Biolubrication enhancement for tissues and biomaterials
Wan, Hongping
DOI:
10.33612/diss.135598825
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Publication date: 2020
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Wan, H. (2020). Biolubrication enhancement for tissues and biomaterials: Restoration of natural lubricant function by biopolymers. University of Groningen. https://doi.org/10.33612/diss.135598825
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The lubricating molecules found in saliva, tear or synovial fuid adsorb on tissue surfaces to give rise to a conditioning film. A lack of lubricant molecules or their structural mutation is associated with the altered water-holding ability of the conditioning films. This yields poor lubrication at sliding tissue interface in vivo resulting in pain and discomfort1. Restoring biolubrication for patients is important because prolonged deficient lubrication can lead to tissue wear and necrosis. In this thesis, we propose a new strategy to enhance the lubrication where the aberrant conditioning film recruits natural lubricating molecules through the addition of adhesive biopolymers. To validate this concept, we have chosen the oral cavity and knee joint, as the models for understanding the role of adhesive biopolymer in lubrication enhancement at the tissue-tissue interface. High lubrication at the tissue-tissue-biomaterial (implant) interface is essential for the normal function and longevity of implants and medical devices. Thus, we designed a coating for polycarbonate urethane (PCU), which is a popular material for meniscus implant. The coating recruited biomacromolecules from the surrounding synovial fluid to enhance the lubrication of PCU while sliding against cartilage. Various surface analytical equipments were adopted to elucidate the mechanism of biolubrication enhancement, including Quartz crystal microbalance with dissipation monitoring (QCM-D), colloidal probe Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), tribochemistry, Attenuated total reflectance - Fourier transform infrared spectroscopy(ATR-FTIR), Scanning electron microscopy (SEM), and Universal mechanical tester(UMT).
Oral lubrication enhancement
Saliva and salivary conditioning film2,3 help lubricate the oral cavity and protect the tooth surface from erosion. Due to lower saliva production or mutation in mucin structure, xerostomia patients suffer from a subjective dry mouth feel. In the present day, dry mouth patients4 prescribed with saliva substitutes, which overwhelm the oral cavity with exogenous mucin, carboxy-methylcellulose, hydroxyethylcellulose, xanthan gum, etc. aims to optimize the viscosity. However, there is only little correlation between viscosity and lubrication function 4. In the oral cavity of dry mouth patients, aberrant salivary conditioning film2,3 (SCF) together with the saliva fluid, shows limited
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lubrication property. In Chapter 2&3, we have demonstrated that the use of two biopolymers, the supercharged polypeptide (SUP) and chitosan catechol (Chi-C), enhances the lubrication by changing the structural softness and composition of SCF with the saliva from xerostomia patient.
In Chapter 2, the lubrication enhancement ability of SUP was taken from an in vitro nanoscale proof of principle2 to the macroscale using a tongue-enamel friction system and saliva from patients suffering from Sjögren’s syndrome. SUPs are known to adsorb and rigidify an existing salivary condition film and then recruit mucins from the surrounding saliva to give rise to a very soft overlayer, increasing the water-holding ability of the SCF2. Effect of increasing molecular weight of SUP either by increasing the repeat units or dimerization due to cysteine termination was evaluated for lubrication enhancement in vitro at the nanoscale. The higher molecular weight SUP (K108cys) recruited higher amounts of negatively charged mucin from the limited saliva, yielding high glycosylation and softer overlayer to establish a layered and robust SCF (Chapter 2 Figure 1). The layered SCF by K108cys was able to prolong the duration of lubrication (relief period) for saliva from patients suffering from Sjögren’s syndrome (Chapter 2 Figure 3).
Intrigued by the positive effects of SUP on the SCF, a simpler mucoadhesive molecule derived from a natural source, i.e., chitosan catechol (Chi-C) was explored for its enhancing effect on salivary lubrication in Chapter 3. The effect of an increasing conjugation degree, i.e., Chi-C7.6%, Chi-C14.5%, and Chi-C22.4%., on lubrication enhancement was studied. A higher conjugation degree increased the mucoadhesive nature of Chi-C to mucin through catechol-mediated covalent reaction, as was previously found by others5. Chi-C showed a similar mechanism of action as SUPs. It showed that the high conjugation yielded a softer SCF and lower friction. Besides enhancing salivary lubrication, Chi-C was found to be antimicrobial and also reduced dental erosion.
