastomer blend stabilizes in a melted state with stable viscosity. At point E, when the curing agent is added to the melt, crosslinking of the rubber phase begins, leading to increased viscosity and torque. Once crosslinking is completed at point F, the phase inversion occurs; however, the rubber domains are relatively large. As mixing continues, these crosslinked rubber phases break into smaller domains, causing a slight decrease in viscosity at point G.

In method 1, the curing agents are added to the pre-mixed rubber/plastic blend after it has cooled to room temperature, before DV begins. This allows for better distribution of the curing agents, primarily within the rubber phase of the pre-mixed blend.

A brief overview of the advantages and disadvantages of each processing route for TPV production is provided below in Table 1. It can serve as a helpful guide for selecting the appropriate processing method for TPV preparation.

Once a rubber compound formulation has been designed, there are two primary methods for converting elastomers into final consumer products: conventional and advanced manufacturing technologies. Conventional production techniques include extrusion, calendaring, and molding.

The primary function of an extruder is to shape rubber compounds into a specific profile, which may either serve as the final step or as a pre-processing stage before the final rubber processing. There are two main types of extruders: ram extruders and screw extruders [83].

Calendars are used to produce rubber sheets of varying thicknesses. They are often employed to apply rubber sheets to textiles or other backing materials, or to process fabrics with rubber compounds, which impart friction properties.

Molding involves shaping and curing rubber compounds using heat and pressure in a mold with specific dimensions. The three primary molding methods are compression molding [84], transfer molding [85], and injection molding [86].

Traditional manufacturing techniques are well-established and suitable for the mass production of elastomeric materials with simple designs and geometries. However, modern industries demand customized and personalized products to meet specific customer needs and applications. Early stages of product development often involve rapid prototyping to test and refine designs. However, conventional manufacturing methods are costly, with high expenses associated with machinery, molds, and tooling, making them unsuitable for prototyping, producing customer-specific or personalized products [87,88].

Furthermore, complex geometries are often essential for advanced applications such as metamaterials, scaffolds, and wearable devices. AM offers a higher level of design freedom compared to traditional methods [89]. By manufacturing complex parts in a single step and machine, AM enables the creation of intricate designs that are difficult or impossible to achieve with conventional techniques.

The ability of AM to produce complex geometries has led businesses to consider it as a viable option for final product manufacturing. As a result, the integration of AM technology has evolved from rapid prototyping to rapid tooling and now towards direct manufacturing. This progression has transformed AM from a special technique, primarily used for prototyping, into a mainstream manufacturing method capable of producing high-quality, customized products [87].

## 4. Material extrusion in additive manufacturing

Additive Manufacturing (AM), also known as Rapid Prototyping (RP), Solid Freeform Fabrication (SFF), or 3D Printing, is a process used to convert a CAD into a physical object by adding material layer by layer

<table><caption>**Table 1**

Advantages and disadvantages of using static or dynamic vulcanization for TPV preparation.</caption><thead><tr><th>Processing method</th><th>Advantages</th><th>Disadvantages</th></tr></thead><tbody><tr><td>Static<br>Vulcanization</td><td><ul><li>greater oversight and management of the vulcanization process, ensuring that the rubber component is properly cured.</li><li>The cross-linking agent primarily influences the rubber phase without altering the thermoplastic phase.</li></ul></td><td><ul><li>The distribution of the rubber phase in the thermoplastic matrix may not be well-developed.</li><li>Reduced chance of rubber particles breaking or fragmenting.</li><li>The rubber phase may not achieve the minimum particle size needed for optimal performance.</li><li>Compromised Mechanical Properties.</li><li>Lack of chemical reactions at the interface between the thermoplastic and rubber phases.</li><li>Greater likelihood of incompatibility between the thermoplastic and elastomeric phases.</li><li>Extra processing steps are required for the rubber phase, increasing production complexity and time.</li><li>Decreased ability to manage and monitor the vulcanization process</li><li>The cross-linking agent may also influence the thermoplastic phase, potentially altering its properties</li></ul></td></tr><tr><td>Dynamic<br>Vulcanization</td><td><ul><li>The rubber phase is more evenly distributed within the thermoplastic matrix</li><li>The application of shear stress during the vulcanization process results in more rubber particle breakage, creating smaller rubber domains</li><li>Enhanced mechanical properties</li><li>Greater chance of chemical reactions occurring between the rubber and thermoplastic phases, which improves compatibility</li><li>The process is straightforward and can be completed in a single step</li></ul></td><td></td></tr></tbody></table>
[90]. This process can utilize various materials such as plastics [91], ceramics [92], metals [93], biomaterials [94], composites [95,96], and even food [97], to produce a wide range of objects [98,99]. Since its introduction in the 1980s, AM has rapidly found applications across various industries, from entertainment to producing final industrial products. Its use extends from the healthcare [100] sector to the automotive industries [101].

The American Society for Testing and Materials (ASTM) classifies AM processes into seven distinct categories [102]: Vat Photopolymerization (VP), Material Jetting (MJ), Binder Jetting (BJ), Material Extrusion (MEX), Sheet Lamination (SL), Powder Bed Fusion (PBF), and Directed Energy Deposition (DED) [103,104]. These categories are organized based on their energy consumption. For instance, Vat Photopolymerization requires the least energy, typically using a laser power of 40–50 mW to cure layers. On the other hand, Directed Energy Deposition consumes significantly more energy, generally ranging between 4 and 6 kW [105,106]. Only a few of the seven categories of AM are suitable for polymer printing. Vat photopolymerization and material jetting are primarily used for photocurable thermoset polymers. Among the other AM methods, MEX and PBF are the most used for thermoplastic materials. TPVs merge the processability of thermoplastics with the elasticity of elastomers, making them suitable for AM primarily through PBF and MEX. However, there are several limitations when using PBF techniques to process TPVs. Fig. 5 summarizes the disadvantages of MEX and PBF and shows why MEX is considered the better option for processing TPVs with AM techniques.

According to the ISO/ASTM 52900 standard, MEX is defined as “a process in which material is selectively dispensed through a nozzle or orifice” [107]. Based on this definition, several AM processes can be identified under this category. These processes include FDM or FFF, robocasting, Direct Ink Writing (DIW), Fused Granular Fabrication (FGF) or direct pellet printing, freeform 3D printing, and other metal and ceramic extrusion processes known by various brand names.

The first extrusion-based AM process, known as FDM, was developed in 1989 by S. Scott Crump, one of the co-founders of Stratasys Inc. [108]. Crump invented the FDM process and introduced the use of acrylonitrile butadiene styrene (ABS) filament in FDM printers. Consequently, Stratasys and 3D Systems emerged as leading companies in the global 3D printing industry [109].

The widespread adoption of FDM and MEX-based AM technology has been increasingly evident in the 3D printing community since its initial introduction. This groundbreaking technology revolutionized the manufacturing world by offering an accessible and cost-effective method for creating prototypes and functional parts [110]. The ability to produce complex geometries using FDM has opened endless opportunities across various industries. One of the key advantages of FDM is its simplicity and affordability. Setting up and operating FDM printers is relatively easy, making them accessible to a wide range of users [111].

MEX-based AM processes can be classified into solid, liquid, and powder-based processes, depending on the initial physical state of the raw materials. For instance, solid materials may take the form of filaments, wires, pastes (suspended granular materials), granules (small compressed masses), liquids, or powders. Each of these forms can be fed into the extrusion chamber using a different feeding mechanism.

MEX is one of the most widely used AM processes in terms of raw materials. FDM, also known as FFF, was the first MEX process introduced and remains the most popular technique for printing polymers [112]. These two terms are used interchangeably in the literature, and both refer to filament-based MEX processes. However, some sources point to a slight distinction between FDM and FFF [113]. The main difference is that FDM takes place in a thermally insulated and enclosed chamber that is maintained at a constant processing temperature. This results in better mechanical properties in the printed materials and stronger layer adhesion, but limits the maximum size of printable parts. In contrast, the newer FFF technology operates in open air [114], which causes the materials to experience varying temperatures during their

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Disadvantages of MEX and PBF for processing TPVs, illustrating why MEX is considered the better option.</figcaption></figure>
thermomechanical cycle. This can lead to undesirable residual stress and weaker layer adhesion compared to FDM.

