3D Printing and Nanotechnologies in Biofilms: History
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Subjects: Surgery

Biofilms remain one of the most pervasive complications of the medical field, representing 50–70% of all nosocomial infections and up to 80% of total microbial infections. Since biofilms contain intricately small matrices, different microenvironments, and accumulations of biodiverse microorganisms of different resistances, these structures end up being difficult to target.

  • biofilms
  • nosocomial
  • 3D printing
  • nanotechnology

1. Introduction

With the advent of 3D printing and nanotechnologies, the possibilities to develop more versatile materials in the medical industry have vastly increased. Polymer printing and nanomaterials in the medical field provide cost-effective means to quickly produce a wide variety of highly customizable products, which include bone tissue scaffolds of enhanced strength, cardiovascular tissues and stents, patient-specific anatomical models for precision medicine, and microbe-resistant devices [1,2,3,4]. Nanoparticles have already proven themselves to be powerful and effective tools in the diagnostic and therapeutic targeting of illnesses such as chronic kidney disease [5]. Although there are concerns about the potential for some toxic adverse effects upon collection of these materials in the human body, new green or ecologically clean forms of nanoparticles help prevent or alleviate these concerns to be able to capitalize on their valuable medical potential [6,7]. Due to the versatility of such materials, this has opened up new opportunities in the development of medical equipment and implants that can prevent the formation of biofilms [8].
Biofilms are large multi-species matrices of bacteria or other microorganisms collecting together on surfaces through extracellular polymeric substances (EPS), creating enhanced cohesion and layers of highly resistant protection [9]. These biofilms tend to include bacteria that can thrive on both abiotic and biotic surfaces, with abilities to evade antibiotics or disinfectants through physical shielding as well as a variety of resistance genes obtained through lateral gene transfer in the biofilm community [10]. When cells are part of a biofilm matrix community, they have been found to have nearly 100 to 1000 times greater resistance to antimicrobials (antibiotics and disinfectants/antiseptics) compared to when they are found in their planktonic form [11,12,13]. This resistance is of growing concern in the medical field, as infections that arise from hospitalization or medical treatments are labeled as nosocomial infections, and it is estimated by the NIH and CDC that more than sixty percent of nosocomial infections are caused by biofilms [14,15]. Biofilms present many dangers in the medical field, including chronic infections with implanted devices, intravenous access line-induced bloodstream infections, surgically implanted mesh infections, orthodontic complications, and orthopedic complications [16,17,18,19,20]. Due to these significant problems, various mechanisms have been studied to reduce the dangers people face from these persistent pathogens. One area of study that has been explored is to interfere with cyclic dimeric guanosine monophosphate (c-di-GMP) metabolism and signaling, as this molecule serves as a second messenger mediating bacterial processes, including the formation of biofilms [21].