From Chapters 2&3, two strong candidates emerged, i.e., K108cys and Chi-C22.4% for future translation towards clinical use. Both candidates were studied at the same concentration of 0.5mg/ml2 to test their lubrication enhancement. The SCF became more rigid with a lower value of the ratio between dissipation and
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frequency shift (∆D3/∆f3) (<0.08 in Chapter 2 Figure 1) after treatment with K108cys compared to SCF treatment with Chi-C22.4% with a relative higher ∆D3/∆f3 (>0.08 in chapter 3 figure 1). The more rigid structure could be caused by the more strong electrostatic force exerted by SUP due to a higher zeta potential (10 mV in Chapter 1) as compared to Chi-C (7.6 mV in Chapter 4). Although the lower zeta potential of Chi-C, after the recruitment of mucins from saliva, the structural softness of the SCF became very similar (∆D3/∆f3 around 0.17) with both two molecules. It indicates that besides electrostatic attraction between mucin and Chi-C, there are also present a certain amount of hydrogen bonding and covalent bonding5.
Both SUP and Chi-C work synergistically with salivary mucins, they consolidated the existing SCF and help recruit mucins which enhances lubrication. It demonstrates that our strategy works and will be helpful for xerostomia patients. K108cys and Chi-C22.4% performed similarly in lubrication enhancement ex-vivo by extending the relief period of patient SCF to 21±7.3 min and 25±4.8 min, respectively, as compared to 3.3±1.3 min for untreated SCF. Considering the commercial application, K108cys requires a recombinant route of synthesis using Escherichia coli, whereas Chi-C is chemically synthesized, making it easier to obtain in large amounts. Furthermore, the antimicrobial and anti-erosion capability of Chi-C makes it a more suitable molecule to help treat xerostomia patients.
Enhancing biolubrication in knee joint
In the knee joint6, an ultralow coefficient of friction (µ~0.005) 7,8 attributes to the biphasic structure of cartilage6,9, presence of an adsorbed film of proteins, polysaccharides, glycoproteins and lipids, i.e., lamina splendens9 and the synovial fluid6,9. Arthritis10 causes changes in the synovial fluid and lamina splendens composition associated with lubrication dysfunction. Intraarticular injection of hyaluronic acid for arthritis patients to enhance cartilage lubrication gives temporary pain relief11,12.
Thus, in Chapter 4, we applied our strategy of recruiting natural lubricating molecules through the addition of adhesive biopolymers and adopted Chi-C12.7% to stabilize the lamina splendens. The results show that Chi-C12.7% can
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efficiently recruit lubricin (PRG4) and other negatively-charged molecules like hyaluronic acid (HA)and albumin from the SF, increasing the softness, and enhancing lubrication. Our strategy demonstrates for the first time that lubricating molecules from the surrounding milieu can be recruited on the cartilage surface to enhance lubrication. Our strategy using the existing natural lubricants is different from immobilization of exogenous polyoxazoline graft-copolymers13, or biomimetic diblock copolymers14 to mimic the natural lubricants to the surface of degraded cartilage.
The stabilized lamina splendens enhance the lubrication of cartilage through recruitment of lubricating molecules from the surrounding milieu by Chi-C, suggesting that the presence of lubricating molecules in the fluid phase is not enough to enhance lubrication, but requires adsorption on the surface in the form of a conditioning film. A similar approach was reported by Singh et al.15 who used HA binding peptides to specifically recruit HA from the synovial fluid to enhance the lubrication of degraded cartilage. The difference is that our strategy is not limited to the specific binding of one type of molecule but is based on a more general binding of molecules in the synovial fluid as mediated by electrostatic interactions. Majd et al. 16 reported that the albumin present in the SF blocks the interaction between PRG4 and HA, leading to high friction. In our study, the Chi-C works through physical and chemical attraction, while albumin does not seem to block the interactions16.