Filament-based MEX is the most common method in the MEX process. FFF printers are available in various sizes, ranging from small desktop models found in offices, laboratories, and schools to larger industrial printers located in factories. Initially, these printers were mainly used for prototyping purposes. However, in recent years, they have gained attention for final product manufacturing as well. FFF is comparable to injection molding, particularly in mass production and customized manufacturing, as it enables the production of personalized items in large quantities without the cost of mold and tooling [115]. Additionally, it allows for the creation of complex shapes and structures while reducing material waste, making it increasingly attractive in both research and industry.

Filament-based MEX printers use thermoplastic filament as their primary material. PLA and ABS are the most commonly used materials for creating filaments in FFF [116]. It is important to note that the filament must be a thermoplastic so that it can be melted to construct the desired object. The FFF technique follows a straightforward process. First, a 3D digital model of the object to be printed is created using CAD software. The model is then sliced into thin layers using slicing software, and a toolpath is generated for the printer. FFF printers consist of a nozzle that heats the thermoplastic filament to its melting point. The filament is then extruded in a controlled manner through the nozzle. The molten filament is deposited layer by layer on the print bed, following the toolpath generated by the slicing software. As each layer is deposited, it rapidly solidifies, forming a solid part. For overhanging or complex geometries, support structures made from different or the same materials are often added during the printing process to prevent sagging or collapse of the part during printing [117,118]. After the printing process, the final part needs to undergo post-processing, which includes removing any support material. The entire printing process is shown in Fig. 6, where the filament-based MEX feeding mechanism continuously feeds the filament to the extrusion nozzle by two rollers. These rollers are driven by stepper motors [119]. This continuous feeding process is a significant advantage of filament-based MEX techniques, allowing the production of larger parts. Other features of FFF printers are similar to those found in other MEX printers.

Filament-based MEX is not the only category of material feeding in MEX processes. MEX can be divided into four main categories based on the feeding mechanism, each with its own advantages and disadvantages. All these categories, along with their merits and demerits, are also

illustrated in Fig. 6.

As shown in Fig. 6, there are two other feeding mechanisms for MEX processes. One of the most well-known mechanisms is the screw-based feeding system, which uses a motor-driven screw to mix and press the material through the nozzle [120]. In this case, there is no need for a filamentation process, as polymer granules are fed directly into the hot chamber. This allows for a wider variety of materials, not just those available as commercial filaments. Additionally, eliminating the filamentation process saves time and energy and avoids problems related to filaments, such as uneven diameter or buckling. The process can be continuous because hoppers allow you to add material to the chamber while printing is in progress. However, control over feeding is limited, and it can be challenging to print detailed models.

Another feeding mechanism is the syringe-based or piston-based system. This mechanism can be divided into two distinct categories based on the type of material being printed. If the material is liquid or printed at room temperature, it is called robocasting or Direct Ink Writing (DIW) [121]. In the other case, if the material is in pellet or granule form (solid) and printed at elevated temperatures, this method, along with screw-based printing, is referred to as direct pellet printing or fused granule fabrication (FGF). In this mechanism, thermoplastic material is placed in a chamber and heated until it melts. Then, a piston, driven by fluid pressure or motor pressure, pushes the melted material through the nozzle. This mechanism can improve printing quality because it allows for better control over feeding. However, since the chamber volume is limited, this method has challenges when printing large models in one load. As a result, feeding is not continuous.

## 5. Challenges and solutions of MEX in TPV printing

In filament-based MEX processes, only thermoplastic filaments can be used. One of the primary challenges encountered when attempting to print elastomers with these systems is the inability to print chemically cross-linked elastomers (Thermosets), a common elastomer type. These systems can only process elastomers with physical cross-links (Thermoplastics). This limitation is prevalent across most MEX processes, except for DIW, which can print thermoset elastomers [122]. However, TPVs can be printed because they can be processed like thermoplastics and have the flexibility of thermoset elastomers.

Another challenge in filament-based MEX processes is the filament fabrication process itself. Converting polymer pellets into filaments can be time-consuming and costly, leading to material and energy waste.

<figure><img src="image_8.png" /><figcaption>Fig. 6. Schematic representation of the 3D printing process using filament-based MEX and different material feeding mechanisms.</figcaption></figure>
This process also subjects the polymer to additional thermomechanical cycles, which can degrade its mechanical properties in subsequent processes. Filaments require proper storage and warehousing to prevent degradation. They should be stored in a dry environment, protected from UV light and high temperatures. Failure to do so may result in moisture absorption, hydrolysis, shortening of polymer chains, accumulation of dust and dirt, mechanical damage, thermal deformation, or melting [123]. Filament fabrication for elastomers, including TPEs with physical cross-links, like TPVs, is particularly challenging due to the difficulty in producing round and uniform filaments from these materials. The filaments may often have varying diameters along their length or an elliptical cross-sectional shape. Having inconsistent and uneven filaments can interrupt the printing process or cause filament breakage.

Nonetheless, in academia, researchers have been trying to produce and print elastomeric filaments. In recent years, elastomeric thermo-plastic filaments, such as TPU, have become increasingly available commercially. As a result, numerous studies have focused on printed TPU, the most used elastomeric filament.

A major challenge in printing elastomeric filaments, such as TPVs, in filament-based MEX systems is filament buckling. This occurs when the filament experiences excessive pressure or improper feeding during the printing process, which can lead to uneven extrusion, gaps in the printed object, or even print failure. This limitation arises from the relatively low modulus of elastomeric filaments. A typical FFF printer has a roller feeding system and a heated nozzle tip located a certain distance apart. It has been observed that the filament bends as it passes through the drive wheels or gears. If the filament enters the feeding system incorrectly, it may fall into the gap between the drive gears and the nozzle inlet. Additionally, tangled or twisted filaments are prone to getting stuck in this gap. Furthermore, if the drive gear speed exceeds the filament extrusion rate, a compressive force is applied to the filament, causing it to buckle under the load [38]. This phenomenon is illustrated in Fig. 7a.

Kim et al. [126] proposed a simple solution to prevent filament buckling during the 3D printing of TPU. They achieved this by placing a rigid tube with an internal diameter similar to the filament's diameter between the filament feeding mechanism and the nozzle inlet.

The maximum compressive stress a filament can withstand without

buckling can be calculated using Equation (1) [127]:

$$ \sigma = \frac{k\pi^2}{\left(\frac{l}{r}\right)^2} \qquad (1) $$

Where $\sigma$ is the critical stress that a filament can withstand, $k$ is the compressive elastic modulus, $l$ is the length of the free column (distance between the drive wheels and the nozzle inlet), and $r$ is the radius of the filament.

To prevent filament buckling, Elkins et al. [124] also proposed three solutions.

1. Shortening the free column length between the drive gear and the nozzle inlet can prevent buckling but may lead to filament leakage (Fig. 7b). To address this, they extended the nozzle inlet upwards and closed the gap between the wheels and the nozzle inlet (Fig. 7c).

2. Dual anti-rotation drive wheels with peripheral grooves are used for filament guidance and force supply. The force applied in this mechanism depends on the friction between the wheels and the filament and the normal force exerted by the wheels. It was observed that softer filaments result in lower forces on the extruder, and rubber wheels generate the highest driving force.

3. Finally, using a thermoelectric cooler to cool the filament enhances its column strength, thereby preventing buckling.