2. Use of 3D Printing to Target Biofilms in Dentistry Industry

While organ or tissue transplant and implant technologies are critical areas prone to biofilm infiltration, it is important to note that dental and orthodontic procedures also carry a major potential for biofilm risks. Dentures, retainers, occlusal splints, crowns, bridges, and other oral prostheses are all highly susceptible to microbial biofilm colonization [23,24]. The use of 3D printing technology in this field has a multi-fold benefit as it provides versatility to produce patient-specific prostheses while also providing more highly resistant material options to reduce infections in a more cost-effective and time-efficient manner [23].
In the dental industry, polymethyl methacrylate (PMMA) has been one of the most commonly used polymers for oral prostheses since its introduction in 1937 [23,25]. PMMA has long provided the benefits of natural aesthetics matching original dental characteristics, widespread availability, biocompatibility, light-curability, cost efficiency, and easy malleability of processing and repair [23,25]. Unfortunately, however, PMMA also suffers from the downside of high susceptibility to microbial growth, including its prime surface propensity for biofilm formation [23]. Enhancing antimicrobial resistance of 3D printed materials is critical, as it has been shown that while 3D printing allows for the benefit of developing personalized, well-fitted implants, in cases of printing plates for dental and craniofacial jaw or dental implants, the roughness and pores produced compared to commercial plates or implants increase bacterial adhesion for biofilms, especially with contamination during surgery [26]. To overcome this weakness, one study utilized 3D printing to produce a PMMA-based formulation that incorporated 2-methacryloyloxyethyl phosphorylcholine (MPC) and sulfobetaine methacrylate (SB), which are zwitterionic materials [23]. The zwitterionic nature of these materials means they have cationic and anionic portions with an overall neutral charge and electrostatic interactions that lead to protein-repellent and antimicrobial properties [23]. Protein-repellent properties are significant for biofilm prevention as proteins, such as CdrA extracellular adhesin, are a major component of the extracellular polymeric substances that comprise the biofilm matrix [27,28]. When tested against early biofilm colonizers, Streptococcus mutans, Staphylococcus aureus, Klebsiella oxytoca, and Klebsiella pneumoniae, as well as human saliva-derived biofilms, 3D printed MPC/SB-enhanced PMMA showed significantly lower amounts of attached bacteria than the control (PMMA before addition of any MPC or SB), as well as minimal biofilms compared to the robust amount on the controls (thickness and biomass were reduced with MPC/SB) [23]. Furthermore, while adding biofilm resistance, the 3D-printed MPC/SB-enhanced PMMA did not have any large losses of flexural strength or elastic modulus, which demonstrates great potential for these materials to increase antimicrobial properties without decreasing the durability or reliability of the products [23].
As already mentioned, one of the major concerns regarding 3D-printed medical devices and the potential for biofilm formation is the surface properties of printed materials [29]. It has become important to test various printing methods to ensure that low surface roughness is maintained, thus reducing bacterial adhesion and biofilm formation [29]. Testing different materials and timepoints after dental events, such as brushing or artificial aging, one study demonstrated that 3D printed resin (methacrylic oligomers and phosphine oxides) exhibited similar roughness to acrylic resin (PMMA), and bisacryl resin (dimethacrylate polymer, Bis-GMA, zirconium particles, silica, and silane) at the initial timepoint and after brushing, but after artificial aging, the printed resin significantly decreased in roughness, while acrylics significantly increased in roughness [29]. The long-term improvement in 3D printed items compared to standard materials is quite promising, especially considering the increased potential of manual and automixing manipulation defects of bubbling and pores that are more commonly observed in the standardly prepared dental resins [29]. The need to reduce pore size in dental implants to best minimize biofilm surface formation has also made nano-engineered dental implants a promising means of development [30]. For instance, electrochemical anodization has allowed for the development of anodized nano-engineered zirconia dental implants with aligned nanopores providing ideal surface topography to prevent biofilms from forming [30]. Important to dental implant placement, this use of nanopore surfaces or nanoscale surface production also increases osseointegration with the natural oral matrixes of the patient, which increases the potential for these techniques in the industry [30]. The successful antimicrobial observations of anodized nanosurfaces against common pathogens, such as Escherichia coli, Listeria monocytogenes, Staphylococcus aureus, and Staphylococcus epidermidis, have been credited to electrostatic and acid-base forces being modified due to the high density of the small pores [11]. In addition to the printing process itself, post-production processing can also be valuable in enhancing the potential of printed materials to resist biofilm formation [22]. In one study, polishing and glazing 3D-printed NextDent UV-curable resin for crowns, bridges, and prosthetics reduced the surface roughness average, which in turn reduced the affinity for bacterial adhesion and colonization [22]. Additionally, glazing alone seemed to be sufficient in reducing surface roughness and biofilm formation, including resistance against streptococci, staphylococci, and Candida, in some polymer substances, such as Mazic Temp 3D-printed materials [22]. The addition of polymerization inhibitors, such as mequinol, to the printed materials, as well as the use of resins that lack long polymerization times, has been shown to further enhance the antimicrobial benefits of 3D-printed glazed or polished materials, as extended polymerization times increase roughness and thus biofilm formation [22].
While some 3D printing focuses on the whole production of implants or devices, some of the value of 3D printing simply lies in the versatility of improving fillers that can be effectively added to the produced dental appliances, such as the printing of intra-oral PMMA appliances with nanodiamonds added as fillers [31]. The use of 3D-printed nanodiamond-enhanced oral appliances not only improved resistance to biofilms of Streptococcus mutans but also improved wear resistance and friction resistance [31]. Even more impressive in dental filler additives is the ability to now fabricate model compounds with 3D-printing technologies using known drugs from the dental field [32]. For instance, customized molds have been made with the antimicrobial tinidazole (TNZ) through thermal pressing of 3D molds to form release and compression molds capable of “on-demand” sustained release of TNZ while maintaining strong mechanical integrity [32]. Since dental caries require proper disinfection when first placed, this new method of 3D-printing custom antimicrobial modes could help provide fillers that are produced with lower cost, greater accessibility and personalization, and increased effectiveness [32]. The promising potential of 3D-printing custom implants in the dental field also expands to the orthodontic and maxillofacial fields, including facial epithesis, with far less strain or complications for patients [33].

