Recent advances in 3D printed wound dressings
Cite as: AIP Conference Proceedings 2344, 020021 (2021); https://doi.org/10.1063/5.0047183 Published Online: 23 March 2021
Muhammad Hanif Nadhif, Hanif Assyarify, Muhammad Irsyad, Arindha R. Pramesti, and Muhammad Suhaeri
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Recent Advances in 3D Printed Wound Dressings
Muhammad Hanif Nadhif
1,2,a), Hanif Assyarify
1, Muhammad Irsyad
1, Arindha R. Pramesti
1, Muhammad Suhaeri
1,31Medical Technology Cluster, Indonesia Medical Education and Research Institute (IMERI), Faculty of Medicine, Universitas Indonesia, Jl. Salemba Raya No. 6, DKI Jakarta 10430, Indonesia
2Department of Medical Physics, Faculty of Medicine, Universitas Indonesia, Jl. Salemba Raya No. 6, DKI Jakarta 10430, Indonesia
3Unit of Education, Research, and Training, Universitas Indonesia Hospital, Universitas Indonesia, Jl. Salemba Raya No. 6, DKI Jakarta 10430, Indonesia
a)Corresponding author: [email protected]
Abstract. Chronic and acute wounds interfere with personal aesthetics and appearance. Regarding the problematics, researchers enhance the functionalities of wound dressings. One of the enhancement approaches is using 3D printing technology. The use of 3D printing has enabled many types of wound dressings, including biodegradable multi-material, cell-laden, and drug-eluting wound dressings. The use of 3D printing even allows for a wound dressing with a 3D construct, facilitating the wound dressing placement at the wound bed. Unfortunately, the 3D printed wound dressing technology has never been reviewed, to the extent of our knowledge. In this report, we aim to reflect on the recent advances in 3D printed wound dressings. Reports regarding the 3D printed wound dressing were collected from the Google Scholar database. The search was limited from 2015 to 2020 with a logic search: “3D printed” AND “wound dressing,” which words can be found in the entire article. Irrelevant reports were excluded manually, thereby resulting in the 24 selected articles. The selected articles were subsequently reviewed in terms of the materials-efficacy analysis and 3D printing mechanism. The extracted information from all articles concluded that alginate is the most used material for 3D printed wound dressing, while the most used 3D printing mechanism is the pneumatic FDM. In terms of efficacy, wound dressings presented an antimicrobial performance against Gram-positive and Gram-negative bacteria, as well as biocompatibility to epidermis and dermis cells.
INTRODUCTION
Humans may experience wounds, either in the form of acute or chronic wounds. Acute wounds, for example, can be generated when humans encounter a traffic accident [1]. Unfortunately, the wound may not be recovered completely when the wound is severe. Instead, scar tissues may be formed at the wound location. Chronic wounds, which result from the failure of normal healing stages, may bring problems to the affected persons since this type of wound cannot be repaired in an orderly and timely manner [2]. The two examples of chronic wounds are the pressure ulcer stages 3 and 4, in which opening of the wound reaches subcutaneous tissues and bones, respectively, forming a wound cavity. The untreated acute and chronic wounds may compromise the aesthetics, appearance, and movement.
The two types of wounds are usually treated using three approaches: wound irrigation, wound dressing, and negative pressure wound therapy (NPWT). Wound irrigation is a technique to perfuse a steady flow of solution across an open wound surface to achieve wound hydration, remove deeper debris, and assist the visual examination [3]. The types of solutions vary, starting from saline water, tap water, castile soap, to chloride [4,5]. After irrigation, a wound dressing is settled at the wound bed to provide an enclosure. The enclosure is intended to protect the wound from foreign bodies, thereby maintaining wound healing [2]. Wound dressings appear in various forms. Traditional wound dressings are in the form of gauze, lint, plasters, and bandages [2], while modern wound dressings are in the form of films, foams, hydrogels, and hydrocolloids [6]. Modern wound dressings are categorized as inert, interactive, and bioactive, which explanations about the three categories can be found at Dhivya et al. [2]. NPWT is used to provide sub-atmospheric pressure to the wound, removing exudates and maintaining humidity [7,8]. Recent NPWT devices
The 5th Biomedical Engineering’s Recent Progress in Biomaterials, Drugs Development, and Medical Devices AIP Conf. Proc. 2344, 020021-1–020021-10; https://doi.org/10.1063/5.0047183
Published by AIP Publishing. 978-0-7354-4086-9/$30.00
incorporate a battery to support the portability of the device [9]. The use of NPWT involves a wound dressing as the interface between the device and the wound, which is mostly in the form of foam, gauze, and sponge [10–12].
