Influence of thermocycling and coffee immersion on surface roughness and bacterial adherence of additively manufactured permanent resin based materials

The Sticky Situation: Why Your Tooth Crown Might Be a Bacterial Hotel

Imagine your mouth is a bustling, chaotic city. It's warm, wet, and constantly changing temperature as you sip hot coffee or bite into an ice cream. In this city, tiny microscopic tenants—bacteria—are always looking for a place to crash. Some of these tenants are troublemakers; they love to stick to surfaces, build messy neighborhoods called biofilms, and cause cavities. For a long time, dentists used to carve these "buildings" (crowns and bridges) out of solid blocks of material, but now, they are increasingly using 3D printers to build them layer by layer, like stacking pancakes.

However, just because a building is 3D printed doesn't mean it's perfect. Over time, the heat of a hot drink followed by the cold of a winter breeze (a process scientists call thermocycling) can stress the material, making it crack or get rougher. And if you drink coffee, the dark, sticky liquid can stain the surface and change its texture. The big question is: Does this roughening turn a smooth, easy-to-clean crown into a bumpy, sticky trap that bacteria love to cling to? If the surface gets rough, do the bacteria get happier? This study dives into that exact scenario, testing three different 3D-printed crown materials to see how they hold up against the daily grind of hot coffee and temperature swings, and just how many bacteria decide to move in.

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The Experiment: A 3D-Printed Race Against Time and Coffee

In this study, researchers set up a high-stakes race between three different 3D-printed resin materials used for permanent dental crowns: VarseoSmile TriniQ, SprintRay Ceramic Crown Resin, and Tera Harz C&B TC-80DP. They didn't just look at them; they put them through a grueling training camp designed to mimic years of real-life wear in a single month.

First, they created tiny, disc-shaped samples of each material, polishing them until they were as smooth as a freshly paved road. Then, they split them into three groups. The first group was the "control" team, left in warm water for a day. The second group went through 10,000 cycles of thermocycling, which means they were dunked back and forth between freezing cold water (5°C) and scalding hot water (55°C). This simulates the stress of drinking hot coffee and then cold soda. The third group got the "double whammy": they endured the 10,000 temperature cycles and then were soaked in a strong cup of instant coffee for 30 days.

After this torture test, the researchers measured two things. First, they used a super-precise laser microscope to measure surface roughness (how bumpy the road is). Second, they introduced two types of bacteria: Streptococcus mutans (the notorious cavities-causing troublemaker) and Streptococcus sanguinis (an early colonizer that helps build the initial biofilm). They counted exactly how many bacteria stuck to each disc.

The Results: Smooth Wins, Rough Loses

The findings were clear and consistent, like a story with a predictable but important ending.

1. The Roughness Race
The study found that aging definitely made the surfaces bumpier. The smoothest surfaces were the ones that hadn't been aged at all. Once the materials went through the 10,000 temperature cycles, they got rougher. But the real champions of roughness were the ones that went through the coffee soak after the heat cycles.

• The Winner: Tera Harz C&B TC-80DP stayed the smoothest of the bunch, even after the abuse. Its roughness (measured as Ra) only went up to 0.11 ± 0.03 µm after the coffee soak.
• The Loser: VarseoSmile TriniQ ended up with the roughest surface, reaching 0.18 ± 0.04 µm after the coffee treatment.
• The Middle: SprintRay Ceramic Crown Resin landed in the middle with 0.15 ± 0.03 µm.

The researchers noted that while the materials reacted differently, the pattern was the same for all of them: heat made them rougher, and adding coffee made them even rougher. Interestingly, the interaction between the specific material and the aging process wasn't a complex puzzle; the aging just made everything rougher in a similar way.

2. The Bacterial Invasion
Here is where the story gets sticky. The researchers found a direct link: the rougher the surface, the more bacteria stuck to it. It's like a velcro strip; the more hooks (roughness) you have, the more loops (bacteria) can catch.

• The Bacterial King: Streptococcus mutans (the cavity-causing bacteria) was much stickier than Streptococcus sanguinis. Across all materials and conditions, S. mutans showed higher adhesion.
• The Worst Case Scenario: The material with the highest bacterial adhesion was VarseoSmile TriniQ after thermocycling and coffee immersion, with 4.20 ± 0.28 log₁₀(CFU/mm²).
• The Best Case Scenario: Tera Harz C&B TC-80DP had the lowest bacterial adhesion, with 3.15 ± 0.20 log₁₀(CFU/mm²) for S. mutans at baseline, rising to 3.65 ± 0.24 after the coffee soak.

The study explicitly rejected the initial guess (the null hypothesis) that material type or aging wouldn't matter. The data proved the opposite: both the type of material and the aging protocol significantly changed how rough the surface was and how many bacteria stuck to it. However, the study also found that the interaction between the material and the aging didn't create a unique, unpredictable surprise for any specific pair; the aging just made things worse for everyone, just to different degrees.

The Takeaway: Smooth is Sweet

This study suggests that if you are getting a 3D-printed permanent crown, the material matters, but so does how well it is finished. The Tera Harz C&B TC-80DP material proved to be the most resistant to becoming rough and sticky, while VarseoSmile TriniQ was the most prone to getting bumpy and attracting bacteria.

The researchers concluded that the rougher the surface gets (due to heat and coffee), the more likely it is to become a playground for bacteria, especially the cavity-causing S. mutans. While the numbers showed a strong connection between roughness and stickiness, the study also reminded us that it's not just about bumps; the chemical makeup of the material also plays a role. But for now, the message is clear: keep those 3D-printed crowns as smooth as possible, because a smooth surface is a surface that bacteria find much harder to grab onto.