Researchers have also proposed another method to prevent filament buckling. Patil et al. [128] developed core-shell filaments consisting of an outer layer of TPE and an inner core of ABS to increase stiffness and prevent buckling during 3D printing. They conducted experiments to evaluate material flow behavior, optimize extrusion settings, and assess the mechanical properties of the printed components. The results demonstrated that incorporating an ABS core improved fine print details and enhanced properties such as flexibility, impact resistance, stiffness, and tensile strength in printed structures. Components produced using filaments containing 30–40 % ABS showed superior durability and impact strength compared to those made with either ABS or TPE alone. Furthermore, a helical spring printed with the ABS + TPE combination

<figure><img src="image_9.png" /><figcaption>Fig. 7. (a) Schematic representation of the challenges of printing elastomers by the filament MEX process (Reproduced with permission from Ref. [40]). (b) Schematic of the filament extrusion process showing the length of the free column, and (c) Elkins et al.'s first solution to prevent filament buckling (Reproduced from Ref. [124]). (d) Schematic representation of Direct pellet printing with cylinder-piston mechanism (Reproduced with permission from Ref. [125]).</figcaption></figure>
exhibited a reversible extension of up to 120 mm, which is double the expansion capacity of a spring coil made from ABS.

The most efficient method for printing soft and elastomeric materials using MEX processes is recommended to be direct pellet printing or FGF. Direct granulate printing with a screw mechanism is one of the earliest approaches used for printing soft materials. Kumar et al. [129] developed a CNC-assisted material deposition system (MDT) to overcome the limitations of FFF in processing elastomers. Specifically, they focused on processing ethylene-vinyl acetate (EVA) in granulate form instead of filament. An experimental study was conducted to determine the optimal process parameters for part fabrication, including screw speed and heating chamber temperature. The measured responses were the melt flow rate and filament diameter before and after deposition. The results showed that the developed system could produce flexible parts with rubber-like properties, making it suitable for various end-use applications. This study demonstrated that a granulate-based system can overcome the limitations of FFF in elastomer processing and enable the fabrication of flexible components. Optimal process parameters were identified, and a variety of flexible 3D-printed parts were successfully produced.

In addition to the screw-based mechanism, a piston-based method has also been used to print elastomeric materials. Bayati et al. [125] developed a piston-cylinder-based direct pellet printing method, using pneumatic pressure to fabricate a highly stretchable propylene-based elastomer. This mechanism is shown in Fig. 7d. By feeding polymer granules into a direct printer and adjusting the pneumatic pressure, precise control over material flow was achieved, resulting in a uniform microstructure. Tensile tests showed that the 3D-printed elastomer exhibited extraordinary elongation of over 4000 %, with a tensile strength of 3.7 MPa. Scanning Electron Microscopy (SEM) confirmed a seamless interlayer bond, which contributed to the material's stretchability. The exceptional stretchability was attributed to the unique molecular structure of the propylene-based elastomer, precise control of printing parameters, polymer chain alignment in the direction of strain, and the absence of air bubbles in the structure. This technique opens new possibilities for 3D-printed soft, stretchable devices.

Recent research in the field of direct pellet printing focused on the 3D printing of elastomers, elastomer blends, and elastomeric composites. These are summarized in Table 2.

## 6. 3D printing of TPVs

TPVs have processability due to their thermoplastic phase, making them suitable for 3D printing. Despite the advancements in the 3D printing of elastomers, research on TPVs in this context remains limited. The existing studies utilize filament-based mechanisms for printing these materials. This section reviews the key studies on TPV 3D printing and explores the primary reasons for their limited application in this area. A summary of previous articles in the field of 3D printing of TPVs is presented in Table 3.

In one of the earliest studies, Gao et al. [146] synthesized a biocompatible TPV by combining PLA copolymers with poly(1, 4-butanediol/2,3-butanediol/succinate/itaconic acid) (PBBSI). Using a melting polycondensation method, PBBSI was first synthesized with different ratios of 2,3-BDO from 50 to 90 %mol, and their molecular weight, chemical structures, mechanical, and thermal properties and copolyester compositions were characterized via GPC, NMR, TGA, DCS, WAXD, FTIR, and tensile test techniques.

A TPV was prepared by combining PLA and PBBSI, with DCP used as a crosslinking agent. The resulting TPV exhibited a sea-island morphology, indicating good dispersion of the rubber phase within the thermoplastic matrix (Fig. 8a). The final TPV was compared with PLA and PBBSI in terms of thermal stability, crystalline properties, mechanical properties, thermomechanical properties, and rheological properties. The results indicated that, in terms of mechanical properties (Fig. 8b), the stress-strain curve of the PBBSI/PLA TPV fell between

<table><caption>**Table 2**
Printed elastomers using the direct granulate printing method.</caption><thead><tr><th>No.</th><th>Printed material</th><th>Material type</th><th>Printing mechanism</th><th>Reference</th></tr></thead><tbody><tr><td>1</td><td>EVA</td><td>Elastomer</td><td>Screw</td><td>Kumar et al. [130]</td></tr><tr><td>2</td><td>TPU</td><td>Elastomer</td><td>Screw</td><td>Leng et al. [131]</td></tr><tr><td>3</td><td>Cured rubbers such as NR, NBR, EPDM</td><td>Elastomer</td><td>Screw</td><td>Drossel et al. [132]</td></tr><tr><td>4</td><td>PBAT</td><td>Elastomer</td><td>Piston-Cylinder</td><td>Karimi et al. [112]</td></tr><tr><td>5</td><td>POE</td><td>Elastomer</td><td>Piston-Cylinder</td><td>Zhang et al. [133]</td></tr><tr><td>6</td><td>EVA</td><td>Elastomer</td><td>Piston-Cylinder</td><td>Luo et al. [134]</td></tr><tr><td>7</td><td>PCL</td><td>Elastomer</td><td>Piston-Cylinder</td><td>Bonetti et al. [135]</td></tr><tr><td>8</td><td>POE/LDPE</td><td>Elastomer blend</td><td>Piston-Cylinder</td><td>Liu et al. [136]</td></tr><tr><td>9</td><td>NBR/PP</td><td>Elastomer blend</td><td>Screw</td><td>Ghoreishy et al. [137]</td></tr><tr><td>10</td><td>PBAT/Wood Flour</td><td>Elastomeric composite</td><td>Screw</td><td>Singamneni et al. [138]</td></tr><tr><td>11</td><td>PETG/PBAT</td><td>Elastomer blend</td><td>Piston-Cylinder</td><td>Ali et al. [139]</td></tr><tr><td>12</td><td>PLA/PBAT</td><td>Elastomer blend</td><td>Piston-Cylinder</td><td>Rahmatabadi et al. [140]</td></tr><tr><td>13</td><td>PETG/EVA</td><td>Elastomer blend</td><td>Piston-Cylinder</td><td>Ali et al. [141]</td></tr><tr><td>14</td><td>EVA/Graphite</td><td>Elastomeric composite</td><td>Screw</td><td>Kumar et al. [129]</td></tr><tr><td>15</td><td>SEBS</td><td>Elastomer</td><td>Screw</td><td>Georgopoulou et al. [142]</td></tr><tr><td>16</td><td>Vistamaxx® 6202</td><td>Elastomer</td><td>Piston-Cylinder</td><td>Bayti et al. [143]</td></tr><tr><td>17</td><td>TPS/CIP/CB</td><td>Elastomeric composite</td><td>Screw</td><td>Costi et al. [144]</td></tr><tr><td>18</td><td>PLA-PBAT-Fe<sub>3</sub>O<sub>4</sub></td><td>Elastomeric composite</td><td>Piston-Cylinder</td><td>Yousefi et al. [145]</td></tr></tbody></table>

those of PBBSI and PLA, reflecting its combined elastomeric and thermoplastic features. The Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA) results (Fig. 8c) showed that the resulting TPV had two distinct glass transition temperatures (Tg) between those of PLA and PBBSI, indicating a biphasic structure. Additionally, rheometry tests (Fig. 8d) revealed that the PBBSI/PLA TPV behaved as a non-Newtonian fluid with shear-thinning properties. The melt index of the PBBSI/PLA TPV, measured using a melt-flow indexer, was 32.8 g/min, demonstrating its excellent rheological properties for processing with 3D printers.