The importance of wound dressings in wound healing encouraged researchers to formulate a better wound dressing in terms of antimicrobial properties, enhanced tissue proliferation, and shorter healing periods. The significant disruption of 3D printing in the field of regenerative medicine [13] and advanced polymers [14] has gained interest from researchers to also utilized the technology in the search for better wound dressings. Many forms of 3D printed wound dressings are studied. The studies also vary regarding the material compositions, 3D printing mechanisms, bactericidal properties, and reactivity of wound dressings.
Unfortunately, there is still a lack of review about 3D printed wound dressings. In this report, we aim to reflect on the recent advances in 3D printed wound dressings in terms of materials-efficacy analysis and 3D printing mechanisms. The reviewed articles were collected from the Google Scholar database published from 2015 to 2020.
METHODS
The review was started by collecting papers form a Google Scholar database with a search logic “3D printed”
AND “wound dressing,” in which the two phrases can be found anywhere in the article. The published years were limited from 2015 to 2020. The collection was also limited from page 1 to page 6 of the Google Scholar search results, which page contained ten articles, thereby resulting in 60 articles. Furthermore, the 60 articles were sorted manually in terms of relevance to the topic. Only original articles are discussed in this report, thereby neglecting review papers.
After sorting, 24 original articles were selected.
RESULTS
From the 24 articles, 3D printed wound dressings are discussed according to materials-efficacy analysis and 3D printing mechanism.
Materials DQG Efficacy
Materials for 3D printed wound dressings can be either extracted from nature or synthesized in factories. Here we elaborate upon the efficacy of materials used for wound dressing for each selected article.
Natural 3olymers
The most common natural polymers as the main materials are cellulose, alginate (ALG), and chitosan (CS).
However, other natural polymers, such as gellan gum (GG), silk sericin (SS), and pectin (PEC), are also reported.
For wound dressings, cellulose was usually modified or pre-processed. The modification of cellulose can be found as nanocellulose [15–17]. Rees et al. characterized two types of nanocellulose (NC): NC pre-treated with a combination of carboxymethylation and periodate oxidation (CPNC) and with TEMPO-mediated oxidation (TEMPO- NC) [15]. The CPNC resulted in shorter nanofibrils than TEMPO-NC [15]. Unlike CPNC, which formed a more solid construction and profound tracks, TEMPO-NC was prone to collapse [15]. The CPNC layer deposited on top of the TEMPO-NC film provided wound dressing features, including a highly porous structure [15]. Both TEMPO-NC and CPNC exhibited the ability to inhibit the growth of P. aeruginosa, a common wound pathogen [15]. Chinga-Carrasco et al. and Espinosa et al. also 3D-printed TEMPO-NC grids [16,17]. However, the group combined the ink with ALG and cross-linked the ink with CaCl2 (Ca2+ ions) before printing, which produced be more robust, stable, and free- standing grids [16] with increased moisture absorption capacity [17]. The microscopic observation suggested the correlation between the ALG content increase and the decline of the 3D printing resolution [17]. However, the TEMPO-NC/ALG grid could hold a large fraction of liquid [17]. Toxicity tests showed that the TEMPO-NC/ALG grid was nontoxic to mouse fibroblasts [16]. Finally, the TEMPO-NC structures were proven sufficiently solid to be applied as dressings, attaching, and conforming easily to the surface of the skin without disintegrating [17].
The cellulose-based wound dressings can also be in the form of carboxymethyl cellulose (CMC) [18] and hydroxypropyl methylcellulose (HPMC) [19]. In Maver et al., CMC was combined with alginate (ALG) as a hydrogel.
The ALG/CMC hydrogel was proven well suited for releasing diclofenac sodium (DCS), a type of nonsteroidal anti- inflammatory drugs (NSAIDs) [18], which is desirable in wound treatment. In Wu and Hong, HPMC and polyacrylamide (PAM) were combined as hydrogel dressing with the addition of silver nanoparticles (AgNP) [19].
The AgNP-crosslinked dressing showed cytocompatibility and antibacterial activity against S. aureus. Furthermore, the AgNP-crosslinked dressing presented the thickest new tissue regeneration with the smallest scar width in the animal study than the blank wound, native PAM/HPMC dressing, and PAM/CS dressing [19].