Compared to the strategy that enhances lubrication by replacing the existing system with exogenous lubricating molecules, our strategy which utilizes the natural lubricants to stabilize the conditioning film by the widely used biocompatible biopolymers is safer and more durable. The exogenous lubricants may trigger inflammation and can be cleared up by the host leading to a short relief. Thus, multiple doses are necessary, e.g., repeated sprays of artificial saliva or multiple intraarticular injections of HA, which is frustrating for the patient. The fact is that the impaired lubrication system has reduced or altered natural lubricants still continuously secreted by the host that can use to build the new conditioning film with the help of an adhesive biopolymer restoring lubrication. Overall this research work has set the stage to provide
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better therapeutics/strategy for oral and knee joint lubrication enhancement that may be applicable for other body sites like eyes as well.
Biomaterial coating based on the recruitment strategy
For biomaterials employed at sliding interfaces with tissue, e.g., polycarbonate urethane (PCU) as meniscus implant, contact lenses on cornea surface 17, a lack of lubrication ability leads to pain and tissue degradation. Surface modification18 of biomaterials/implants is regarded as a promising way to improve the lubrication function of medical devices19.
In Chapter 5, taking the meniscus implant (PCU) as the example, we demonstrated, for the first time, that lubrication between the implant (PCU) and tissue (cartilage) can be improved through a coating, which recruits lubricating molecules from its surrounding to the surface. This film (PLL-HADN) contains the positively charged poly-L-lysine (PLL) and negatively charged mussel-inspired hyaluronic acid dopamine conjugate (HADN). The dopamine modification increases the adhesive strength of the film onto PCU, meanwhile, HADN shows a high affinity to PRG4 and can recruit the PRG4 from the synovial fluid to enhance lubrication and reduce wear of cartilage (Chapter 5 Figure 6). Majd et al. 16 found that compared to PCU, the mPCU with a more hydrophobic surface yielded a higher friction force as was measured with colloidal probe AFM, while the way to improve the lubrication function of PCU remained unclear. Some studies try to improve the lubrication of implant/medical devices by grafting exogenous lubricants like poly (2‐methacryloyloxyethyl phosphorylcholine (MPC)‐co‐n‐butyl methacrylate (BMA))18, Poly (2-methacryloyloxyethyl phosphorylcholine) 19, and mucin20 while it is inadequate on a lasting basis due to the clearing of the exogenous lubricants by the host. Our strategy seems more durable and persistent because of the continuous natural lubricant secreted by the host.
Future perspectives
With the increasing average age, our society faces a growing number of age-related chronic diseases, like dysfunction of lubrication, i.e., dry mouth, dry eye, and arthritis that decrease the quality of life21–23. The current clinical
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treatment with commercial artificial lubricants, i.e., artificial saliva, artificial tear, only shows temporary effects. Thus, a need for development of new therapeutic strategies for patients with friction-related disease is necessary. Herein, two strategies are utilized: (1) explore biomimetic lubricating fluid with exogenous lubricating molecules to overwhelm the natural system, (2) utilize the existing impaired lubricants to repair the lubrication system. New lubricating molecules with glycoprotein-based structure24 like brush polymer25, nanospheres26, and block copolymer14,27 have been designed for decades to mimic the natural lubricating fluid while none of them have made it to the clinic. This also suggests that the interaction between the exogenous lubricants and the naturally occurring conditioning film is important for their in vivo application. Furthermore, before a new compound can be used in the clinic, extensive safety assessments are required, which take a long time. In the long term, exogenous biomimetic lubricating molecules working with the natural lubrication system may yield good perspective for clinical application. However, in the short term, the strategy of utilizing the limited natural lubricants to repair the existing lubrication system with the help of ‘safe’ biopolymers seems much easier and more effective for clinical translation. With an increasing number of people that suffer from lubrication dysfunction-related diseases, the need to develop new exogenous lubricating molecules may be less urgent than the need to focus on clinical translation of available strategies, as presented in this thesis.
In this thesis, we evaluated the lubrication enhancement by various kinds of biopolymers through the creation/recruitment of a coating with natural lubricants in oral and knee joint. We did not investigate the eye because of the scarcity in proper ex-vivo models and limited availability of fluid (tear). Thus, setting up a relevant eye model in future work is needed to investigate if our strategy also works in the dry eye patient. Furthermore, future work should include in vivo testing in animal models to stimulate the clinical translation.
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