This study primarily focused on two main steps: first, fabricating and characterizing PBBSI, and second, combining it with PLA to create TPV. The study concluded by noting that the prepared TPV had desirable flowability based on rheometric tests. However, a comprehensive evaluation of its 3D printing performance was not conducted. There are no SEM images of the printed layers, no details about the printing conditions, and no information about filament preparation. It is only mentioned that the TPV was printed at 170 °C using a Wiiboox ONE 3D printer (Wiiboox Technology Co Ltd., Nanjing, China), and figures of three printed parts is presented as evidence of the printability of the TPV (Fig. 8e). Despite these limitations, this study represents a significant step in demonstrating that TPVs can be 3D printed, providing a promising foundation for further research on printing TPVs.

In another study by the same research group [147], they looked at TPVs made from bio-elastomers called poly(lactate/butanediol/sebacate/itaconate) (PLBSI) and PLA. The process used to create these TPVs was the same as in the previous study. First, the elastomer and thermoplastic were mixed together in a mixer. Then, this mixture was combined with DCP on a two-roll mill at room temperature. Finally, the mixture was dynamically vulcanized using heat and shear in a mixer. A key part of this study was that samples from different mixing stages were
<table><caption>**Table 3**
A summary of previous articles in the field of TPV 3D printing according to content, printing method, cross-linking agent, and achievements.</caption><thead><tr><th>Composition</th><th>Preparing Methods</th><th>Crosslinking agent</th><th>3D Printing Methods</th><th>Achievements</th><th>Ref</th></tr></thead><tbody><tr><td>PLA-PBBSI</td><td>Melt mixing</td><td>DCP</td><td>FDM</td><td>Good flowing property and compatibility with high toughness, Tensile strength: 13.03 MPa, Elongation at break: 231.3 %</td><td>Gao et al. [146]</td></tr><tr><td>PLBSI-PLA</td><td>Melt mixing</td><td>DCP</td><td>FDM</td><td>Good processability, excellent degradability and biocompatibility, Tensile strength: 19.6 MPa, Elongation at break: 314 %</td><td>Hu et al. [147]</td></tr><tr><td>BIIR-PP</td><td>Melt mixing</td><td>HVA-2, ZnO and Stearic Acid</td><td>FDM</td><td>Superior rheology and 3D printability, Tensile strength ~ 10 MPa, Elongation at break over 400 %</td><td>Hou et al. [148]</td></tr><tr><td>PLA-NR</td><td>Melt mixing</td><td>DCP</td><td>FDM</td><td>Co-continuous structure, excellent biodegradability, good mechanical and shape memory properties</td><td>Gong et al. [149,150]</td></tr><tr><td>PP-EPDM</td><td>Melt mixing</td><td>Sulfur</td><td>FFF</td><td>Tensile strength: 20–30 MPa, Elongation at break: 700–1100 %</td><td>Innes et al. [151]</td></tr></tbody></table>

<figure><img src="image_10.png" /><figcaption>**Fig. 8.** PLA/PBBSI TPV: (a) Atomic Force Microscopy (AFM) image of morphology, (b) stress-strain curves, (c) DMA and DSC results, (d) rheological curves and (e) 3D printed TPV with different shapes (Reproduced with permission from Ref. [146]).</figcaption></figure>

taken from the melt and analyzed using SEM (Fig. 9a). This analysis, along with the torque-time curve, helped to show how the microstructure of the TPV developed. The study examined various properties of the prepared TPVs, including thermal, mechanical, hysteresis, and dynamic mechanical characteristics. It also tested for cytotoxicity and biodegradability. The results indicated that these TPVs were classified as level 1 based on their RGR (Relative Growth Rate), meaning they have low toxicity and could be suitable for biomedical applications.

In the 3D printing experiments, tensile samples with different levels of porosity (25 %, 50 %, and 75 %) were printed by altering the infill density settings on the 3D printer. The tensile tests of the 3D-printed PLBSI/PLA TPVs showed that as porosity decreased, the tensile strength increased (Fig. 9b).

Although the presence of pores in the 3D-printed samples could

weaken them, adjusting the porosity allowed for control over the strength, resulting in a variety of mechanical performances. Importantly, there were no significant differences in elongation at break among the samples, indicating that desirable elasticity was maintained after the 3D printing process. The examination of fractured surfaces (shown in Fig. 9c to e) revealed visible interconnected pores within channel networks. No defects related to polymer dispersion were found, and the layers were uniformly stacked with a thickness of 150 µm, showing a clear layer-by-layer structure. Several 3D-printed products (Fig. 9f) were also printed, demonstrating that it is feasible to create TPVs with porous, pre-designed structures using 3D printing. While previous study focused more on preparing the elastomeric phase, this study concentrated on characterizing TPVs with different ratios of r/p. Additionally, it emphasized the examination of 3D-printed materials
<figure><img src="image_11.png" /><figcaption>**Fig. 9.** TPVs made of PLBSI/PLA: (a) microstructure development of prepared TPV, (b) mechanical properties of printed samples with different porosity, (c–e) SEM images of printed layers and (f) printed TPVs (Reproduced with permission from Ref. [147]).</figcaption></figure>

through tensile tests and the investigation of print porosity. This study can be considered one of the first to explore the application of TPVs in 3D printing. However, it is important to note that the main focus was not solely on the 3D printing procedure.

As previously discussed, the mixing method and order during TPV preparation play a crucial role in the material's properties and rubber dispersion within the plastic matrix. This procedure can directly affect the quality of TPV filaments, their viscosity, and consequently the 3D-printed samples. Hou et al. [148] developed a high-performance 3D-printable TPV filament composed of bromobutyl rubber (BIIR) and PP, using maleic anhydride-grafted polypropylene (PP-g-MA) as a compatibilizer. For comparison, TPVs were prepared using three different methods (Fig. 10a): (I) by pre-mixing and (II) by the master-batch method. The difference between the (II) and (III) samples was the presence or absence of a compatibilizer.

The 3D printing properties, rheological, mechanical, and phase morphology characteristics of three types of TPVs were compared. In the masterbatch method, the uniform dispersion of curing agents and extended curing times contributed to the homogeneous cross-linking of BIIR and the efficient formation of small BIIR particles (dark regions)

and the transformation from co-continuous to sea-island morphology (Fig. 10b to d). The incorporation of PP-g-MA in the BIIR/PP interface improved their compatibility. The superior rheology and lower viscosity of the TPV containing PP-g-MA, produced via the masterbatch method, can be attributed to the lower viscosity of PP-g-MA and the smaller particle size of BIIR. The low viscosity and higher polarity of this material effectively enhanced the bond strength between adjacent layers and decreased defects of the printed product (Fig. 10e to g). The three filaments produced by these different methods are shown in Fig. 10h to j. As it can be observed, the filament produced using the masterbatch method and compatibilizer (Fig. 10j) has a uniform surface and consistent diameter, which improves the printing process. The filaments of pTPV occasionally caused the printer to jam. In contrast, the printing process for mTPV-MA10 was smooth and uninterrupted.

This study specifically examined the printing of TPVs, particularly how viscosity affects printing quality based on different preparation methods. The study systematically explained the entire printing process, including filament preparation, printing parameters, and the characterization of the printed parts. This is different from previous studies, which did not focus on the printing aspect.
<figure><img src="image_12.png" /><figcaption>**Fig. 10.** BIIR/PP TPVs (a) different preparation methods, (b) TEM image of pTPV, (c) TEM image of mTPV, (d) TEM image of mTPV-MA, (e) SEM image of printed pTPV cross section, (f) SEM image of printed mTPV cross section, (g) SEM image of printed mTPV-MA cross section, (h) pTPV filament, (i) mTPV filament, and (j) mTPV-MA filament (Reproduced with permission from Ref. [148]).</figcaption></figure>

In two other studies, conducted by Gong et al. [149,150], the fabrication of TPV from PLA/NR at an 80/20 ratio and the use of DCP as a cross-linking agent significantly increased the impact resistance and toughness of PLA. They selected SiO₂ nanoparticles, a widely used reinforcement, as the compatibilizer. In one study, 3-(trimethoxysilyl) propyl methacrylate, also known as KH570, was used to modify SiO₂, while in the other, Bis-(3-triethoxysilylpropyl) tetrasulfide, known as TESPT, was employed. Due to surface tension, the modified SiO₂ nanoparticles migrated from the NR phase to the PLA/NR interface. These modified nanoparticles, located at the interface of the two phases, formed bonds with the NR and PLA chains, acting as a bridge to improve interfacial compatibility. The preparation routes of these two TPVs are illustrated in Fig. 11a and b.