Apart from the combination with cellulose-based compounds, alginate was also mixed with polyethylene glycol (PEG), polylactic co-glycolic acid (PLGA), polyvinyl alcohol (PVA), and chitosan to generate PEG/ALG scaffold [20], PLGA/ALG bilayer membrane [21], PVA/ALG bilayer membrane [22], and asymmetric construct (3D SAC) [23], respectively. Ilhan et al. added Satureja cuneifolia (SC) plant extract to the PEG/ALG composite as an antimicrobial agent. The PEG/ALG scaffold had a high swelling capacity and burst drug release effect, which enabled exudate absorption and facilitated wound healing in the inflammatory phase, respectively [20]. Furthermore, the scaffold also indicated high antimicrobial effects on bacteria, especially on the Gram-positive group [20]. On the other hand, the PLGA/ALG and PVA/ALG composites were formed in the means of a bilayer membrane (BLM) [21,22].
Wang et al. reported that the PLGA/ALG BLM had high tensile strength due to the PLGA nanofiber structure. The PLGA/ALG BLM was also reported to prevent bacterial (S. aureus) invasion, retain the humidity of the underlying hydrogel in vitro, and accelerate wound healing process [21]. The PVA/ALG BLM, despite swelling ability, had an adverse effect on the cell viability due to the large pore of the dressing [22]. Furthermore, the macro-porous surface structure of the 3D printed PVA/ALG BLM did not provide proper bio-adhesion properties for wound care since the dressing was hard to detach from the damaged tissue [22].
The 3D SAC was similar to the PLGA/ALG BLM in terms of bilayer structure. The 3D SAC consisted of a 3D printed CS/ALG construct and an electrospun polycaprolactone/silk sericin layer, reconstituting a dermis and an epidermis layer, respectively [23]. The 3D SAC exhibited morphology, porosity, mechanical properties, wettability, antimicrobial activity, and cytotoxic profile that were suitable for the wound healing process [23].
Alginate was also incorporated in GelDerm, a multifunctional dressing with pH sensors and drug-eluting scaffolds, as the main material [24]. Alginate was combined with glycerol (GLY), a plasticizer, to enhance the flexibility and mechanical integrity of the dressings, reduce the degree of swelling and dehydration rate, and increase the response time of the sensors [24]. Overall, GelDerm showed no toxicity towards keratinocytes and fibroblasts, as well as the capability to release gentamicin sulfate (GS), an antibiotic agent against Gram-positive and Gram-negative bacteria, at the wound site [24].
Chitosan, as mentioned earlier, has been used as wound dressing materials. Apart from the combination with alginate, chitosan-based wound dressings could be found in many forms. According to Long et al., the chitosan-pectin (CS/PEC) hydrogels attached with a PLA scaffold showed excellent printability, dimensional integrity, flexibility, self-adhesion to the skin, and ability to absorbs exudates [25]. Lidocaine hydrochloride (LDC), the medication for pain prevention and relief, attached on the scaffold presented a fast but controlled release [25]. In Hafezi et al., chitosan wound dressing film utilized genipin (GE) as a crosslinker, while GLY and PEG acted as plasticizers [6]. The chitosan- based film could swell and release a cheap model soluble drug, fluorescein sodium (FS), allowing for exudate management and moist maintenance [6]. Moreover, the film presented good adhesion with a model mucosal substrate6. The film was also proven nontoxic by cytotoxicity assay [6].
The least common natural polymers, gellan gum (GG) and silk sericin (SS), presented good performance as wound dressings as well. The degradation rate of 3D printed GG scaffolds could be tuned by altering the surface area:mass ratio, which was well-suited as drug-eluting wound dressings [26]. As for silk sericin, besides the application as a dermis-like layer, it was also fabricated as a transparent and microporous hydrogel scaffold with the addition of gelatin methacrylate (GelMA) [27]. Due to the hydrophilicity of the SS/GelMA hydrogel, the water uptake of the hydrogel was significant. The water absorption was most profound when the ratio of SS and GelMA was 1:2 [27]. The SS/GelMA hydrogel also presented good cell adhesion, as well as the proliferation of fibroblasts and keratinocytes, which allowed for re-epithelization [27].
Synthetic 3olymers
Various synthetic polymers were used, which included polycaprolactone (PCL), PAM, polydimethylsiloxane (PDMS), polyethylene glycol, polyvinyl alcohol (PVA), polyurethane (PU), polylactic acid (PLA), and translucent rubber (TR).
