Background: saliva versus innovative biomaterials versus oral ecosystem

The oral fluid named 'saliva' is recognised for possessing several beneficial properties in the oral ecosystem, nutrition, digestion and systemic human health.1 Characteristics such as lubrication, digestion, protection and re-mineralisation are of paramount importance for a stable systemic life. Saliva is an oral fluid found in the majority of people, excluding only people possessing extremely rare systemic diseases that compromise the production of saliva, demanding the use of artificial saliva.

From the clinical perspective, saliva is uninterruptedly exhibited in oral surgical procedures applying biomaterials and it is constantly interacting with oral tissues/oral fluids.1,2 Dental clinicians usually employ several techniques to prevent the direct interaction of saliva with dental procedures; however, the capacity of this oral fluid to spread quickly on the oral tissues in addition to the huge number of minor/major salivary glands present in the human mouth/gingiva suggest a persistent interaction with biomaterials applied in surgeries.1,2

The complexity to develop oral biomaterials promoting high biocompatibility and significant antibacterial properties has induced researchers and companies to manufacture biomaterials with responsive surfaces, stimulating faster interactions with beneficial cells.3,4 Dental implants possessing hydrophilic surfaces for rapid cell adhesion or antibacterial surfaces to prevent infection, membranes for bone regeneration with biodegradable properties and bone substitutes structured with different porosities are examples of these complexities aiming for better healing conditions in surgical procedures.3,4,5

Discussion and 'open question'

The application of these innovative biomaterials in conjunction with oral procedures may result in saliva contact before the completion of the surgical procedure. Previous studies from our group have shown that saliva may negatively influence the physical-chemical properties of hydrophilic dental implants,4 plus reduced biocompatibility of osteogenic cells related to hard/soft tissues regeneration.4,6 Other studies have shown that saliva-contaminated biomaterials have demonstrated to stimulate bacterial colonisation.7,8 An in vivo study applying dental implants and biomaterials for bone augmentation procedures where local saliva contamination (peri-implantitis contamination) was induced demonstrated an intense reduction in the osseointegration status and bone regeneration quality.9

This arrangement of findings suggests a significant negative role of the early/late saliva interaction with innovative biomaterials for oral regenerative procedures causing limited biocompatibility and increased bacterial colonisation (Fig. 1).

Fig. 1
figure 1

Illustration showing examples of previous studies which revealed negative effects of saliva contamination on biomaterials for oral regenerative procedures. a) Reduced cellular viability on hydrophilic surfaces contaminated with saliva compared to non-contaminated hydrophilic surfaces (* = statistical significant [p <0.05]). Figure adapted from Kunrath M F, Correia A, Teixeira E R, Hubler R, Dahlin C, 'Superhydrophilic Nanotextured Surfaces for Dental Implants: Influence of Early Saliva Contamination and Wet Storage', Nanomaterials 2022; 12: 2603, under the Creative Commons Attribution Licence.4 b) Different biomaterials applied in oral procedures and the differences in the wettability caused by saliva interaction (yellow bar = control, blue bar = saliva-contaminated). Reprinted from Dental Materials, Vol 29, Issue 10, Schweikl et al., 'Salivary protein adsorption and Streptococccus gordonii adhesion to dental material surfaces', pages 1080-1089, Copyright 2013, with permission from Elsevier.10 c) Negative morphological alterations of osteogenic cells caused by saliva contamination on modified-surfaces developed for dental implants. Reprinted from Materials Science and Engineering: C, Vol 100, Hirota et al., 'Impaired osteoblastic behaviour and function on saliva-contaminated titanium and its restoration by UV treatment', pages 165-177, Copyright 2019, with permission from Elsevier.6 d) Clinical image demonstrating saliva excretion by oral ducts in the oral environment, generating the possibility of interaction with biomaterials applied in oral regeneration

On the other hand, advanced studies have promoted the concept of applying isolated saliva proteins on surfaces in order to functionalise biomaterials and stimulate biocompatibility. Sun and co-authors revealed the application of isolated Histatin-1 (saliva protein) on titanium surfaces and into collagen membranes demonstrating an induced spreading of bone-related cells over the surfaces and promotion of higher taxes of newly formed bone in vivo, respectively.11,12 Interestingly, some saliva proteins have been reported as direct antimicrobial/antibacterial molecules due to their isolated action, such as adrenomedullin, statherin, histatins, lysozyme (antibacterial) and α-Defensins, β-Defensins, lactoferricin, lactoperoxidase, myeloperoxidase (antimicrobial).13,14 These investigations reveal that specific proteins present in the saliva composition may be promoters of immune/innate defences and might be applied for beneficial defensive outcomes in oral surgical procedures or to functionalise biomaterials.

The challenges regarding saliva research are broad due to the diverse spectrum of saliva compositions and different proteomic mapping/quantification in each person. Moreover, patients' nutrition, age and systemic/local diseases play huge roles in the saliva composition.15 Additionally, the protocols and techniques applied for human saliva collection, storage and application methods in preclinical and clinical biomaterials research are highly distinct.1 Therefore, most of the research demonstrated in the current literature cannot be totally intercorrelated, opening a clear need for investigations into this topic (Table 1).

Table 1 Challenges for saliva research in fundamental, preclinical, and clinical studies

Concluding remarks and future perspectives

Current studies have suggested avoiding saliva interaction in early and late stages in order to prevent infections or loss of biomaterials' properties, inducing that saliva contamination is considered harmful for oral regenerative procedures associated with biomaterials. However, this information is not completely 'scientifically clear' in the literature based on a large number of complex studies, resulting in the requirement for further advanced basic and clinical studies.

This knowledge 'gap' generates wide opportunities for future research that may clarify these questions involving dental procedures/saliva/microbiology/biomaterials. Based on our experience, innovative biomaterials have demonstrated intense interaction with saliva due to their surface sensitivity, changing important characteristics related to biocompatibility. Additionally, the applied protocols for saliva collection, storage and application should be as similar as possile to the clinical reality, trying to explore the saliva properties directly after collection.

Modern techniques and molecular data, such as proteomics, genomics, meta-transcriptomics and meta-metabolomics, are the perspectives to understand the deep influence of saliva on the beneficial cells and bacteria, thus, subsequently, the outcomes in the clinical environment, revealing if there are positive pathways to apply saliva proteins in oral regenerative procedures or if the saliva interaction must be completely avoided in all the clinical procedures. Lastly, promising functionalised biomaterials using isolated saliva proteins might be developed in the near future, exploring the beneficial effects of specific proteins to improve clinical outcomes.