3. Use of 3D Printing to Target Biofilms in Reusable Medical Devices

The use of 3D-printed reusable medical devices in areas other than dentistry has also brought about a new industry of medical devices that are now safer to implement without the fear of post-operative infections occurring. Chronic infections due to bacterial biofilm formation on implanted medical devices are a major concern in the field of medicine and the healthcare industry [16]. Various pathogens can cause infections in humans, such as viruses, fungi, and bacteria, but infections from bacteria are the most common type of infection, causing both acute and chronic infections in the population [34]. Bacterial infections are also becoming more untreatable due to the alarming rise of antibiotic-resistant bacteria strains [35,36,37,38]. Bacteria exist in two forms: planktonic, which is a free-floating state, and sessile, which adheres to the surface. Both states result in the production of a protective barrier that works as an endogenous defense system, making it difficult for antibiotics to rid the bacteria of infection. This exopolysaccharide matrix barrier, or “slime”, along with the accumulated microbial cell community, is what is now referred to as “biofilm” [39].
Both Gram-positive and Gram-negative bacteria form biofilms on medical devices, but some are more prevalent than others. The most common forms are Enterococcus faecalis, Staphylococcus aureus, Staphylococcus epidermidis, Streptococcus viridans, E. coli, Klebsiella pneumoniae, Proteus mirabilis, and Pseudomonas aeruginosa [38]. Approximately two-thirds of medical device infections are caused by the Staphylococcus species, which can infect devices such as prosthetic heart valves and catheters, causing potential hospitalizations [40,41,42,43]. According to the National Institute of Health, biofilms account for up to 80% of the total number of microbial infections, such as endocarditis, cystic fibrosis, periodontitis, osteomyelitis, and kidney infections [44,45,46]. Biofilms are particularly challenging to treat due to their difficult diagnosis and lack of biomarkers. Due to the complexity of biofilm communities and their antibiotic-resistant nature, the problem requires new material science to find and implement solutions, particularly biofilm-resistant materials and conventional antibiotics [35,47]. Sustainable innovations in antifouling being explored include targeting bacterial processes, such as quorum sensing, biofilm-related gene expression, secondary messengers, and regulatory RNA, as well as blocking initial adhesions by using green technology, such as silicon oil-infused substrates from plant models [48,49].
A prominent medical device example is the use of hearing aid devices [50]. Approximately 5% of the world’s population suffers from hearing loss, including one-third of the population over the age of 65 years which requires the use of hearing aid devices [50]. As a result of prolonged use of these medical devices, it is common for the ear microbiota to be altered or increased, increasing the risk of fungal and bacterial infections [50]. However, this is not just a limited issue for hearing aids alone. This has been a common occurrence with most internally placed or implanted medical devices—especially implanted medical devices used in joint replacements, catheters, stents, and prosthetics [47].
To date, 3D-printed medical devices with antibiofilm properties are not very prevalent but are being researched and developed for multipurpose use [51]. Common practices for removing biofilm development on hearing aid devices require removing the device for extensive cleaning. This becomes even more complicated when an infection has already taken root. In the case of ear infections, topical antibiotics, and systemic or topical fluoroquinolones are the most effective in administering treatment [50]. Two of the most common fluoroquinolones used for ear infections are ciprofloxacin and ofloxacin [47,50]. With new 3D-printed hearing aids that can act as drug-loaded platforms, patients would avoid discontinuing the use of hearing aids due to infection [52,53].