In Muwaffak et al. and Seol et al., wound dressings were fabricated to mimic the anatomical curvature of facial organs. Muwaffak et al. developed a 3D nose-like and ear-like wound dressings made of PCL [28]. The PCL dressing was mixed with silver, zinc, and copper antimicrobial agents [28]. The results showed that the bactericidal properties of Ag-PCL and Cu-PCL dressings are most significant against S. aureus, a common bacterium to cause skin infection [28]. Meanwhile, Seol et al. developed a porous PU BioMask wound dressing, mimicking facial curvature. PU was
combined with a hydrogel made of hyaluronic acid (HA), GLY, gelatin (GEL), and fibrinogen (FIB) [29]. The hydrogel was loaded with keratinocytes and fibroblasts to mimic an epidermis and a dermis layer, respectively [29].
BioMask showed the desired flexibility and did not affect cell viability [29]. In animal studies, BioMask was able to be implemented in the skin wound since the wound region was entirely covered with the newly regenerated skin [29].
PDMS wound dressings were made of pure PDMS and PDMS-SH. PDMS-SH was formulated by a co-hydrolysis condensation reaction of 3-mercaptopropylmethyldimethoxysilane (MPDMSi) and dimethyl diethoxylsilane (DMDESi) with H20 and HCl [30]. Unlike the other found articles that utilized a fused deposition modeling printer, the 3D printed PSDMS-SH was fabricated using stereolithography processes [30]. The PDMS-SH dressing exhibited antibacterial ability against Gram-positive (S. aureus) and Gram-negative (E. coli) bacteria, good cytocompatibility, and histocompatibility, as well as enhanced wound healing process [30]. Shi et al., on the other hand, made several configurations of PDMS wound dressing: pure PDMS, PDMS with silicone oil (iPDMS), PDMS with AgNPs (PDMS/AgNPs), and PDMS with AgNPs and silicone oil (iPDMS/AgNPs) [31]. The iPDMS/AgNPs dressing produced the most bactericidal performance against S. aureus and E. coli compared with other configurations [31].
The iPDMS/AgNPs dressing also demonstrated no cytotoxicity on fibroblasts and resulted in the longest epithelization and thickest granulation, which accelerated wound healing [31].
In the natural wound dressing section, PEG was stated to strengthen the mechanical integrity of chitosan- and alginate-based wound dressings. However, the derivative of PEG, poly(ethylene glycol) diacrylate (PEGDA), was also used as a wound dressing. Two PEGDA-based wound dressings were discovered in the literature. One wound dressing was made of pure PEGDA, while the other one was in the form of a porous polymeric network made using the high internal phase emulsion technique (polyHIPE). The pure PEGDA wound dressing was developed by Cereceres et al. using an FDM 3D printer [32]. Cereceres et al. loaded gallium maltolate (GaM) in the hydrogel.
Accordingly, GaM was evident as a bactericidal agent to inhibit the growth of bacteria (S. aureus), which was previously proven to prevent biofilm formation and wound infection [33]. In addition, the resulted PEGDA hydrogel dressing had hierarchical porosity to enhance the water uptake ability and provide more rapid moisture balance [32].
In terms of polyHIPE, Streifel et al. synthesized polyHIPE from sodium acrylate, poly(ethylene glycol) diacrylate (PEGDA), poly(ethylene glycol) methacrylate (PEGMA), Pluronic F-127, and ammonium persulfate (APS) [34].
Subsequently, polyHIPE was combined with kaolin to initiate hemostasis [34]. The resulted PolyHIPE foam had highly regular porous structures and noncytotoxic properties to human dermal fibroblasts [34]. The experimental works, which demonstrated that the 100 wt% kaolin formulation reduced the blood clotting time to 41% compared with the pureblood, confirmed the hemostasis-inducing properties of kaolin [34].
Other common polymers used in 3D printing, PVA and PLA, were also applied as wound dressing materials.
Clohessy et al. incorporated the 3D printed porous PVA as a sacrificial element for a wound dressing made of polyglycolic acid and polyethylene glycol (PGA/PEG) so that an internal reservoir for cells could be realized [35].
Nonetheless, the wound dressing seemed to hinder the proliferation and remodeling by native cells, despite the satisfying cell attachment, reepithelization, and improved angiogenesis [35]. In Domínguez-Robles et al., PVA was also used for the second layer of the wound dressing. Meanwhile, the first layer of the wound dressing was in the form of a PLA/LIG mesh made of PLA filament pre-mixed with lignin (LIG), tetracycline (TC), and the addition of castor oil for the LIG-TC attachment to the PLA pellets [36]. Curcumin was applied on the surface of the dressing as a model drug. The results showed that TC lowered the bacterial adherence significantly, while the presence of LIG reduced the maximum load the composite can hold before fracture [36]. Furthermore, the PVA second layer was evident to delay the curcumin release and provide a moist environment to the wound [36].