By adding only, a small amount of filler and modifier, the impact resistance of the fabricated TPV increased to 95.96 kJ/m² (without fracture), which is 38 times greater than that of pure PLA. Even at low temperatures of -30 °C, the impact resistance remained at 7.93 kJ/m². Ultimately, the continuous structure and excellent toughness, achieved with a small amount of stiffening agent, demonstrate the significant potential of TPVs for 3D printing applications (with a printing speed of 110 m/min). As previously mentioned, one of the major drawbacks of PLA, as the most used FDM printing material, is its high brittleness, low toughness, and high moisture absorption, which limit its applications.

These studies briefly mentioned 3D printing but did not test the printed samples or conduct a thorough investigation of the printing process. They only looked at the toughness and moisture absorption of the filament made from this TPV and compared it to PLA. However, an important point from these studies is that TPVs can be used to create tough prints, unlike traditional brittle 3D printing filaments like PLA. The printed samples made with PLA (Fig. 11c) and TPV (Fig. 11d) showed comparable quality.

A more recent study by Innes et al. [151] examined the 3D printing of the most well-known TPV, PP/EPDM. This study used a 50/50 thermoplastic/elastomer ratio, sulfur as a cross-linking agent, and a twin-screw extruder to fabricate the filament. They also prepared three types of TPVs with different curing packages to investigate how the curing package affects TPV performance. In rheometry tests, as shown in Fig. 12a, they observed that pure PP has lower viscosity compared to its

TPVs. This suggests that the higher viscosity of TPVs may pose challenges for printability. In their initial attempt using standard equipment, only one of the samples was successfully printed, while the others failed due to high viscosity and nozzle clogging (nozzle diameter = 0.4 mm). To address this issue, several upgrades were made, including upgrading the extruder to an E3D Hemera (direct drive, 1.75 mm version) and replacing the nozzle with a 1.75 mm E3D V6 nozzle with a 0.8 mm internal diameter. These upgrades allowed the pinch roller system to provide higher torque and friction. Additionally, they adjusted the filament cross-sectional area by changing the R parameter in the G-code, which defines the extruder motor's rpm. This approach resolved the issue of diameter variation. The samples also exhibited warping, which was mitigated by preparing the build surface with a sheet of EPDM rubber to improve adhesion. The mechanical properties of the printed TPVs were compared to those of pure PLA and PP in both parallel and perpendicular directions to the print path.

As illustrated in Fig. 12d to f, the results for mechanical properties parallel to the print direction indicated that TPVs exhibited elongation exceeding 700 %, while pure PLA and PP showed elongation between 20 % and 50 %. Additionally, the tensile strength of the printed TPV was comparable to that of printed PP. SEM images of the printed TPVs' cross-sections (Fig. 12g to i) and peel test results (Fig. 12b and c) reveal that modifying the curing package significantly enhances interlayer adhesion and overall print quality. Additionally, Fig. 12j to l presents SEM images of the printed TPVs' microstructure, providing a detailed visualization of the material's morphology and phase distribution.

This study systematically examined the process of printing TPVs using FDM printers, identified challenges, and proposed innovative solutions to enhance the printing process. Considering the challenges and solutions presented in this study, 3D printing high-quality parts from TPVs is not out of reach, but there are still fundamental challenges that may vary from one material to another. These challenges have prevented the development of a reliable printing method for these materials, and further research is needed to achieve a stable and dependable printing process.
<figure><img src="image_13.png" /><figcaption>**Fig. 11.** PLA/NR TPV: (a) preparation route for first article (Reproduced with permission from Ref. [150]), (b) preparation route for second article, (c) printed part by commercial PLA filament and (d) printed part by PLA/NR TPV filament (Reproduced with permission from Ref. [149]).</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>**Fig. 12.** Printed PP/EPDM: (a) rheological curves, (b) peel test, (c) adhesion results, (d) tensile strengths, (e) tensile modulus, (f) elongation at break, (g–i) SEM images from cross section of printed TPVs, and (j–l) SEM images from the microstructure of the printed TPVs (Reproduced with permission from Ref. [151]).</figcaption></figure>

## 7. Challenges and proposed solutions for successful TPV printing

As discussed in this review article, TPVs, due to their combination of elastomeric properties and thermoplastic recyclability, are highly versatile and green materials that are expected to replace thermoset elastomers, which currently account for a large portion of rubber consumption in industry. However, these materials require a reliable, efficient, and flexible production method to enable both batch

production and customization for emerging industries. 3D printing, as an advanced manufacturing technique, has significant potential to meet these requirements. However, the 3D printing of TPVs presents numerous challenges. One of the most critical challenges, common to all TPEs, is filament buckling during printing, which is caused by the softness and low modulus of TPVs, as discussed before, making the filament extrusion process unsuccessful. Filament buckling can be mitigated using the methods proposed in this review, with direct pellet printing appearing to be the most efficient and reliable approach.
As noted in previous studies, the incompatibility between the two phases of TPV blends presents another challenge in printing these materials. In cases of phase incompatibility, the produced filament and the printed part exhibit delamination, with layers failing to adhere properly. Phase separation leads to cracks and defects in the printed parts, causing stress concentration and reduced strength in the final product. Additionally, producing filament with a consistent diameter and quality under such conditions is highly challenging. To address this issue, finding a suitable compatibilizer can be an effective solution. In certain cases, DV leads to grafting reactions between the two phases, which can enhance compatibility.

Due to the presence of cross-links, TPVs exhibit high viscosity, which prevents the material's melt flow, leading to nozzle clogging during printing and uneven extrusion. Furthermore, the melting temperature increases significantly when the rubber content is high, and in some cases, where the cross-link density is high, the material may become thermoset and fail to melt entirely. Cross-link density and rubber content have a significant impact on processability, layer adhesion, and the final material strength. More favorable conditions for printability can be achieved by using plasticizers or reducing the content of rubber and cross-linking agents. In some cases, using a low-viscosity thermoplastic as the matrix can decrease viscosity and increase the flowability of the molten TPV. Since flowability is crucial in 3D printing, it is recommended to conduct rheological tests to ensure the developed TPV has the desired viscosity before printing. Additionally, printing parameters significantly impact print quality. For example, increasing the nozzle temperature can enhance flowability and reduce viscosity. Increasing the nozzle diameter can often reduce the likelihood of nozzle clogging and produce more even material flow. In piston-based 3D printing, piston pressure can also increase material flowability. However, there are limitations associated with these parameters.

Another potential challenge is warping of the printed TPV. Although TPVs are less prone to warping compared to polymers with higher crystallinity, if warping occurs, it can be addressed using a heated bed or printing chamber.

Another issue with parts produced by AM methods is their static nature, which prevents them from adapting to environmental changes or modifying their function. The advent of 4D printing in recent years has addressed this issue for many printed materials. This technology will be discussed further as a potential application.

# 8. Future directions

## 8.1. Printed TPVs for greener future

The world is undergoing a significant transformation towards a circular economy, a model that emphasizes the reduction of waste and the maximization of the reuse and recycling of materials for as long as possible [152]. This shift is particularly urgent in the context of polymer waste, which has become a major environmental concern. As shown in Fig. 13, Polymer waste can contaminate soil, water, and even human bodies, leading to severe health and ecological issues [153]. The need to avoid single-use and non-recyclable polymers has become a pressing priority, prompting a search for sustainable alternatives that can reduce polymer pollution. One of the key strategies in this quest for sustainability is the use of recyclable and biodegradable materials [154].