The drugs chosen for the 3D-printed devices were based on the two most common medications used to treat ear infections. A combination of ciprofloxacin and fluocinolone acetonide were both incorporated into the hearing aid as controlled-release drugs, and this combination was then evaluated against the two most common biofilm formers—Pseudomonas aeruginosa and Staphylococcus aureus—both involved in chronic ear infections [52,53]. Flexible resin and engineering hard resin (ENG) were used to prepare the photoreactive solutions [50]. The results for 3D printed hearing aids proved to be highly efficient and cost-effective when the volume of production was kept low when compared to other manufacturing methods used, such as molding, which requires molds that come at a higher cost due to materials, machinery, and labor [54,55]. The DLP 3D-printed hearing aids preventing or treating biofilm-related infections were successful when prepared using both flexible and ENG hard resins with different drug loads. In vitro drug release studies showed that the hearing aids were able to provide sustained drug release for over seven days for both drugs, successfully demonstrating antibiofilm properties against P. aeruginosa and S. aureus [50].
While resin composite hearing aids with antibiotics are one method of combating antibiofilm resistance, it is not the only angle of approach scientists are taking. Material sciences are also playing a role by researching certain composite materials that show promising results with antibiofilm resistance. Such a material is a 3D-printed composite of polylactic acid (PLA) with the addition of MgB2 particles [56]. The PLA composite, along with MgB2, was shown to have strong antimicrobial activity and is a great candidate for future medical devices due to its range of biomedical applications and its biodegradability and biocompatibility properties [56,57]. Another application of polylactic acid (PLA) is in filament compositions along with modified carbon nanomaterials such as bidimensional graphene (PLA-G), which improves the performance of 3D-printed medical devices. Together, this composition can also be used in the production of personal protective equipment due to its properties, which allow it to be sterilized by near-infrared light exposure within three minutes. This method has even been considered for combating the efforts in the SARS-CoV-2 epidemic [58].
Another approach has been modified 3D-printed polycaprolactone (PCL) scaffolds along with antimicrobial peptides (AMPs). Currently, in Phase I and Phase II human clinical trials, melamine, a chimeric cationic peptide, was immobilized onto the surface of a 3D printed medical-grade polycaprolactone (mPCL) scaffold and showed promising results with a ~78.7% reduction of Staphylococcus aureus compared to the control sample [59,60,61]. Melimine has some interesting properties and might have many applications in the medical implant field. According to multiple studies, it has been shown to reduce bacteria adhesion and biofilm formation when covalently bound to different non-degradable materials, such as silicone, glass, and titanium [62,63,64,65].
A promising new method for orthopedic surgeries involving bone regeneration and defects involves a dual-functional 3D-printed composite scaffold. The quaternized chitosan (HACC)-grafted polylactide-co-glycoside (PLGA)/hydroxyapatite (HA) scaffold (PLGA/HA/HACC) generated by 3D-printing technology exhibited significant antimicrobial and osteoconductive properties in vitro [66,67]. The study evaluated the bone-repairing effects of the 3D-printed scaffolds using infected cortical and cancellous bone defects. The study used 80 female Sprague–Dawley rats and 36 female New Zealand white rabbits. X-ray, micro-CT, microbiological, and histopathological analyses were used to assess the anti-infection and bone-repairing potential of the dual-functional porous scaffolds, and it was observed that the HACC-grafted PLGA/HA scaffolds exhibited significantly enhanced anti-infection and bone regeneration capability in different infected bone defect models [66,67,68]. The scaffold also exhibited enhanced anti-infection and bone regeneration capability in different infected bone defect models, showing promising results in applications for repairing different types of bone defects, even under infection [66,68,69].

This entry is adapted from the peer-reviewed paper 10.3390/hygiene3030024

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