Apart from the form of foams, films, and hydrogel, 3D printed synthetic polymers were fabricated into a smart, flexible bandage. The patches were made of a photosensitive TangoPlus FLX930 translucent rubber (Stratasys, Ltd.), renowned for the exceptional elastomeric properties [37,38]. Mostafalu et al. fabricated the smart, flexible bandage using a jet dispensing 3D printer [37,38]. The earlier report embedded an electronic system, an oxygen sensor, and a wireless telemetry module to the smart bandage, thereby allowing for remote wound monitoring [37]. The 3D printed bandage had high strength, flexibility, and stretchability to serve a mechanically robust wound bandage due to the elastomeric-based material [37]. In the latter report, with the same material, the smart, flexible bandage was improved as a drug delivery bandage using rhodamine isocyanate (RhIC) for the release study [38]. The drug delivery module was enveloped in an alginate hydrogel layer inside the bandage, comprising thermo-responsive drug microcarriers and a heater to stimulate the microparticles [38]. The drug release mechanism started with the infuse of the drugs into the microparticles, which subsequently shrank in diameter and released the drugs when the heater rose the temperature [38].
TABLE 1. 3D printing mechanisms and materials used in the found articles 3D Printing
Mechanism
Materials
References Main Addition Bactericide/
Drug Cell
Roller Extrusion
PCL Ag, Zn, Cu - Muwaffak et al. [28]
PVA PGA/PEG - Clohessy et al. [35]
PAM/HPMC PLA AgNPs - Wu and Hong [19]
PLA/LIG PVA TC, CUR - Domínguez-Robles et al. [36]
Pneumatic
CS/PEC PLA LDC - Long et al. [25]
PVA/ALG Copier film/Al foil - - Palo et al. [22]
CS GE, GLY, PEG FS - Hafezi et al. [6]
NC - - - Rees et al. [15]
ALG/CMC - LID, DCS - Maver et al. [18]
GG - - - Yu et al. [26]
TR - - - Mostafalu et al. [37]
TR ALG RhIC - Mostafalu et al. [38]
PU HA/GLY/GEL/FIB - Keratinocytes
, fibroblasts Seol et al. [29]
Piston based
PDMS AgNPs,
silicone oil - Shi et al. [31]
NC ALG - - Chinga-Carrasco et al. [16]
NC ALG - - Espinosa et al. [17]
CS/ALG PCL, SS - - Miguel et al. [23]
PEG/ALG SC - Ilhan et al. [20]
Modified FDM
PolyHIPE Kaolin - Streifel et al. [34]
PEGDA GaM - Cereceres et al. [32]
ALG GLY GS - Mirani et al. [24]
E-Jet PLGA/ALG - - - Wang et al. [21]
SLA PDMS-SH - - - Xiang et al. [30]
Unspecified SS GelMA - - Chen et al. [27]
3D Printing Mechanism
Among the 24 articles, 6 3D printing mechanisms were discovered: roller extrusion FDM, pneumatic FDM, piston based FDM, modified FDM, electrohydrodynamic jet (E-jet), and stereolithography (SLA). However, there is one report that did not provide detailed information about the printing mechanism, thereby emerging the unspecified category. The 3D printing mechanisms are illustrated in Fig. 1.
FIGURE 1. 3D printing mechanisms: roller extrusion FDM (a), pneumatic FDM (b), piston based FDM (c), electrohydrodynamic jet (d), and stereolithography (e)
Roller Extrusion FDM
Roller extrusion FDM refers to the conventional FDM, which uses a stepper motor equipped with a roller gear to feed a filament through a heated element to fabricate a 3D part [39]. Muwaffak et al. fabricated a PCL filament from PCL pellets loaded with antibacterial agents using a Filabot filament hot-melt extruder (Filabot Inc., Barre, USA) [28].
Meanwhile, Domínguez-Robles et al. extruded a PLA/LIG filament from LIG-PLA coated pellets explained in Weisman et al. using a Next 1.0 filament extruder (3devo, Utrecht, The Netherlands) [36]. Clohessy et al. used a PVA commercial filament [35]. Wu and Hong, unfortunately, did not mention the PLA filament fabrication [19]. Muwaffak et al. and Clohessy et al. utilized a Replicator 2X™ desktop 3D printer (MakerBot, New York, USA), while Wu &
Hong and Domínguez-Robles et al. used JGaurora (Filabot Inc., Barre, USA) and Ultimaker 3 (Ultimaker B.V., Geldermalsen, The Netherlands) printers, respectively.