Biodegradable materials can naturally degrade after their useful life, significantly reducing long-term environmental pollution. Biodegradable polymers, such as PLA, are derived from renewable resources and can be easily composted, making them an attractive option for reducing waste [17]. However, while biodegradable materials offer a promising solution, they also have limitations, such as lower durability and higher production costs compared to traditional plastics. Therefore, integrating biodegradable materials into existing manufacturing processes and product designs requires careful consideration of these factors [155].

AM plays a pivotal role in the transition to a circular economy [156]. AM enables the production of complex geometries and customized products with minimal waste generation, aligning well with the principles of sustainability. Unlike traditional manufacturing methods, which often result in significant material waste, AM allows for the precise use of materials, reducing excess and promoting efficiency [157]. Moreover, AM can utilize recycled materials, further reducing the demand for virgin polymers and supporting the circular economy by closing material loops [158].

TPVs combine the elasticity of thermoset elastomers with the recyclability of thermoplastic polymers, making them a crucial component in sustainable manufacturing. They can replace non-recyclable thermoset rubbers, and TPVs can be designed to be recyclable and reusable, reducing waste and the environmental impact associated with traditional elastomers [159]. Furthermore, TPVs can be formed using a range of biodegradable polymers, such as PLA, polybutylene succinate (PBS), polybutylene succinate adipate (PBSA), and natural rubber (NR) [160]. This enables us to explore new uses for environmentally friendly TPVs made from biopolymers. Additionally, there is extensive research on using thermoset rubber waste in TPVs as the elastic component [161,

<figure><img src="image_15.png" /><figcaption>Fig. 13. Ecological importance of using TPVs instead of thermoset rubbers.</figcaption></figure>
[162]. This indicates that TPVs not only have the potential to be recycled themselves multiple times [20], but they can also help reduce thermoset rubber waste, like tire waste, and promote their use as green materials in the future.

By using TPVs in AM, products can be created by minimizing waste that can be reused after their working life. The integration of TPVs into AM processes also allows for the creation of complex products with tailored properties, which can be particularly beneficial in applications where both flexibility and strength are required. This is a comprehensive approach to creating sustainable products that align with circular economy goals. By leveraging these technologies, industries can develop products that are not only environmentally friendly but also meet the performance requirements of various applications.

To effectively implement a circular economy for polymers, several strategies must be employed. First, unnecessary and problematic plastic items should be eliminated. Second, innovation should focus on developing materials and products that are reusable, recyclable, or biodegradable. Finally, materials should be kept in circulation, ensuring they remain in use and are recycled or reused to minimize waste. By integrating TPVs into AM, we can create a more sustainable future where polymer waste is significantly reduced and resources are utilized efficiently.

## 8.2. Research gaps

Incorporating TPVs into AM techniques, especially MEX, creates many research opportunities for 3D printed TPVs. As discussed in this article, there are very few studies on printing TPVs, and most of them do not focus on the printing process itself. This means that this area of research is largely unexplored, allowing researchers to investigate it in depth. This section discusses some potential research topics related to 3D printing TPVs, but it does not cover all possible future directions.

### 8.2.1. Development of new TPVs as feedstock

This review article provides a detailed overview of studies on 3D printing with TPVs and introduces the materials used as feedstock, all in the form of filaments. However, many different TPVs have not yet been tested as filament or feedstock materials. By combining a thermoplastic

matrix with a crosslinkable elastomer, new TPVs can be developed that may be suitable for 3D printing. Various crosslinking agents and additives can also be included to modify the properties of TPVs. Different ratios of rubber to plastic can be experimented with to find the best combination for 3D printing applications. As shown in Fig. 14, a wide variety of TPVs can be created using different thermoplastics and elastomers.

It is important to note that some combinations may lead to lower-quality TPVs, but they are still worth investigating. Many of these combinations have been studied before, but not specifically as materials for 3D printing. Exploring existing literature can be very helpful in identifying suitable TPVs for printing purposes.

As mentioned, TPVs can be made from various thermoplastics and elastomers. Additionally, TPVs can be used to develop composites with desirable electrical [163], mechanical [164], thermal [165], and biological properties [166]. Composites are materials composed of two or more components with distinct chemical or physical properties. TPVs can be mixed with fibers and particles to create new materials for 3D printing. They can also be incorporated into laminate composites. Both natural fibers (e.g., plant-based, wood, and animal-derived fibers) [167] and synthetic fibers (e.g., glass, carbon, Kevlar, and nylon) can be added to the TPV matrix in various forms, such as short, long, or continuous fibers. Particles can also be added to modify TPVs. These particles can include nanoparticles or 0D particles (like graphene quantum dots, metal nanoparticles [168], carbon black [169], and nano-silica [170]), 1D particles (such as nanorods, nanowires, carbon nanotubes [22], and graphene nanoribbons), 2D particles (like graphene and MXenes [171]), and 3D particles. Although many TPV composites have been studied, a significant number remain unexplored in the literature. Currently, none of the TPV composites, except for PLA/NR/SiO₂, are used in 3D printing. This gap in research can be addressed through future studies.

In recent years, the desire to develop biomaterials compatible with body organs has received much attention among researchers for biological applications. TPVs, due to their softness and excellent flexibility, can be strong candidates for use in biomedical applications, especially in cases where the flexibility of the material is important [172]. By developing the 3D printing capability of TPVs, it is possible to achieve personalized printed bio-TPVs with tailored biological applications

<figure><img src="image_16.png" /><figcaption>Fig. 14. Different combinations of thermoplastic and elastomer can be used to develop new TPVs which can be used as 3D printing feedstock.</figcaption></figure>
[173]. But first, it is very important to prepare the bio-TPV feedstocks. Providing adequate feedstock, they can be more easily 3D or 4D printed using the methods mentioned in this paper. An important point in preparing bio-TPVs is that both the thermoplastic and elastomer components should be selected from biocompatible materials. Also, the curing method should be chosen so that it does not cause toxicity. Methods for examining cytotoxicity, as well as in vitro and in vivo tests, are used to evaluate the biocompatibility of the developed TPVs.

Many researchers have studied the preparation of biocompatible TPVs and tested their biocompatibility. However, there are very few studies focused on biocompatible TPVs that are 3D printed. This represents a research gap that could be addressed in future work. In a previous study, Zhang et al. [174] designed and prepared silicone rubber (SiR)/polycaprolactone (PCL) TPV with added polyurethane-polysiloxane copolymer (PU-co-PSi) to improve compatibility and reduce SiR phase size. They used dynamic vulcanization to achieve phase inversion and fine SiR dispersion. TPVs' cytocompatibility is confirmed by supporting cell adhesion, proliferation, and spreading. The resulting TPVs showed excellent mechanical properties, elasticity, processability, recyclability, and cytocompatibility, making them promising biomaterials for tissue engineering and biomedical devices. Kang et al. [175] fabricated bio-TPVs with tunable elasticity, high strength, and good cytocompatibility using an in situ dynamic vulcanizing strategy. They melt-blended biobased polyester elastomer (BPE) and PCL with a crosslinker, causing phase inversion. The BPE provided flexibility and elasticity, while PCL contributed strength and processability. The resulting bio-TPV showed excellent tensile properties, processability, and supported MC3T3 cell adhesion and proliferation. This makes these materials promising for biomedical and engineering applications. Li et al. [176] fabricated a novel bio-based TPV composed of PLA and a polymeric slide ring material (SeRM) via isocyanate-induced dynamic vulcanization. They analyzed its microscopic morphology, thermal and mechanical properties, and biocompatibility using TEM, DSC, cytotoxicity tests, tensile testing, and molecular dynamics simulations. The optimal phase inversion occurred at a 70/30 SeRM/PLA ratio, resulting in improved interface compatibility, enhanced tensile strength (14.7 MPa), elongation (164 %), and non-toxicity to L929 cells. Finally, Hu et al. [147] developed fully renewable PLBSI/PLA TPVs for 3D printing and evaluated their cytotoxicity using MTT colorimetry on L929 cells. The relative growth rates (RGR) of cells exposed to TPV extracts remained above 80 % after 3 days, corresponding to level 1 cytotoxicity, indicating the materials are non-toxic and suitable as biomaterials. Additionally, the TPVs demonstrated good in vitro degradability and supported healthy cell growth, confirming their potential as sustainable, biocompatible materials for biomedical applications.

