• At least 18 years old - Diagnosis of rheumatoid arthritis as defined by ACR/EULAR 2010 classification criteria - At least moderate disease activity, defined as ≥4 tender joints (28-joint count) and ≥4 swollen joints (28-joint count) and a DAS28-CRP >3.2 at the baseline visit - hsCRP > 0.3 mg/dL at the last qualifying visit, baseline or retest - If on background DMARD therapy, must be on stable dose (see

) - Able and willing to comply with all study-related procedures, including daily treatment sessions in the study vehicle, research site visits, and assessments Exclusion Criteria: - Unable to provide informed consent - Current or planned participation in another interventional clinical trial - Inflammatory joint disease other than RA (including but not limited to gout, systemic lupus erythematosus, psoriatic arthritis, axial spondyloarthritis including ankylosing spondylitis and non-radiographic axial spondyloarthritis, reactive arthritis, overlap connective tissue diseases, scleroderma, polymyositis, dermatomyositis. Current diagnosis of secondary Sjogren's Syndrome is permitted. - Prior use of >2 biologic or targeted synthetic DMARDs for RA where the primary reason for discontinuation was efficacy - Conventional synthetic DMARDs: - Initiation within 12 weeks prior to enrollment - Dose adjustment or discontinuation within 4 weeks prior to enrollment - If on a stable dose, inability to maintain the stable dose during the study period - Biologic DMARDs: - Initiation or dose adjustment within 12 weeks prior to enrollment - Discontinuation within 4 weeks prior to enrollment - If on a stable dose, inability to maintain the stable dose during the study period - JAK inhibitors: - Initiation, dose adjustment, or discontinuation within 4 weeks prior to enrollment - If on a stable dose, inability to maintain the stable dose during the study period - Corticosteroids: - Initiation, dose adjustment, or discontinuation within 4 weeks prior to enrollment - Current dose >10 mg/day prednisone (or equivalent) - If on a stable dose, inability to maintain the stable dose during the study - NSAIDs: - Initiation, dose adjustment, or discontinuation of high-dose NSAIDs (≥1200 mg/day ibuprofen) within 14 days of informed consent. Over-the-counter use of NSAIDs is permissible. - Pregnant or planning to become pregnant during the study period - Known hypersensitivity to ultrasound gel or membrane components - History of splenic disorders, including splenectomy, congenital asplenia, or splenomegaly on baseline visit ultrasound - Rash, wound, or skin infection overlying the spleen - History of vagal nerve injury, vagotomy, or known autonomic neuropathy - Recent abdominal surgery or trauma within 30 days of screening - Skin to spleen hilum depth >7 cm as measured by ultrasound at the baseline visit - Abdominal anatomy or condition precluding adequate ultrasound targeting of the spleen - Uncontrolled fibromyalgia or other diffuse pain syndromes that may confound symptom reporting - Any condition that, in the investigator's judgment, would preclude safe participation or compliance with study procedures