Pneumatic FDM
Pneumatic FDM was used in 9 articles. Pneumatic FDM means that the material deposition was realized using pneumatic air pressure. Long et al. and Palo et al. used a BioBots 1 pneumatic FDM (Biobots Inc., Philadelphia PA, USA, currently known as Allevi) with a 25G needle as a nozzle [22,25]. Long et al. started the material preparation by diluting LDC in PEC solution [25]. The solution was subsequently mixed with CS solution, thereby generating LDC-laden CS/PEC gel structure ready to print [25]. The group fabricated LDC-laden CS/PEC hydrogels with BioBots 1, while used a roller extrusion printer to fabricate PLA scaffolds [25]. Palo et al., on the other hand, prepared the PVA/ALG gel by mixing the PVA solution with the SA solution [22]. The PVA/ALG gel was inserted in the syringe of the BioBots 1 printer afterwards [22]. Hafezi et al. pre-processed the gel made of dissolved CH powder and GLY-PEG plasticizer in acetic acid [6]. The gel was then mixed with GE before transferred to the syringe of DispenseMate® D-583 (Nordson-Asymtek, Maastricht, The Netherlands) [6]. Rees et al. derived two types of nanocellulose (CPNC and TEMPO-NC) from Pinus radiata, which were processed to generate the two suspensions are mentioned earlier [15]. Rees et al. deposited CPNC porous scaffolds and a TEMPO-NC film with a 3D Bioplotter®
unit (EnvisionTEC GmbH, Gladbeck, Germany). Similar to Rees et al., Maver et al. mixed a cellulose-based solution (CMC) with another solution (ALG solution) to form an ALG/CMC bioink [18]. After ALG/CMC bioink was formed, two drugs, lidocaine (LID) and diclofenac sodium (DCS), were added to the bioink and subsequently placed in a syringe of a BioScaffolder 3.1 printer (GeSiM, Radeberg, Germany). Yu et al. formulated a GG hydrogel by molding a heated mixture of GG powder in ultra-pure water [26]. To start the printing, the hydrogel was poured into a syringe of an Inkredible bioprinter (Cellink, Gothenburg, Sweden) [26]. Mostafalu et al. did not say much about the 3D printing mechanism. The group only provide the information about the 3D printer and the material, which were an Objet Connex500™ 3D printer and a TangoPlus FLX930 translucent rubber (Stratasys, Ltd.), respectively [37,38].
Another report using a pneumatic FDM, which was more complicated than other reports, was from Seol et al. [29]
As previously mentioned, Seol’s group constructed BioMask, comprising 3 layers: porous PU layer as wound dressing, keratinocytes-loaded hydrogel layer as an epidermis, and fibroblast-loaded hydrogel as a dermis. The printing of the three layers incorporated an inhouse 3D integrated tissue-organ printing (ITOP) system, which was equipped with multiple syringes [40]. The preparation of the PU layer started with the loading of PU Tecoflex LM-95A pellets (Lubrizol, Wickliffe, OH, USA) into a syringe of the ITOP system [29]. To prepare the keratinocyte- and fibroblast- hydrogel, the cells were amassed with trypsin-EDTA and spun down to make a cell pellet, which pellet was mixed with a hydrogel-based bioink composed of HA, GLY, GEL, and FIB [29]. The hydrogel was subsequently loaded into another syringe of the ITOP system [29].
Piston based FDM
Piston based FDM relies on a piston to inject a bioink to the printing bed so that a 3D construct can be built. Same as the roller-extrusion FDM that uses a stepper motor, the piston based FDM also used a stepper motor to maintain the piston movement. Shi et al. prepared an iPDMS/AgNPs wound dressing by mixing the PDMS base: curing agent (1:1) with AgNPs and loading the mixture in a syringe of an HKable 3D bioprinter (Hkable 3D Biologic Printing Technology Co., Ltd., Chongqing, China) [31]. After printing, the 3D printed PDMS membrane was immersed in a bath of 5 Cst silicone oil [31]. Chinga-Carrasco et al. and Espinosa et al. produced TEMPO-NC ink from soda bagasse fibers [16] and Pinus radiata [17], respectively. Before printing with a Regemat3D (v1.0 in Chinga-Carrasco et al.
and v1.8 in Espinosa et al.) printer (Regemat3D, Granada, Spain), the TEMPO-NC ink was blended with ALG [16,17].