### 8.2.2. Using new printing techniques

As mentioned in the review, all printed TPVs have been produced using filament-based MEX. However, there is significant potential for research into using TPVs with other printing techniques. Addressing the challenges of filament buckling in filament-based extrusion could help identify more suitable printing methods for these materials. For example, preparing TPV powders for use in PBF could create new opportunities for printing high-precision TPV materials. This technique does not face the same issues with filament buckling since the material is in powder form. Although developing TPV powders requires thorough investigation, it could be a valuable area of study.

Additionally, alternative feeding mechanisms in MEX, such as screw-based and piston-based direct pellet printing, could eliminate the need for filaments and may be effective ways to print TPVs. It is important to address this challenge of finding reliable printing techniques for TPVs to support future advancements in AM of these materials.

### 8.2.3. Printing parameters optimization

Printing parameters significantly impact the quality of printed parts. Each parameter can affect print quality. For instance, increasing the

nozzle temperature can enhance mechanical properties by improving material flowability up to a certain point. Beyond this point, material degradation occurs, reversing the trend. Identifying the optimal value for these parameters can improve print quality. However, this optimal value is influenced by other printing parameters such as print speed, layer height, and nozzle diameter. In the case of printing TPVs, where producing high-quality prints poses significant challenges, optimizing printing parameters is crucial to achieve the best properties and print quality. It has been demonstrated that optimizing printing parameters can also enhance other properties, such as shape memory performance [177]. Effective methods for optimization include design of experiments and optimization methodologies like the Taguchi method [143], Response Surface Methodology (RSM) [178], and Artificial Neural Networks (ANN) [179]. These approaches can help identify the optimal combination of printing parameters.

Additionally, in recent years, machine learning techniques have been employed to optimize the printing process. These techniques address various aspects, including design customization, material selection, process control, and quality assurance [180].

## 8.3. Potential applications

TPVs are already widely used in various industries, including construction and automotive. However, their applications can be further expanded by utilizing TPVs in AM techniques. This section highlights some potential applications that researchers may focus on in the future. It is important to note that the range of potential applications is extensive and cannot be fully covered in this section.

### 8.3.1. Application in 4D printing

The advent of 4D printing marks a new era in AM by integrating smart materials, especially shape memory polymers (SMPs), with traditional 3D printing techniques [28]. This method adds a fourth dimension, time, using shape memory effects to transform static 3D-printed parts into dynamic systems. With the development of this technique, it is possible to create dynamic systems that can change configuration in response to external stimuli such as heat [181,182], light [183,184], magnetic fields [185,186], and humidity [187,188], and revert to a pre-programmed state. In some cases, multi-stimuli responsiveness can be achieved, where the configuration of 4D systems can be altered by various stimuli, known as multi-stimuli-responsive materials. Adding multi-stimuli responsiveness is often achieved by incorporating other particles into the material.

The Venus flytrap plant is a good example of 4D behavior [189]. It reacts quickly by closing its leaves in about 100 ms to catch insects [190]. This reaction happens when part of the insect touches the leaf of the plant. The 4D printing process is based on three main ideas. First, it uses smart materials that can change when exposed to certain stimuli. Second, a specific stimulus is applied to these materials to observe their unique reaction. Third, the focus is on how long this reaction lasts and how the shape of the material changes. These shape changes can include folding, bending, twisting, expanding or contracting, surface twisting, and changes in surface texture. 4D printing has introduced new avenues for smart materials and self-assembly structures to researchers across various fields, including medicine, robotics, wearables and automotive industries. While 4D printing is a relatively new technique, it has already gained significant attention by combining shape memory materials with AM methods [186,191].

Although TPVs have demonstrated excellent shape memory properties in previous studies, 4D printing for these materials has not yet been investigated. Previous studies have demonstrated that TPVs exhibit high shape recovery [192] and shape fixity ratios [193] due to their cross-linked three-dimensional network within a rigid thermoplastic matrix. Additionally, TPVs have shown rapid recovery capabilities [194]. Many studies have also shown that TPVs can respond to multiple stimuli, making them suitable for use in 4D printing applications [195]. For
example, in a recent study [196], researchers used tannin as a biological curing agent to create fully bio-based PLA/ENR TPVs. They developed a dual cross-linking network and a co-continuous morphology, enhancing the TPVs' mechanical robustness and multi-stimuli-responsive shape

memory properties, including response to heat, near-infrared light, and sunlight (Fig. 15a to c).

With their impressive shape memory properties, using TPVs in 4D printing could greatly change how they are applied in various industries.

<figure><img src="image_17.png" /><figcaption>**Fig. 15.** Multi-responsiveness of TPVs: (a) heat responsive, (b) NIR responsive, and (c) sun light responsive behavior (Reproduced with permission from Ref. [196]).</figcaption></figure>
The potential applications arising from TPVs in 3D and 4D printing are illustrated in Fig. 16.

### 8.3.2. Adoptive, self-repairing and sustainable clothing

Recent advancements in 3D printing are transforming the fashion industry, enabling the production of fabrics and garments that are both innovative and sustainable [197]. For 3D-printed clothing, it is crucial that materials are soft and comfortable, as they come into direct contact with the skin [198]. The evolution to 4D printing introduces adaptive clothing that can change its size and properties based on environmental factors like humidity and temperature [199,200]. This represents a significant step forward for everyday wear and specialized activities, such as sports, where garments can adjust to fit the user's body. TPVs are particularly promising for future adaptive clothing due to their softness and excellent shape memory characteristics. Additionally, TPVs exhibit remarkable self-healing capabilities [201] that could be used in self-repairing garments. For instance, if a piece of clothing tears during an activity, it could be repaired simply by applying heat. Furthermore, at the end of their lifecycle, TPV-based clothing can be easily recycled due to the recyclability of the materials. This allows the fabric to be

repurposed for printing new garments. If the TPVs used in these adaptive clothes are made from biodegradable materials, they can be composted after use without harming the ecosystem. This approach not only contributes to reducing clothing waste significantly but also promotes a more sustainable fashion industry.

### 8.3.3. Space exploration applications

The growing interest in space exploration over recent decades necessitates the development of essential infrastructures, suits, and facilities for space travel. 3D and 4D printing technologies are particularly advantageous in these areas due to their design flexibility and ability to create complex smart structures [202]. TPVs possess excellent properties such as flexibility, durability, flame retardancy [203], oil resistance [204], self-healing capabilities, and gas barrier performance [205], making them highly suitable for future space missions, such as in the following areas.

* **Elastic Space Suits:** Next-generation 3D-printed space suits made from TPVs can provide superior flexibility and durability. Their

<figure><img src="image_18.png" /><figcaption>**Fig. 16.** Schematic representation of potential applications arising from 3D and 4D printing of TPVs.</figcaption></figure>
excellent gas barrier properties and durability will protect astronauts from harsh space conditions while allowing for free movement.

* **Lunar or Martian Habitats:** TPVs can be utilized to create flexible, airtight seals for modular habitats on the Moon or Mars. These habitats would be designed to withstand extreme temperature fluctuations.

* **Elastic Space Stations:** Components made from 3D-printed TPVs could allow space stations to expand or contract based on crew needs, facilitating the creation of temporary storage areas or emergency shelters.

* **Shape-Morphing Satellites:** The durability and shape-shifting capabilities of TPVs make them ideal for various parts of satellites that need to adapt their shape for different situations or directions.