After printing, the TEMPO-NC/ALG was crosslinked in a bath of CaCl2 [16,17]. Miguel et al. prepared the CS/ALG hydrogels by mixing the CS and ALG solutions and crosslinking them with 5% CaCl2 [23]. Subsequently, the hydrogel was placed in a syringe of a Fab@Home 3D printer [23] from an open-source RepRap 3D printing platform.
Same as Espinosa et al., Miguel’s group also crosslinked the CS/ALG hydrogels in a bath of CaCl2 [23]. Ilhan et al.
began the fabrication by making a PEG/ALG mixture by mixing the ALG and PEG solutions [20]. Next, the PEG/ALG
mixture was poured to the dissolved SC with the addition of CaCl2 as a crosslinking agent before transferring the mixture into a syringe of an SDS5 3D printer (Hyrel 3D, GA, USA) [20].
Modified FDM
An open-source 3D printing platform (roller extrusion FDM) developed by Prusa has facilitated research institutions and corporates to enhance their outcome. In the separate works, Streifel et al., Cereceres et al., and Mirani et al. also took advantage of this platform by modifying a RepRap Prusa i3 3D printer (Prusa Research, Prague, Czech) to develop their wound dressing research works [24,32,34]. Streifel et al. and Cereceres et al. modified the printer by embedding an open-source RAMPS v1.4 electronics set and external MOSFETs to control the UV cure system [32,34].
The two groups also installed a customized HYREL EMO-25 extruder with a Luer lock adapter and a 22G blunted stainless steel needle (413 μm, 6.35 mm in length, Sigma Aldrich) [32,34]. For material preparation, Streifel et al. and Cereceres et al. altered the HIPE composition and PEGDA hydrogels, respectively, with a LAP photoinitiator to form an emulsion ink to allow for cure-on-dispense 3D printing [32,34]. Once the ink was ready, the ink was poured into the printer’s syringe. Different from the previous reports, Mirani et al. modified a RepRap Prusa i3 3D printer by utilizing a microextruder with a coaxial needle system, which was enabled by two syringe pumps (Harvard Apparatus, Holliston, MA, USA). Mirani et al. prepared the GS-loaded ALG hydrogel in one syringe while loading another syringe with a CaCl2 crosslinker [24]. The ionic crosslinking took place at the tip of the extruder before the hydrogel was deposited on the printing bed [24]. In brief, the three groups reported a modification of a roller extrusion FDM into a piston based FDM.
Electrohydrodynamic -et 3rinting
Electrohydrodynamic jet (E-jet) printing is a micro-to-nano scale liquid deposition (droplet fabrication) technique through micro capillary nozzles [41], in which fluid flows are induced by electrohydrodynamic force [42]. Referring to Sutanto et al., “The applied voltage generates an electric field between the nozzle and the substrate causing concentration of charge on the pendant drop emanating from the tip” [41]. The concentrated charge induces shear stress, which deforms the meniscus to a conical shape, the so-called a Taylor cone [42]. The material deposition is supported by a precision piston-based (syringe) pump [43,44] or a pneumatic system [45]. E-jet printing has been reported to deposit living cells in suspension onto the printing surface [43].
Wang et al. reported the use of E-jet printing to generate a PLGA/ALG bilayer membrane (BLM) scaffold [21].
Wang’s group started with the preparation of PLGA solution and ALG hydrogel by dissolving the PLGA copolymer in Hexafluoro-2-propanol (HFIP) and ALG in water, respectively [21]. The two solutions were transferred to the separate barrels of an E-jet printing machine (Tongli micro-nano technology Co., Ltd., Shenzhen, China) [21]. The PLGA solution was transferred to a barrel loaded in a syringe pump, while the ALG hydrogel was transferred to a barrel loaded in an air compressor [21]. The BLM scaffold printing started with ALG, acting as a bottom layer and subsequently continued with PLGA, acting as a top layer [21]. Finally, the BLM scaffold was placed in a CaCl2 bath to crosslink the ALG hydrogel [21].
Stereolithography
Stereolithography (SLA) is the first 3D printing technique ever developed, pioneered by 3D Systems [46]. The 3D fabrication in SLA is based on the spatially controlled solidification of a photosensitive resin through photo- polymerization [46]. Using the SLA technique, Xiang et al. fabricated a PDMS-SH wound dressing with a preparation technique mentioned earlier [30]. The 3D printing was executed using a desktop Form 2 SLA machine (Formlabs, Somerville, MA, USA) [30]. Next, the 3D construct was rinsed with ethanol and post-cured in an Intelli-Ray 400 UV Curing System [46].
Unspecified
It is worth mentioning a report that did not elaborate upon the 3D printing mechanism or the machine. Chen et al.
mentioned that the primary material, silk sericin (SS), was extracted from Bombyx mori, 185 Nd-s, silk cocoons [27].
The group explained that the extracted SS solution was blended with a GelMA solution; and transferred into a syringe of a bioprinter [27] afterward. Unfortunately, the group did not specify the bioprinter used for the fabrication. Judging
from Figure 1 on the report, it can be assumed that the printing utilized either pneumatic or piston based FDM.
Nonetheless, it is safer to put the report from Chen et al. in this category.
DISCUSSION
The material-efficacy analysis and 3D printing mechanism for each selected paper have been discussed. Among the 24 articles, 6 types of natural material and 8 types of synthetic material were used as main materials.
Alginate is the most used natural material, mentioned in 9 articles, either as the main or additional material. As the main material, alginate was mostly combined with other materials. Only Mirani et al. used ALG solely as the main material [24]. As an additional material, alginate functioned as a mechanical integrity layer [16,17] and an encapsulation layer [38]. Due to the use of ALG, CaCl2 also became the most used crosslinker since the crosslinking of ALG required Ca2+ ions [16,17,20,21,23,24]. The second most common material is cellulose. Six articles mentioned the cellulose-based materials, which were found in many forms: TEMPO-NC [15–17], CPNC [15], CMC [18], and HPMC [19]. The list continues with CHI (3 papers), SS (2 papers), GG (1 paper), and PEC (1 paper). On the other hand, for the additional material, glycerol is the most utilized either as a plasticizer to enhance the flexibility and mechanical strength of the dressing [6,24] or a hydrogel layer to load cells [29].
Synthetic polymers acting as the primary materials comprise PCL, PVA, PAM, PLA, TR, PU, PDMS, and PEG.
PLA and PEG are the most used synthetic materials (4 articles for each polymer), either as the main or additional material. Apart from the 3 reports, the report from Wang et al. used the copolymer of PLA and PGA, forming PLGA [21]. Meanwhile, two reports utilized two distinct derived forms of PEG: PEGDA [32] and polyHIPE [34]. PEGDA is a derivative of PEG with diacrylate chains. PolyHIPE, as mentioned earlier, was synthesized from sodium acrylate, PEGDA, PEGMA, Pluronic F-127, and APS [34].
Other important aspects regarding material-efficacy analysis are bactericidal agents, drugs, cells, and testing bacteria. A novel bactericidal agent, AgNP, was also reported twice [28,31]. The model drugs also commonly behaved as bactericidal agents. Nevertheless, LDC, FS, DCS functioned as pain prevention and relief [25], exudate management and moist maintenance [6], and NSAID [18], respectively. The most common testing cells are keratinocytes and fibroblasts since the two types of cell construct epidermis and dermis, respectively. Seol et al. even developed a bilayer hydrogel dressing loaded with the two types of cell [29]. For testing bacteria, S. aureus (Gram- positive) and E. coli (Gram-negative) are the most mentioned since the two species usually cause skin infection.
However, another type of Gram-negative bacteria (P. aeruginosa) was also used.
In terms of the printing mechanism, the pneumatic FDM is in the 1st place with 9 reports benefitting from the mechanism. The 2nd place is taken by piston based FDM, while the 3rd and the 4th places are taken by the roller extrusion and modified FDM, respectively. E-jet and SLA are only found in one paper for each mechanism.
Unfortunately, the last one paper did not specify the mechanism, albeit mentioning the extrusion-based printing in the paper. However, the most interesting part, in our view, is the modification of a low-cost FDM machine into a bioprinter by changing the extrusion system with a piston-based syringe-needle system. Using this idea, a bioprinter can be installed on a budget, enabling more democratized biomaterials and tissue-engineered products, including the research on 3D printed wound dressings.
To close, we hope that the information in this review will facilitate research groups working on novel 3D printed wound dressings.
ACKNOWLEDGMENT
This report is funded by the PUTI Prosiding Grant 2020 from Universitas Indonesia with a contract number of NKB-915/UN2.RST/HKP.05.00/2020.
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