Using 4D-printed TPV structures with adaptive properties in space exploration can greatly reduce the weight of facilities. This is because their ability to change shape does not require any electrical or chemical energy source. Instead, they can be designed to adapt naturally to environmental conditions.

### 8.3.4. Energy harvesting

3D and 4D printing are widely used in energy harvesting and renewable energy generation [206]. TPVs can also serve as printed materials in these applications.

* **Flexible Solar Panels:** 3D-printed TPVs can be used as the base material for flexible, shape-morphing, and lightweight solar panels. These panels can be integrated into clothing, vehicles, or even rolled up for portable use. TPVs are a better choice than other elastomers because they are more environmentally friendly.

* **Piezoelectric and Thermoelectric Generator TPVs:** TPVs embedded with piezoelectric materials can generate electricity from mechanical and thermal stress, such as footsteps, vibrations, or even body heat. This technology can be used in self-powered wearable devices or infrastructure.

### 8.3.5. Future constructions and infrastructures

Buildings and city infrastructures are expected to change significantly in the future to conserve resources, enhance comfort, and reduce environmental impacts. 3D and 4D printing can play a crucial role in future facilities by increasing the adaptability and flexibility of building functionalities. Some ideas for potential applications of TPVs in future construction could be.

* **Smart Roads and Bridges:** 3D-printed TPV components embedded in roads or bridges could self-heal minor cracks or deformations due to their elastic nature. These materials could also be integrated with sensors to detect stress and repair themselves automatically.

* **Adaptive Building Skins:** TPV-based 3D/4D printed panels could be used for buildings that flex and adapt to weather conditions. For example, they could expand to seal gaps during storms or contract to allow airflow in warm weather.

* **Temporary Infrastructures:** 3D-printed TPV structures could be designed to degrade or change properties over time, such as temporary bridges or shelters that dissolve after a set period. This would reduce the need for manual removal.

* **Breathing Buildings:** Building materials made from TPVs could expand and contract, like lungs, regulating airflow and temperature within the structure without relying on mechanical systems.

### 8.3.6. Soft robotics

Traditional robots are usually seen as machines with rigid bodies and limited movements. Soft robotics is a new area of robotics that seeks to replace hard and inflexible parts with soft and flexible materials, mimicking the movement mechanisms found in living organisms [207]. TPVs, because of their flexible and soft characteristics, can be widely

used in soft robotic components such as grippers, actuators, and artificial skins. TPVs can also be utilized in 3D-printed bio-inspired robots that imitate the movements of animals, like octopus tentacles or snake-like structures. These designs could be particularly useful for applications in search-and-rescue missions or underwater exploration.

In the future, it is likely that emotionally intelligent robots will emerge [208]. One potential application is the use of TPV-based synthetic skin for these robots, which can change texture or temperature to simulate a human-like touch. This feature could enhance emotional connections between robots and users, making interactions feel more natural and engaging.

### 8.3.7. Printed stretchable electronics/wearable devices

TPVs are widely used in research to create stretchable conductive materials by incorporating various fillers, such as graphene [209], multi-walled carbon nanotubes [22,210], carbon black [211,212], and single-walled carbon nanotubes [163]. Some TPVs exhibit a low percolation threshold, making them suitable for different applications [24,213]. Stretchable conductors with good electrical conductivity under large strain have significant potential for use in robotic arm joints, wearable displays, and flexible electronics [214,215]. TPVs are ideal for stretchable conductors due to their high elasticity and stretchability. Additionally, their recyclability is an advantage over thermoset alternatives. Recently, 3D printing has emerged as a precise and flexible manufacturing technique for producing wearable devices and stretchable electronics with diverse applications. Using 3D printing to manufacture TPVs could open up their use in printed stretchable conductors.

### 8.3.8. Application in sensors and actuators

Previous studies have shown that TPVs can be used as wearable sensors to detect human motion and strain [216]. For example, Saleh et al. [217] developed a flexible strain sensor by adding conductive carbon black to TPVs. By adjusting the processing conditions, they achieved a low percolation threshold of about 4 % by weight. The sensor's performance under static and cyclic loading depended on its structure and filler content. The results showed that TPVs with interconnected structures had excellent piezoresistive properties and good sensitivity. Additionally, TPVs can be used as actuators due to their shape memory capabilities. Combining these properties with AM techniques allows for the creation of multifunctional printed parts that can both actuate and sense, which is useful in applications like soft robotics.

### 8.3.9. Application in architected and metamaterials

Archited materials, also known as metamaterials, obtain their properties from the geometric arrangement of their internal structures. Their designs depend on networks of interconnected elements to control the overall mechanical behavior [218]. The choice of material used in manufacturing metamaterials is crucial for adjusting their properties. For example, the shape memory effect can be used to restore metamaterials to their original shape. TPVs offer a wide range of tunability, recyclability, and softness, making them promising candidates for use in metamaterials in the future. Due to their complex geometries, metamaterials can be efficiently produced using 3D printing techniques.

### 8.3.10. Application in printed artificial organs

Due to their softness and compatibility with biocompatible thermoplastics and elastomers, TPVs can effectively mimic biological organs. For example, they can replace thermoset rubbers used in artificial skin.

Another potential application of TPVs is in artificial muscles. Artificial muscles are devices that convert energy into movement, like natural muscles, allowing tasks like lifting or pulling weights. This energy can come from various sources, such as physical or chemical stimuli [219]. Artificial muscles typically respond to stimuli like heat, electricity, pressure, temperature, or light by contracting [220]. These devices often use elastomer components for movement. Recently, 3D printing has become a common method for manufacturing artificial
muscles. Printed TPVs could find applications in this technology.

## 9. Conclusion

TPVs hold remarkable potential as a sustainable and high-performance class of materials suitable for additive manufacturing. Their unique structure, consisting of a dynamically crosslinked elastic-meric phase within a thermoplastic matrix, offers a rare combination of elasticity, processability, and recyclability. These characteristics not only distinguish TPVs from conventional thermoset rubbers but also align them with the global shift toward environmentally responsible design and manufacturing.

However, the transition of TPVs from conventional molding to 3D printing remains constrained by several persistent technical barriers. Main among these are high melt viscosity, phase incompatibility, poor interlayer adhesion, and filament buckling, particularly in material extrusion-based techniques such as FFF. These challenges are intrinsic to the material's dual-phase nature and require targeted interventions in both material formulation and printing strategy.

Despite these obstacles, the prospects for printable TPVs are promising. Advances in compatibilizer chemistry, crosslink density control, and alternative feedstock formats such as pellet-based extrusion have begun to demonstrate pathways toward improved printability without compromising the core mechanical advantages of TPVs. Moreover, the possibility of tailoring TPV formulations for specific functionalities, such as shape memory behavior, self-healing, and electrical conductivity, extends their relevance to next-generation applications in wearable electronics, soft robotics, biomedical devices, and adaptive infrastructure.

What emerges from this study is that TPVs are not inherently incompatible with 3D printing; they are simply not yet optimized for it. With continued focus on material innovation and process adaptation, TPVs can evolve from a promising but challenging elastomer to a cornerstone of sustainable, intelligent additive manufacturing. Their ability to replace less recyclable, rigid polymers while enabling flexible, customizable, and functional components gives TPVs a distinct and necessary role in the future of additive manufacturing.

## CRediT authorship contribution statement

**Abbas Bayati:** Writing – review & editing, Writing – original draft, Visualization, Methodology, Formal analysis, Conceptualization.
**Mohammad Amin Yousefi:** Writing – review & editing, Writing – original draft, Visualization, Methodology, Formal analysis, Conceptualization.
**Davood Rahmatabadi:** Writing – review & editing, Writing – original draft, Methodology, Formal analysis, Conceptualization.
**Majid Baniassadi:** Writing – review & editing.
**Karen Abrinia:** Writing – review & editing.
**Daniel George:** Writing – review & editing.
**Mostafa Baghani:** Writing – review & editing.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

The figures 1, 5, 6, 13 and 16 created with BioRender.com, were accessed on January 2025.

## Data availability

No data was used for the research described in the article.

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